As of now LoadPin restricts loading of kernel files to a single pinned
filesystem, typically the rootfs. This works for many systems, however it
can result in a bloated rootfs (and OTA updates) on platforms where
multiple boards with different hardware configurations use the same rootfs
image. Especially when 'optional' files are large it may be preferable to
download/install them only when they are actually needed by a given board.
Chrome OS uses Downloadable Content (DLC) [1] to deploy certain 'packages'
at runtime. As an example a DLC package could contain firmware for a
peripheral that is not present on all boards. DLCs use dm-verity [2] to
verify the integrity of the DLC content.
This series extends LoadPin to allow loading of kernel files from trusted
dm-verity devices. LoadPin maintains a list of root digests of verity
devices it considers trusted. Userspace can populate this list through an
ioctl on the new LoadPin securityfs entry 'dm-verity'. The ioctl receives
a file descriptor of a file with verity digests as parameter. Verity reads
the digests from this file after confirming that the file is located on the
pinned root. The list of trusted digests can only be set up once, which is
typically done at boot time.
When a kernel file is read LoadPin first checks (as usual) whether the file
is located on the pinned root, if so the file can be loaded. Otherwise, if
the verity extension is enabled, LoadPin determines whether the file is
located on a verity backed device and whether the root digest of that
device is in the list of trusted digests. The file can be loaded if the
verity device has a trusted root digest.
[1] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
[2] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
Changes in v3:
- added securityfs for LoadPin (currently only populated when
CONFIG_SECURITY_LOADPIN_VERITY=y)
- added uapi include for LoadPin
- changed the interface for setting up the list of trusted
digests from sysctl to ioctl on securityfs entry
- added stub for loadpin_is_fs_trusted() to be used
CONFIG_SECURITY_LOADPIN_VERITY is not select
- depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
- updated Kconfig help
- minor changes in read_trusted_verity_root_digests()
- updated commit message
Changes in v2:
- userspace now passes the path of the file with the verity digests
via systcl, instead of the digests themselves
- renamed sysctl file to 'trusted_verity_root_digests_path'
- have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
- updated Kconfig doc
- updated commit message
Matthias Kaehlcke (3):
dm: Add verity helpers for LoadPin
LoadPin: Enable loading from trusted dm-verity devices
dm: verity-loadpin: Use CONFIG_SECURITY_LOADPIN_VERITY for conditional
compilation
drivers/md/Makefile | 1 +
drivers/md/dm-verity-loadpin.c | 80 +++++++++++++
drivers/md/dm-verity-target.c | 33 ++++++
drivers/md/dm-verity.h | 4 +
include/linux/dm-verity-loadpin.h | 27 +++++
include/uapi/linux/loadpin.h | 19 +++
security/loadpin/Kconfig | 16 +++
security/loadpin/loadpin.c | 184 +++++++++++++++++++++++++++++-
8 files changed, 363 insertions(+), 1 deletion(-)
create mode 100644 drivers/md/dm-verity-loadpin.c
create mode 100644 include/linux/dm-verity-loadpin.h
create mode 100644 include/uapi/linux/loadpin.h
--
2.36.0.464.gb9c8b46e94-goog
Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
devices.
This change adds the concept of trusted verity devices to LoadPin. LoadPin
maintains a list of root digests of verity devices it considers trusted.
Userspace can populate this list through an ioctl on the new LoadPin
securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
a file with verity digests as parameter. Verity reads the digests from
this file after confirming that the file is located on the pinned root.
The list of trusted digests can only be set up once, which is typically
done at boot time.
When a kernel file is read LoadPin first checks (as usual) whether the file
is located on the pinned root, if so the file can be loaded. Otherwise, if
the verity extension is enabled, LoadPin determines whether the file is
located on a verity backed device and whether the root digest of that
device is in the list of trusted digests. The file can be loaded if the
verity device has a trusted root digest.
Background:
As of now LoadPin restricts loading of kernel files to a single pinned
filesystem, typically the rootfs. This works for many systems, however it
can result in a bloated rootfs (and OTA updates) on platforms where
multiple boards with different hardware configurations use the same rootfs
image. Especially when 'optional' files are large it may be preferable to
download/install them only when they are actually needed by a given board.
Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
at runtime. As an example a DLC package could contain firmware for a
peripheral that is not present on all boards. DLCs use dm-verity to verify
the integrity of the DLC content.
[1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
[2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
Signed-off-by: Matthias Kaehlcke <[email protected]>
---
Changes in v3:
- added securityfs for LoadPin (currently only populated when
CONFIG_SECURITY_LOADPIN_VERITY=y)
- added uapi include for LoadPin
- changed the interface for setting up the list of trusted
digests from sysctl to ioctl on securityfs entry
- added stub for loadpin_is_fs_trusted() to be used
CONFIG_SECURITY_LOADPIN_VERITY is not select
- depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
- updated Kconfig help
- minor changes in read_trusted_verity_root_digests()
- updated commit message
Changes in v2:
- userspace now passes the path of the file with the verity digests
via systcl, instead of the digests themselves
- renamed sysctl file to 'trusted_verity_root_digests_path'
- have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
- updated Kconfig doc
- updated commit message
include/uapi/linux/loadpin.h | 19 ++++
security/loadpin/Kconfig | 16 +++
security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
3 files changed, 218 insertions(+), 1 deletion(-)
create mode 100644 include/uapi/linux/loadpin.h
diff --git a/include/uapi/linux/loadpin.h b/include/uapi/linux/loadpin.h
new file mode 100644
index 000000000000..d303a582209b
--- /dev/null
+++ b/include/uapi/linux/loadpin.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Copyright (c) 2022, Google LLC
+ */
+
+#ifndef _UAPI_LINUX_LOOP_LOADPIN_H
+#define _UAPI_LINUX_LOOP_LOADPIN_H
+
+#define LOADPIN_IOC_MAGIC 'L'
+
+/**
+ * LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS - Set up the root digests of verity devices
+ * that loadpin should trust.
+ *
+ * Takes a file descriptor from which to read the root digests of trusted verity devices.
+ */
+#define LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS _IOW(LOADPIN_IOC_MAGIC, 0x00, unsigned int)
+
+#endif /* _UAPI_LINUX_LOOP_LOADPIN_H */
diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
index 91be65dec2ab..e319ca8e3f3d 100644
--- a/security/loadpin/Kconfig
+++ b/security/loadpin/Kconfig
@@ -18,3 +18,19 @@ config SECURITY_LOADPIN_ENFORCE
If selected, LoadPin will enforce pinning at boot. If not
selected, it can be enabled at boot with the kernel parameter
"loadpin.enforce=1".
+
+config SECURITY_LOADPIN_VERITY
+ bool "Allow reading files from certain other filesystems that use dm-verity"
+ depends on DM_VERITY=y && SECURITYFS
+ help
+ If selected LoadPin can allow reading files from filesystems
+ that use dm-verity. LoadPin maintains a list of verity root
+ digests it considers trusted. A verity backed filesystem is
+ considered trusted if its root digest is found in the list
+ of trusted digests.
+
+ The list of trusted verity can be populated through an ioctl
+ on the LoadPin securityfs entry 'dm-verity'. The ioctl
+ expects a file descriptor of a file with verity digests as
+ parameter. The file must be located on the pinned root and
+ contain a comma separated list of digests.
diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index b12f7d986b1e..c29ce562a366 100644
--- a/security/loadpin/loadpin.c
+++ b/security/loadpin/loadpin.c
@@ -18,6 +18,9 @@
#include <linux/path.h>
#include <linux/sched.h> /* current */
#include <linux/string_helpers.h>
+#include <linux/device-mapper.h>
+#include <linux/dm-verity-loadpin.h>
+#include <uapi/linux/loadpin.h>
static void report_load(const char *origin, struct file *file, char *operation)
{
@@ -43,6 +46,9 @@ static char *exclude_read_files[READING_MAX_ID];
static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
static struct super_block *pinned_root;
static DEFINE_SPINLOCK(pinned_root_spinlock);
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+static LIST_HEAD(trusted_verity_root_digests);
+#endif
#ifdef CONFIG_SYSCTL
@@ -118,6 +124,24 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
}
}
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+static bool loadpin_is_fs_trusted(struct super_block *sb)
+{
+ struct mapped_device *md = dm_get_md(sb->s_bdev->bd_dev);
+ bool trusted;
+
+ if (!md)
+ return false;
+
+ trusted = dm_verity_loadpin_is_md_trusted(md);
+ dm_put(md);
+
+ return trusted;
+}
+#else
+static inline bool loadpin_is_fs_trusted(struct super_block *sb) { return false; };
+#endif
+
static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
bool contents)
{
@@ -174,7 +198,8 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
spin_unlock(&pinned_root_spinlock);
}
- if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
+ if (IS_ERR_OR_NULL(pinned_root) ||
+ ((load_root != pinned_root) && !loadpin_is_fs_trusted(load_root))) {
if (unlikely(!enforce)) {
report_load(origin, file, "pinning-ignored");
return 0;
@@ -240,6 +265,7 @@ static int __init loadpin_init(void)
enforce ? "" : "not ");
parse_exclude();
security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
+
return 0;
}
@@ -248,6 +274,162 @@ DEFINE_LSM(loadpin) = {
.init = loadpin_init,
};
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+
+enum loadpin_securityfs_interface_index {
+ LOADPIN_DM_VERITY,
+};
+
+static int read_trusted_verity_root_digests(unsigned int fd)
+{
+ struct fd f;
+ void *data;
+ int rc;
+ char *p, *d;
+
+ /* The list of trusted root digests can only be set up once */
+ if (!list_empty(&trusted_verity_root_digests))
+ return -EPERM;
+
+ f = fdget(fd);
+ if (!f.file)
+ return -EINVAL;
+
+ data = kzalloc(SZ_4K, GFP_KERNEL);
+ if (!data) {
+ rc = -ENOMEM;
+ goto err;
+ }
+
+ rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
+ if (rc < 0)
+ goto err;
+
+ ((char *)data)[rc] = '\0';
+
+ p = strim(data);
+ while ((d = strsep(&p, ",")) != NULL) {
+ int len = strlen(d);
+ struct trusted_root_digest *trd;
+
+ if (len % 2) {
+ rc = -EPROTO;
+ goto err;
+ }
+
+ len /= 2;
+
+ trd = kzalloc(sizeof(*trd), GFP_KERNEL);
+ if (!trd) {
+ rc = -ENOMEM;
+ goto err;
+ }
+
+ trd->data = kzalloc(len, GFP_KERNEL);
+ if (!trd->data) {
+ kfree(trd);
+ rc = -ENOMEM;
+ goto err;
+ }
+
+ if (hex2bin(trd->data, d, len)) {
+ kfree(trd);
+ kfree(trd->data);
+ rc = -EPROTO;
+ goto err;
+ }
+
+ trd->len = len;
+
+ list_add_tail(&trd->node, &trusted_verity_root_digests);
+ }
+
+ kfree(data);
+ fdput(f);
+
+ if (!list_empty(&trusted_verity_root_digests))
+ dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
+
+ return 0;
+
+err:
+ kfree(data);
+
+ {
+ struct trusted_root_digest *trd, *tmp;
+
+ list_for_each_entry_safe(trd, tmp, &trusted_verity_root_digests, node) {
+ kfree(trd->data);
+ list_del(&trd->node);
+ kfree(trd);
+ }
+ }
+
+ fdput(f);
+
+ return rc;
+}
+
+/******************************** securityfs ********************************/
+
+static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+{
+ void __user *uarg = (void __user *)arg;
+ unsigned int fd;
+ int rc;
+
+ switch (cmd) {
+ case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
+ rc = copy_from_user(&fd, uarg, sizeof(fd));
+ if (rc)
+ return rc;
+
+ return read_trusted_verity_root_digests(fd);
+
+ default:
+ return -EINVAL;
+ }
+}
+
+static const struct file_operations loadpin_dm_verity_ops = {
+ .unlocked_ioctl = dm_verity_ioctl,
+ .compat_ioctl = compat_ptr_ioctl,
+};
+
+/**
+ * init_loadpin_securityfs - create the securityfs directory for LoadPin
+ *
+ * We can not put this method normally under the loadpin_init() code path since
+ * the security subsystem gets initialized before the vfs caches.
+ *
+ * Returns 0 if the securityfs directory creation was successful.
+ */
+static int __init init_loadpin_securityfs(void)
+{
+ struct dentry *loadpin_dir, *dentry;
+
+ loadpin_dir = securityfs_create_dir("loadpin", NULL);
+ if (IS_ERR(loadpin_dir)) {
+ pr_err("LoadPin: could not create securityfs dir: %d\n",
+ PTR_ERR(loadpin_dir));
+ return PTR_ERR(loadpin_dir);
+ }
+
+ dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
+ (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
+ if (IS_ERR(dentry)) {
+ pr_err("LoadPin: could not create securityfs entry 'dm-verity': %d\n",
+ PTR_ERR(dentry));
+ return PTR_ERR(dentry);
+ }
+
+ return 0;
+}
+
+fs_initcall(init_loadpin_securityfs);
+
+#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
+
/* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
module_param(enforce, int, 0);
MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
--
2.36.0.464.gb9c8b46e94-goog
On May 13, 2022 9:32:12 AM PDT, Mike Snitzer <[email protected]> wrote:
>On Wed, May 04 2022 at 3:54P -0400,
>Matthias Kaehlcke <[email protected]> wrote:
>
>> Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
>> devices.
>>
>> This change adds the concept of trusted verity devices to LoadPin. LoadPin
>> maintains a list of root digests of verity devices it considers trusted.
>> Userspace can populate this list through an ioctl on the new LoadPin
>> securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
>> a file with verity digests as parameter. Verity reads the digests from
>> this file after confirming that the file is located on the pinned root.
>> The list of trusted digests can only be set up once, which is typically
>> done at boot time.
>>
>> When a kernel file is read LoadPin first checks (as usual) whether the file
>> is located on the pinned root, if so the file can be loaded. Otherwise, if
>> the verity extension is enabled, LoadPin determines whether the file is
>> located on a verity backed device and whether the root digest of that
>> device is in the list of trusted digests. The file can be loaded if the
>> verity device has a trusted root digest.
>>
>> Background:
>>
>> As of now LoadPin restricts loading of kernel files to a single pinned
>> filesystem, typically the rootfs. This works for many systems, however it
>> can result in a bloated rootfs (and OTA updates) on platforms where
>> multiple boards with different hardware configurations use the same rootfs
>> image. Especially when 'optional' files are large it may be preferable to
>> download/install them only when they are actually needed by a given board.
>> Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
>> at runtime. As an example a DLC package could contain firmware for a
>> peripheral that is not present on all boards. DLCs use dm-verity to verify
>> the integrity of the DLC content.
>>
>> [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
>> [2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
>>
>> Signed-off-by: Matthias Kaehlcke <[email protected]>
>> ---
>>
>> Changes in v3:
>> - added securityfs for LoadPin (currently only populated when
>> CONFIG_SECURITY_LOADPIN_VERITY=y)
>> - added uapi include for LoadPin
>> - changed the interface for setting up the list of trusted
>> digests from sysctl to ioctl on securityfs entry
>> - added stub for loadpin_is_fs_trusted() to be used
>> CONFIG_SECURITY_LOADPIN_VERITY is not select
>> - depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
>> - updated Kconfig help
>> - minor changes in read_trusted_verity_root_digests()
>> - updated commit message
>>
>> Changes in v2:
>> - userspace now passes the path of the file with the verity digests
>> via systcl, instead of the digests themselves
>> - renamed sysctl file to 'trusted_verity_root_digests_path'
>> - have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
>> - updated Kconfig doc
>> - updated commit message
>>
>> include/uapi/linux/loadpin.h | 19 ++++
>> security/loadpin/Kconfig | 16 +++
>> security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
>> 3 files changed, 218 insertions(+), 1 deletion(-)
>> create mode 100644 include/uapi/linux/loadpin.h
>
>I would certainly need some Reviewed-by:s from security and/or loadpin
>experts if I were to pick this patch up.
Alternatively, since it's mostly touching loadpin, I can carry it in my tree, as long as you've Acked the dm bits. :)
>Did you see the issues the kernel test robot emailed about?
>
>You'd do well to fix those issues up when submitting another revision
>of this patchset.
Agreed.
--
Kees Cook
On Wed, May 04 2022 at 3:54P -0400,
Matthias Kaehlcke <[email protected]> wrote:
> Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> devices.
>
> This change adds the concept of trusted verity devices to LoadPin. LoadPin
> maintains a list of root digests of verity devices it considers trusted.
> Userspace can populate this list through an ioctl on the new LoadPin
> securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> a file with verity digests as parameter. Verity reads the digests from
> this file after confirming that the file is located on the pinned root.
> The list of trusted digests can only be set up once, which is typically
> done at boot time.
>
> When a kernel file is read LoadPin first checks (as usual) whether the file
> is located on the pinned root, if so the file can be loaded. Otherwise, if
> the verity extension is enabled, LoadPin determines whether the file is
> located on a verity backed device and whether the root digest of that
> device is in the list of trusted digests. The file can be loaded if the
> verity device has a trusted root digest.
>
> Background:
>
> As of now LoadPin restricts loading of kernel files to a single pinned
> filesystem, typically the rootfs. This works for many systems, however it
> can result in a bloated rootfs (and OTA updates) on platforms where
> multiple boards with different hardware configurations use the same rootfs
> image. Especially when 'optional' files are large it may be preferable to
> download/install them only when they are actually needed by a given board.
> Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
> at runtime. As an example a DLC package could contain firmware for a
> peripheral that is not present on all boards. DLCs use dm-verity to verify
> the integrity of the DLC content.
>
> [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
> [2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
>
> Signed-off-by: Matthias Kaehlcke <[email protected]>
> ---
>
> Changes in v3:
> - added securityfs for LoadPin (currently only populated when
> CONFIG_SECURITY_LOADPIN_VERITY=y)
> - added uapi include for LoadPin
> - changed the interface for setting up the list of trusted
> digests from sysctl to ioctl on securityfs entry
> - added stub for loadpin_is_fs_trusted() to be used
> CONFIG_SECURITY_LOADPIN_VERITY is not select
> - depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
> - updated Kconfig help
> - minor changes in read_trusted_verity_root_digests()
> - updated commit message
>
> Changes in v2:
> - userspace now passes the path of the file with the verity digests
> via systcl, instead of the digests themselves
> - renamed sysctl file to 'trusted_verity_root_digests_path'
> - have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
> - updated Kconfig doc
> - updated commit message
>
> include/uapi/linux/loadpin.h | 19 ++++
> security/loadpin/Kconfig | 16 +++
> security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
> 3 files changed, 218 insertions(+), 1 deletion(-)
> create mode 100644 include/uapi/linux/loadpin.h
I would certainly need some Reviewed-by:s from security and/or loadpin
experts if I were to pick this patch up.
Did you see the issues the kernel test robot emailed about?
You'd do well to fix those issues up when submitting another revision
of this patchset.
Mike
>
> diff --git a/include/uapi/linux/loadpin.h b/include/uapi/linux/loadpin.h
> new file mode 100644
> index 000000000000..d303a582209b
> --- /dev/null
> +++ b/include/uapi/linux/loadpin.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/*
> + * Copyright (c) 2022, Google LLC
> + */
> +
> +#ifndef _UAPI_LINUX_LOOP_LOADPIN_H
> +#define _UAPI_LINUX_LOOP_LOADPIN_H
> +
> +#define LOADPIN_IOC_MAGIC 'L'
> +
> +/**
> + * LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS - Set up the root digests of verity devices
> + * that loadpin should trust.
> + *
> + * Takes a file descriptor from which to read the root digests of trusted verity devices.
> + */
> +#define LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS _IOW(LOADPIN_IOC_MAGIC, 0x00, unsigned int)
> +
> +#endif /* _UAPI_LINUX_LOOP_LOADPIN_H */
> diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
> index 91be65dec2ab..e319ca8e3f3d 100644
> --- a/security/loadpin/Kconfig
> +++ b/security/loadpin/Kconfig
> @@ -18,3 +18,19 @@ config SECURITY_LOADPIN_ENFORCE
> If selected, LoadPin will enforce pinning at boot. If not
> selected, it can be enabled at boot with the kernel parameter
> "loadpin.enforce=1".
> +
> +config SECURITY_LOADPIN_VERITY
> + bool "Allow reading files from certain other filesystems that use dm-verity"
> + depends on DM_VERITY=y && SECURITYFS
> + help
> + If selected LoadPin can allow reading files from filesystems
> + that use dm-verity. LoadPin maintains a list of verity root
> + digests it considers trusted. A verity backed filesystem is
> + considered trusted if its root digest is found in the list
> + of trusted digests.
> +
> + The list of trusted verity can be populated through an ioctl
> + on the LoadPin securityfs entry 'dm-verity'. The ioctl
> + expects a file descriptor of a file with verity digests as
> + parameter. The file must be located on the pinned root and
> + contain a comma separated list of digests.
> diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> index b12f7d986b1e..c29ce562a366 100644
> --- a/security/loadpin/loadpin.c
> +++ b/security/loadpin/loadpin.c
> @@ -18,6 +18,9 @@
> #include <linux/path.h>
> #include <linux/sched.h> /* current */
> #include <linux/string_helpers.h>
> +#include <linux/device-mapper.h>
> +#include <linux/dm-verity-loadpin.h>
> +#include <uapi/linux/loadpin.h>
>
> static void report_load(const char *origin, struct file *file, char *operation)
> {
> @@ -43,6 +46,9 @@ static char *exclude_read_files[READING_MAX_ID];
> static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
> static struct super_block *pinned_root;
> static DEFINE_SPINLOCK(pinned_root_spinlock);
> +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> +static LIST_HEAD(trusted_verity_root_digests);
> +#endif
>
> #ifdef CONFIG_SYSCTL
>
> @@ -118,6 +124,24 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
> }
> }
>
> +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> +static bool loadpin_is_fs_trusted(struct super_block *sb)
> +{
> + struct mapped_device *md = dm_get_md(sb->s_bdev->bd_dev);
> + bool trusted;
> +
> + if (!md)
> + return false;
> +
> + trusted = dm_verity_loadpin_is_md_trusted(md);
> + dm_put(md);
> +
> + return trusted;
> +}
> +#else
> +static inline bool loadpin_is_fs_trusted(struct super_block *sb) { return false; };
> +#endif
> +
> static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> bool contents)
> {
> @@ -174,7 +198,8 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> spin_unlock(&pinned_root_spinlock);
> }
>
> - if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
> + if (IS_ERR_OR_NULL(pinned_root) ||
> + ((load_root != pinned_root) && !loadpin_is_fs_trusted(load_root))) {
> if (unlikely(!enforce)) {
> report_load(origin, file, "pinning-ignored");
> return 0;
> @@ -240,6 +265,7 @@ static int __init loadpin_init(void)
> enforce ? "" : "not ");
> parse_exclude();
> security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
> +
> return 0;
> }
>
> @@ -248,6 +274,162 @@ DEFINE_LSM(loadpin) = {
> .init = loadpin_init,
> };
>
> +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> +
> +enum loadpin_securityfs_interface_index {
> + LOADPIN_DM_VERITY,
> +};
> +
> +static int read_trusted_verity_root_digests(unsigned int fd)
> +{
> + struct fd f;
> + void *data;
> + int rc;
> + char *p, *d;
> +
> + /* The list of trusted root digests can only be set up once */
> + if (!list_empty(&trusted_verity_root_digests))
> + return -EPERM;
> +
> + f = fdget(fd);
> + if (!f.file)
> + return -EINVAL;
> +
> + data = kzalloc(SZ_4K, GFP_KERNEL);
> + if (!data) {
> + rc = -ENOMEM;
> + goto err;
> + }
> +
> + rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
> + if (rc < 0)
> + goto err;
> +
> + ((char *)data)[rc] = '\0';
> +
> + p = strim(data);
> + while ((d = strsep(&p, ",")) != NULL) {
> + int len = strlen(d);
> + struct trusted_root_digest *trd;
> +
> + if (len % 2) {
> + rc = -EPROTO;
> + goto err;
> + }
> +
> + len /= 2;
> +
> + trd = kzalloc(sizeof(*trd), GFP_KERNEL);
> + if (!trd) {
> + rc = -ENOMEM;
> + goto err;
> + }
> +
> + trd->data = kzalloc(len, GFP_KERNEL);
> + if (!trd->data) {
> + kfree(trd);
> + rc = -ENOMEM;
> + goto err;
> + }
> +
> + if (hex2bin(trd->data, d, len)) {
> + kfree(trd);
> + kfree(trd->data);
> + rc = -EPROTO;
> + goto err;
> + }
> +
> + trd->len = len;
> +
> + list_add_tail(&trd->node, &trusted_verity_root_digests);
> + }
> +
> + kfree(data);
> + fdput(f);
> +
> + if (!list_empty(&trusted_verity_root_digests))
> + dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
> +
> + return 0;
> +
> +err:
> + kfree(data);
> +
> + {
> + struct trusted_root_digest *trd, *tmp;
> +
> + list_for_each_entry_safe(trd, tmp, &trusted_verity_root_digests, node) {
> + kfree(trd->data);
> + list_del(&trd->node);
> + kfree(trd);
> + }
> + }
> +
> + fdput(f);
> +
> + return rc;
> +}
> +
> +/******************************** securityfs ********************************/
> +
> +static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> +{
> + void __user *uarg = (void __user *)arg;
> + unsigned int fd;
> + int rc;
> +
> + switch (cmd) {
> + case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
> + rc = copy_from_user(&fd, uarg, sizeof(fd));
> + if (rc)
> + return rc;
> +
> + return read_trusted_verity_root_digests(fd);
> +
> + default:
> + return -EINVAL;
> + }
> +}
> +
> +static const struct file_operations loadpin_dm_verity_ops = {
> + .unlocked_ioctl = dm_verity_ioctl,
> + .compat_ioctl = compat_ptr_ioctl,
> +};
> +
> +/**
> + * init_loadpin_securityfs - create the securityfs directory for LoadPin
> + *
> + * We can not put this method normally under the loadpin_init() code path since
> + * the security subsystem gets initialized before the vfs caches.
> + *
> + * Returns 0 if the securityfs directory creation was successful.
> + */
> +static int __init init_loadpin_securityfs(void)
> +{
> + struct dentry *loadpin_dir, *dentry;
> +
> + loadpin_dir = securityfs_create_dir("loadpin", NULL);
> + if (IS_ERR(loadpin_dir)) {
> + pr_err("LoadPin: could not create securityfs dir: %d\n",
> + PTR_ERR(loadpin_dir));
> + return PTR_ERR(loadpin_dir);
> + }
> +
> + dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
> + (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
> + if (IS_ERR(dentry)) {
> + pr_err("LoadPin: could not create securityfs entry 'dm-verity': %d\n",
> + PTR_ERR(dentry));
> + return PTR_ERR(dentry);
> + }
> +
> + return 0;
> +}
> +
> +fs_initcall(init_loadpin_securityfs);
> +
> +#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
> +
> /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
> module_param(enforce, int, 0);
> MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
> --
> 2.36.0.464.gb9c8b46e94-goog
>
On May 4, 2022 12:54:18 PM PDT, Matthias Kaehlcke <[email protected]> wrote:
>Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
>devices.
>
>This change adds the concept of trusted verity devices to LoadPin. LoadPin
>maintains a list of root digests of verity devices it considers trusted.
>Userspace can populate this list through an ioctl on the new LoadPin
>securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
>a file with verity digests as parameter. Verity reads the digests from
>this file after confirming that the file is located on the pinned root.
>The list of trusted digests can only be set up once, which is typically
>done at boot time.
>
>When a kernel file is read LoadPin first checks (as usual) whether the file
>is located on the pinned root, if so the file can be loaded. Otherwise, if
>the verity extension is enabled, LoadPin determines whether the file is
>located on a verity backed device and whether the root digest of that
I think this should be "... on an already trusted device ..."
>device is in the list of trusted digests. The file can be loaded if the
>verity device has a trusted root digest.
>
>Background:
>
>As of now LoadPin restricts loading of kernel files to a single pinned
>filesystem, typically the rootfs. This works for many systems, however it
>can result in a bloated rootfs (and OTA updates) on platforms where
>multiple boards with different hardware configurations use the same rootfs
>image. Especially when 'optional' files are large it may be preferable to
>download/install them only when they are actually needed by a given board.
>Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
>at runtime. As an example a DLC package could contain firmware for a
>peripheral that is not present on all boards. DLCs use dm-verity to verify
>the integrity of the DLC content.
>
>[1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
>[2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
>
>Signed-off-by: Matthias Kaehlcke <[email protected]>
>---
>
>Changes in v3:
>- added securityfs for LoadPin (currently only populated when
> CONFIG_SECURITY_LOADPIN_VERITY=y)
>- added uapi include for LoadPin
>- changed the interface for setting up the list of trusted
> digests from sysctl to ioctl on securityfs entry
>- added stub for loadpin_is_fs_trusted() to be used
> CONFIG_SECURITY_LOADPIN_VERITY is not select
>- depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
>- updated Kconfig help
>- minor changes in read_trusted_verity_root_digests()
>- updated commit message
>
>Changes in v2:
>- userspace now passes the path of the file with the verity digests
> via systcl, instead of the digests themselves
>- renamed sysctl file to 'trusted_verity_root_digests_path'
>- have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
>- updated Kconfig doc
>- updated commit message
>
> include/uapi/linux/loadpin.h | 19 ++++
> security/loadpin/Kconfig | 16 +++
> security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
> 3 files changed, 218 insertions(+), 1 deletion(-)
> create mode 100644 include/uapi/linux/loadpin.h
>
>diff --git a/include/uapi/linux/loadpin.h b/include/uapi/linux/loadpin.h
>new file mode 100644
>index 000000000000..d303a582209b
>--- /dev/null
>+++ b/include/uapi/linux/loadpin.h
>@@ -0,0 +1,19 @@
>+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>+/*
>+ * Copyright (c) 2022, Google LLC
>+ */
>+
>+#ifndef _UAPI_LINUX_LOOP_LOADPIN_H
>+#define _UAPI_LINUX_LOOP_LOADPIN_H
>+
>+#define LOADPIN_IOC_MAGIC 'L'
>+
>+/**
>+ * LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS - Set up the root digests of verity devices
>+ * that loadpin should trust.
>+ *
>+ * Takes a file descriptor from which to read the root digests of trusted verity devices.
Maybe add to the comment the securityfs node path here as a helpful hint to the reader, and mention the format (comma separated?)
>+ */
>+#define LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS _IOW(LOADPIN_IOC_MAGIC, 0x00, unsigned int)
>+
>+#endif /* _UAPI_LINUX_LOOP_LOADPIN_H */
>diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
>index 91be65dec2ab..e319ca8e3f3d 100644
>--- a/security/loadpin/Kconfig
>+++ b/security/loadpin/Kconfig
>@@ -18,3 +18,19 @@ config SECURITY_LOADPIN_ENFORCE
> If selected, LoadPin will enforce pinning at boot. If not
> selected, it can be enabled at boot with the kernel parameter
> "loadpin.enforce=1".
>+
>+config SECURITY_LOADPIN_VERITY
>+ bool "Allow reading files from certain other filesystems that use dm-verity"
>+ depends on DM_VERITY=y && SECURITYFS
>+ help
>+ If selected LoadPin can allow reading files from filesystems
>+ that use dm-verity. LoadPin maintains a list of verity root
>+ digests it considers trusted. A verity backed filesystem is
>+ considered trusted if its root digest is found in the list
>+ of trusted digests.
>+
>+ The list of trusted verity can be populated through an ioctl
>+ on the LoadPin securityfs entry 'dm-verity'. The ioctl
>+ expects a file descriptor of a file with verity digests as
>+ parameter. The file must be located on the pinned root and
>+ contain a comma separated list of digests.
>diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
>index b12f7d986b1e..c29ce562a366 100644
>--- a/security/loadpin/loadpin.c
>+++ b/security/loadpin/loadpin.c
>@@ -18,6 +18,9 @@
> #include <linux/path.h>
> #include <linux/sched.h> /* current */
> #include <linux/string_helpers.h>
>+#include <linux/device-mapper.h>
>+#include <linux/dm-verity-loadpin.h>
>+#include <uapi/linux/loadpin.h>
>
> static void report_load(const char *origin, struct file *file, char *operation)
> {
>@@ -43,6 +46,9 @@ static char *exclude_read_files[READING_MAX_ID];
> static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
> static struct super_block *pinned_root;
> static DEFINE_SPINLOCK(pinned_root_spinlock);
>+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
>+static LIST_HEAD(trusted_verity_root_digests);
>+#endif
>
> #ifdef CONFIG_SYSCTL
>
>@@ -118,6 +124,24 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
> }
> }
>
>+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
>+static bool loadpin_is_fs_trusted(struct super_block *sb)
>+{
>+ struct mapped_device *md = dm_get_md(sb->s_bdev->bd_dev);
>+ bool trusted;
>+
>+ if (!md)
>+ return false;
>+
>+ trusted = dm_verity_loadpin_is_md_trusted(md);
>+ dm_put(md);
>+
>+ return trusted;
>+}
>+#else
>+static inline bool loadpin_is_fs_trusted(struct super_block *sb) { return false; };
>+#endif
>+
> static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> bool contents)
> {
>@@ -174,7 +198,8 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> spin_unlock(&pinned_root_spinlock);
> }
>
>- if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
>+ if (IS_ERR_OR_NULL(pinned_root) ||
>+ ((load_root != pinned_root) && !loadpin_is_fs_trusted(load_root))) {
> if (unlikely(!enforce)) {
> report_load(origin, file, "pinning-ignored");
> return 0;
>@@ -240,6 +265,7 @@ static int __init loadpin_init(void)
> enforce ? "" : "not ");
> parse_exclude();
> security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
>+
> return 0;
> }
>
>@@ -248,6 +274,162 @@ DEFINE_LSM(loadpin) = {
> .init = loadpin_init,
> };
>
>+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
>+
>+enum loadpin_securityfs_interface_index {
>+ LOADPIN_DM_VERITY,
>+};
>+
>+static int read_trusted_verity_root_digests(unsigned int fd)
>+{
>+ struct fd f;
>+ void *data;
Probably easier if this is u8 *?
>+ int rc;
>+ char *p, *d;
>+
>+ /* The list of trusted root digests can only be set up once */
>+ if (!list_empty(&trusted_verity_root_digests))
>+ return -EPERM;
>+
>+ f = fdget(fd);
>+ if (!f.file)
>+ return -EINVAL;
>+
>+ data = kzalloc(SZ_4K, GFP_KERNEL);
>+ if (!data) {
>+ rc = -ENOMEM;
>+ goto err;
>+ }
>+
>+ rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
>+ if (rc < 0)
>+ goto err;
>+
>+ ((char *)data)[rc] = '\0';
>+
>+ p = strim(data);
>+ while ((d = strsep(&p, ",")) != NULL) {
Maybe be flexible and add newline as a separator too?
>+ int len = strlen(d);
>+ struct trusted_root_digest *trd;
>+
>+ if (len % 2) {
>+ rc = -EPROTO;
>+ goto err;
>+ }
>+
>+ len /= 2;
>+
>+ trd = kzalloc(sizeof(*trd), GFP_KERNEL);
With the struct change, this could be:
kzalloc(struct_size(trd, data, len), ...)
>+ if (!trd) {
>+ rc = -ENOMEM;
>+ goto err;
>+ }
>+
>+ trd->data = kzalloc(len, GFP_KERNEL);
>+ if (!trd->data) {
>+ kfree(trd);
>+ rc = -ENOMEM;
>+ goto err;
>+ }
>+
>+ if (hex2bin(trd->data, d, len)) {
>+ kfree(trd);
>+ kfree(trd->data);
>+ rc = -EPROTO;
>+ goto err;
>+ }
>+
>+ trd->len = len;
>+
>+ list_add_tail(&trd->node, &trusted_verity_root_digests);
>+ }
>+
>+ kfree(data);
>+ fdput(f);
>+
>+ if (!list_empty(&trusted_verity_root_digests))
>+ dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
>+
>+ return 0;
>+
>+err:
>+ kfree(data);
>+
Maybe add a comment that any load failure will invalidate the entire list?
>+ {
>+ struct trusted_root_digest *trd, *tmp;
>+
>+ list_for_each_entry_safe(trd, tmp, &trusted_verity_root_digests, node) {
>+ kfree(trd->data);
>+ list_del(&trd->node);
>+ kfree(trd);
>+ }
>+ }
>+
>+ fdput(f);
>+
>+ return rc;
>+}
>+
>+/******************************** securityfs ********************************/
>+
>+static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>+{
>+ void __user *uarg = (void __user *)arg;
>+ unsigned int fd;
>+ int rc;
>+
>+ switch (cmd) {
>+ case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
>+ rc = copy_from_user(&fd, uarg, sizeof(fd));
>+ if (rc)
>+ return rc;
>+
>+ return read_trusted_verity_root_digests(fd);
>+
>+ default:
>+ return -EINVAL;
>+ }
>+}
>+
>+static const struct file_operations loadpin_dm_verity_ops = {
>+ .unlocked_ioctl = dm_verity_ioctl,
>+ .compat_ioctl = compat_ptr_ioctl,
>+};
>+
>+/**
>+ * init_loadpin_securityfs - create the securityfs directory for LoadPin
>+ *
>+ * We can not put this method normally under the loadpin_init() code path since
>+ * the security subsystem gets initialized before the vfs caches.
>+ *
>+ * Returns 0 if the securityfs directory creation was successful.
>+ */
>+static int __init init_loadpin_securityfs(void)
>+{
>+ struct dentry *loadpin_dir, *dentry;
>+
>+ loadpin_dir = securityfs_create_dir("loadpin", NULL);
>+ if (IS_ERR(loadpin_dir)) {
>+ pr_err("LoadPin: could not create securityfs dir: %d\n",
>+ PTR_ERR(loadpin_dir));
>+ return PTR_ERR(loadpin_dir);
>+ }
>+
>+ dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
>+ (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
>+ if (IS_ERR(dentry)) {
>+ pr_err("LoadPin: could not create securityfs entry 'dm-verity': %d\n",
>+ PTR_ERR(dentry));
>+ return PTR_ERR(dentry);
>+ }
>+
>+ return 0;
>+}
>+
>+fs_initcall(init_loadpin_securityfs);
>+
>+#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
>+
> /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
> module_param(enforce, int, 0);
> MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
Otherwise looks good! The only other thing I can think of is pondering more about more carefully failing closed. E.g. instead of just throwing away all the other hashes on a file load failure, maybe lock out future attempts to set it too? I'm not sure this is actually useful, though. :P it shouldn't be possible to corrupt the file, etc. But in the universe where things like DirtyCOW happens, I've gotten even more paranoid. ;)
--
Kees Cook
On Fri, May 13, 2022 at 12:32:12PM -0400, Mike Snitzer wrote:
> On Wed, May 04 2022 at 3:54P -0400,
> Matthias Kaehlcke <[email protected]> wrote:
>
> > Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> > devices.
> >
> > This change adds the concept of trusted verity devices to LoadPin. LoadPin
> > maintains a list of root digests of verity devices it considers trusted.
> > Userspace can populate this list through an ioctl on the new LoadPin
> > securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> > a file with verity digests as parameter. Verity reads the digests from
> > this file after confirming that the file is located on the pinned root.
> > The list of trusted digests can only be set up once, which is typically
> > done at boot time.
> >
> > When a kernel file is read LoadPin first checks (as usual) whether the file
> > is located on the pinned root, if so the file can be loaded. Otherwise, if
> > the verity extension is enabled, LoadPin determines whether the file is
> > located on a verity backed device and whether the root digest of that
> > device is in the list of trusted digests. The file can be loaded if the
> > verity device has a trusted root digest.
> >
> > Background:
> >
> > As of now LoadPin restricts loading of kernel files to a single pinned
> > filesystem, typically the rootfs. This works for many systems, however it
> > can result in a bloated rootfs (and OTA updates) on platforms where
> > multiple boards with different hardware configurations use the same rootfs
> > image. Especially when 'optional' files are large it may be preferable to
> > download/install them only when they are actually needed by a given board.
> > Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
> > at runtime. As an example a DLC package could contain firmware for a
> > peripheral that is not present on all boards. DLCs use dm-verity to verify
> > the integrity of the DLC content.
> >
> > [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
> > [2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
> >
> > Signed-off-by: Matthias Kaehlcke <[email protected]>
> > ---
> >
> > Changes in v3:
> > - added securityfs for LoadPin (currently only populated when
> > CONFIG_SECURITY_LOADPIN_VERITY=y)
> > - added uapi include for LoadPin
> > - changed the interface for setting up the list of trusted
> > digests from sysctl to ioctl on securityfs entry
> > - added stub for loadpin_is_fs_trusted() to be used
> > CONFIG_SECURITY_LOADPIN_VERITY is not select
> > - depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
> > - updated Kconfig help
> > - minor changes in read_trusted_verity_root_digests()
> > - updated commit message
> >
> > Changes in v2:
> > - userspace now passes the path of the file with the verity digests
> > via systcl, instead of the digests themselves
> > - renamed sysctl file to 'trusted_verity_root_digests_path'
> > - have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
> > - updated Kconfig doc
> > - updated commit message
> >
> > include/uapi/linux/loadpin.h | 19 ++++
> > security/loadpin/Kconfig | 16 +++
> > security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
> > 3 files changed, 218 insertions(+), 1 deletion(-)
> > create mode 100644 include/uapi/linux/loadpin.h
>
> I would certainly need some Reviewed-by:s from security and/or loadpin
> experts if I were to pick this patch up.
Yes, I think Kees (LoadPin maintainer) is generally on board with the idea,
but a more in depth review is still pending.
I'll send out a new revision which addresses the current outstanding
comments soon.
> Did you see the issues the kernel test robot emailed about?
>
> You'd do well to fix those issues up when submitting another revision
> of this patchset.
Yes, I plan to address those in the next revision. Thanks for the reminder!
> >
> > diff --git a/include/uapi/linux/loadpin.h b/include/uapi/linux/loadpin.h
> > new file mode 100644
> > index 000000000000..d303a582209b
> > --- /dev/null
> > +++ b/include/uapi/linux/loadpin.h
> > @@ -0,0 +1,19 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +/*
> > + * Copyright (c) 2022, Google LLC
> > + */
> > +
> > +#ifndef _UAPI_LINUX_LOOP_LOADPIN_H
> > +#define _UAPI_LINUX_LOOP_LOADPIN_H
> > +
> > +#define LOADPIN_IOC_MAGIC 'L'
> > +
> > +/**
> > + * LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS - Set up the root digests of verity devices
> > + * that loadpin should trust.
> > + *
> > + * Takes a file descriptor from which to read the root digests of trusted verity devices.
> > + */
> > +#define LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS _IOW(LOADPIN_IOC_MAGIC, 0x00, unsigned int)
> > +
> > +#endif /* _UAPI_LINUX_LOOP_LOADPIN_H */
> > diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
> > index 91be65dec2ab..e319ca8e3f3d 100644
> > --- a/security/loadpin/Kconfig
> > +++ b/security/loadpin/Kconfig
> > @@ -18,3 +18,19 @@ config SECURITY_LOADPIN_ENFORCE
> > If selected, LoadPin will enforce pinning at boot. If not
> > selected, it can be enabled at boot with the kernel parameter
> > "loadpin.enforce=1".
> > +
> > +config SECURITY_LOADPIN_VERITY
> > + bool "Allow reading files from certain other filesystems that use dm-verity"
> > + depends on DM_VERITY=y && SECURITYFS
> > + help
> > + If selected LoadPin can allow reading files from filesystems
> > + that use dm-verity. LoadPin maintains a list of verity root
> > + digests it considers trusted. A verity backed filesystem is
> > + considered trusted if its root digest is found in the list
> > + of trusted digests.
> > +
> > + The list of trusted verity can be populated through an ioctl
> > + on the LoadPin securityfs entry 'dm-verity'. The ioctl
> > + expects a file descriptor of a file with verity digests as
> > + parameter. The file must be located on the pinned root and
> > + contain a comma separated list of digests.
> > diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> > index b12f7d986b1e..c29ce562a366 100644
> > --- a/security/loadpin/loadpin.c
> > +++ b/security/loadpin/loadpin.c
> > @@ -18,6 +18,9 @@
> > #include <linux/path.h>
> > #include <linux/sched.h> /* current */
> > #include <linux/string_helpers.h>
> > +#include <linux/device-mapper.h>
> > +#include <linux/dm-verity-loadpin.h>
> > +#include <uapi/linux/loadpin.h>
> >
> > static void report_load(const char *origin, struct file *file, char *operation)
> > {
> > @@ -43,6 +46,9 @@ static char *exclude_read_files[READING_MAX_ID];
> > static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
> > static struct super_block *pinned_root;
> > static DEFINE_SPINLOCK(pinned_root_spinlock);
> > +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> > +static LIST_HEAD(trusted_verity_root_digests);
> > +#endif
> >
> > #ifdef CONFIG_SYSCTL
> >
> > @@ -118,6 +124,24 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
> > }
> > }
> >
> > +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> > +static bool loadpin_is_fs_trusted(struct super_block *sb)
> > +{
> > + struct mapped_device *md = dm_get_md(sb->s_bdev->bd_dev);
> > + bool trusted;
> > +
> > + if (!md)
> > + return false;
> > +
> > + trusted = dm_verity_loadpin_is_md_trusted(md);
> > + dm_put(md);
> > +
> > + return trusted;
> > +}
> > +#else
> > +static inline bool loadpin_is_fs_trusted(struct super_block *sb) { return false; };
> > +#endif
> > +
> > static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> > bool contents)
> > {
> > @@ -174,7 +198,8 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> > spin_unlock(&pinned_root_spinlock);
> > }
> >
> > - if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
> > + if (IS_ERR_OR_NULL(pinned_root) ||
> > + ((load_root != pinned_root) && !loadpin_is_fs_trusted(load_root))) {
> > if (unlikely(!enforce)) {
> > report_load(origin, file, "pinning-ignored");
> > return 0;
> > @@ -240,6 +265,7 @@ static int __init loadpin_init(void)
> > enforce ? "" : "not ");
> > parse_exclude();
> > security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
> > +
> > return 0;
> > }
> >
> > @@ -248,6 +274,162 @@ DEFINE_LSM(loadpin) = {
> > .init = loadpin_init,
> > };
> >
> > +#ifdef CONFIG_SECURITY_LOADPIN_VERITY
> > +
> > +enum loadpin_securityfs_interface_index {
> > + LOADPIN_DM_VERITY,
> > +};
> > +
> > +static int read_trusted_verity_root_digests(unsigned int fd)
> > +{
> > + struct fd f;
> > + void *data;
> > + int rc;
> > + char *p, *d;
> > +
> > + /* The list of trusted root digests can only be set up once */
> > + if (!list_empty(&trusted_verity_root_digests))
> > + return -EPERM;
> > +
> > + f = fdget(fd);
> > + if (!f.file)
> > + return -EINVAL;
> > +
> > + data = kzalloc(SZ_4K, GFP_KERNEL);
> > + if (!data) {
> > + rc = -ENOMEM;
> > + goto err;
> > + }
> > +
> > + rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
> > + if (rc < 0)
> > + goto err;
> > +
> > + ((char *)data)[rc] = '\0';
> > +
> > + p = strim(data);
> > + while ((d = strsep(&p, ",")) != NULL) {
> > + int len = strlen(d);
> > + struct trusted_root_digest *trd;
> > +
> > + if (len % 2) {
> > + rc = -EPROTO;
> > + goto err;
> > + }
> > +
> > + len /= 2;
> > +
> > + trd = kzalloc(sizeof(*trd), GFP_KERNEL);
> > + if (!trd) {
> > + rc = -ENOMEM;
> > + goto err;
> > + }
> > +
> > + trd->data = kzalloc(len, GFP_KERNEL);
> > + if (!trd->data) {
> > + kfree(trd);
> > + rc = -ENOMEM;
> > + goto err;
> > + }
> > +
> > + if (hex2bin(trd->data, d, len)) {
> > + kfree(trd);
> > + kfree(trd->data);
> > + rc = -EPROTO;
> > + goto err;
> > + }
> > +
> > + trd->len = len;
> > +
> > + list_add_tail(&trd->node, &trusted_verity_root_digests);
> > + }
> > +
> > + kfree(data);
> > + fdput(f);
> > +
> > + if (!list_empty(&trusted_verity_root_digests))
> > + dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
> > +
> > + return 0;
> > +
> > +err:
> > + kfree(data);
> > +
> > + {
> > + struct trusted_root_digest *trd, *tmp;
> > +
> > + list_for_each_entry_safe(trd, tmp, &trusted_verity_root_digests, node) {
> > + kfree(trd->data);
> > + list_del(&trd->node);
> > + kfree(trd);
> > + }
> > + }
> > +
> > + fdput(f);
> > +
> > + return rc;
> > +}
> > +
> > +/******************************** securityfs ********************************/
> > +
> > +static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> > +{
> > + void __user *uarg = (void __user *)arg;
> > + unsigned int fd;
> > + int rc;
> > +
> > + switch (cmd) {
> > + case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
> > + rc = copy_from_user(&fd, uarg, sizeof(fd));
> > + if (rc)
> > + return rc;
> > +
> > + return read_trusted_verity_root_digests(fd);
> > +
> > + default:
> > + return -EINVAL;
> > + }
> > +}
> > +
> > +static const struct file_operations loadpin_dm_verity_ops = {
> > + .unlocked_ioctl = dm_verity_ioctl,
> > + .compat_ioctl = compat_ptr_ioctl,
> > +};
> > +
> > +/**
> > + * init_loadpin_securityfs - create the securityfs directory for LoadPin
> > + *
> > + * We can not put this method normally under the loadpin_init() code path since
> > + * the security subsystem gets initialized before the vfs caches.
> > + *
> > + * Returns 0 if the securityfs directory creation was successful.
> > + */
> > +static int __init init_loadpin_securityfs(void)
> > +{
> > + struct dentry *loadpin_dir, *dentry;
> > +
> > + loadpin_dir = securityfs_create_dir("loadpin", NULL);
> > + if (IS_ERR(loadpin_dir)) {
> > + pr_err("LoadPin: could not create securityfs dir: %d\n",
> > + PTR_ERR(loadpin_dir));
> > + return PTR_ERR(loadpin_dir);
> > + }
> > +
> > + dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
> > + (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
> > + if (IS_ERR(dentry)) {
> > + pr_err("LoadPin: could not create securityfs entry 'dm-verity': %d\n",
> > + PTR_ERR(dentry));
> > + return PTR_ERR(dentry);
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +fs_initcall(init_loadpin_securityfs);
> > +
> > +#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
> > +
> > /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
> > module_param(enforce, int, 0);
> > MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
> > --
> > 2.36.0.464.gb9c8b46e94-goog
> >
>
Hi Kees,
thanks for the review!
On Fri, May 13, 2022 at 03:36:26PM -0700, Kees Cook wrote:
>
>
> On May 4, 2022 12:54:18 PM PDT, Matthias Kaehlcke <[email protected]> wrote:
> >Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> >devices.
> >
> >This change adds the concept of trusted verity devices to LoadPin. LoadPin
> >maintains a list of root digests of verity devices it considers trusted.
> >Userspace can populate this list through an ioctl on the new LoadPin
> >securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> >a file with verity digests as parameter. Verity reads the digests from
> >this file after confirming that the file is located on the pinned root.
> >The list of trusted digests can only be set up once, which is typically
> >done at boot time.
> >
> >When a kernel file is read LoadPin first checks (as usual) whether the file
> >is located on the pinned root, if so the file can be loaded. Otherwise, if
> >the verity extension is enabled, LoadPin determines whether the file is
> >located on a verity backed device and whether the root digest of that
>
> I think this should be "... on an already trusted device ..."
It's not entirely clear which part you want me to substitute. 'an already
trusted device' makes me wonder whether you are thinking about reading the
list of digests, and not the general case of reading a kernel file, which
this paragraph intends to describe.
> >device is in the list of trusted digests. The file can be loaded if the
> >verity device has a trusted root digest.
> >
> >Background:
> >
> >As of now LoadPin restricts loading of kernel files to a single pinned
> >filesystem, typically the rootfs. This works for many systems, however it
> >can result in a bloated rootfs (and OTA updates) on platforms where
> >multiple boards with different hardware configurations use the same rootfs
> >image. Especially when 'optional' files are large it may be preferable to
> >download/install them only when they are actually needed by a given board.
> >Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
> >at runtime. As an example a DLC package could contain firmware for a
> >peripheral that is not present on all boards. DLCs use dm-verity to verify
> >the integrity of the DLC content.
> >
> >[1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
> >[2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
> >
> >Signed-off-by: Matthias Kaehlcke <[email protected]>
> >---
> >
> >Changes in v3:
> >- added securityfs for LoadPin (currently only populated when
> > CONFIG_SECURITY_LOADPIN_VERITY=y)
> >- added uapi include for LoadPin
> >- changed the interface for setting up the list of trusted
> > digests from sysctl to ioctl on securityfs entry
> >- added stub for loadpin_is_fs_trusted() to be used
> > CONFIG_SECURITY_LOADPIN_VERITY is not select
> >- depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
> >- updated Kconfig help
> >- minor changes in read_trusted_verity_root_digests()
> >- updated commit message
> >
> >Changes in v2:
> >- userspace now passes the path of the file with the verity digests
> > via systcl, instead of the digests themselves
> >- renamed sysctl file to 'trusted_verity_root_digests_path'
> >- have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
> >- updated Kconfig doc
> >- updated commit message
> >
> > include/uapi/linux/loadpin.h | 19 ++++
> > security/loadpin/Kconfig | 16 +++
> > security/loadpin/loadpin.c | 184 ++++++++++++++++++++++++++++++++++-
> > 3 files changed, 218 insertions(+), 1 deletion(-)
> > create mode 100644 include/uapi/linux/loadpin.h
> >
> >diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> >index b12f7d986b1e..c29ce562a366 100644
> >--- a/security/loadpin/loadpin.c
> >+++ b/security/loadpin/loadpin.c
>
> ...
>
> >+static int read_trusted_verity_root_digests(unsigned int fd)
> >+{
> >+ struct fd f;
> >+ void *data;
>
> Probably easier if this is u8 *?
Maybe slightly, it would then require a cast when passing it to
kernel_read_file()
> >+ int rc;
> >+ char *p, *d;
> >+
> >+ /* The list of trusted root digests can only be set up once */
> >+ if (!list_empty(&trusted_verity_root_digests))
> >+ return -EPERM;
> >+
> >+ f = fdget(fd);
> >+ if (!f.file)
> >+ return -EINVAL;
> >+
> >+ data = kzalloc(SZ_4K, GFP_KERNEL);
> >+ if (!data) {
> >+ rc = -ENOMEM;
> >+ goto err;
> >+ }
> >+
> >+ rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
> >+ if (rc < 0)
> >+ goto err;
> >+
> >+ ((char *)data)[rc] = '\0';
> >+
> >+ p = strim(data);
> >+ while ((d = strsep(&p, ",")) != NULL) {
>
> Maybe be flexible and add newline as a separator too?
Sure, I can add that. I'd also be fine with just allowing a newline as
separator, which seems a reasonable format for a sysfs file.
> >+ int len = strlen(d);
> >+ struct trusted_root_digest *trd;
> >+
> >+ if (len % 2) {
> >+ rc = -EPROTO;
> >+ goto err;
> >+ }
> >+
> >+ len /= 2;
> >+
> >+ trd = kzalloc(sizeof(*trd), GFP_KERNEL);
>
> With the struct change, this could be:
>
> kzalloc(struct_size(trd, data, len), ...)
Will change
> >+ if (!trd) {
> >+ rc = -ENOMEM;
> >+ goto err;
> >+ }
> >+
> >+ trd->data = kzalloc(len, GFP_KERNEL);
> >+ if (!trd->data) {
> >+ kfree(trd);
> >+ rc = -ENOMEM;
> >+ goto err;
> >+ }
> >+
> >+ if (hex2bin(trd->data, d, len)) {
> >+ kfree(trd);
> >+ kfree(trd->data);
> >+ rc = -EPROTO;
> >+ goto err;
> >+ }
> >+
> >+ trd->len = len;
> >+
> >+ list_add_tail(&trd->node, &trusted_verity_root_digests);
> >+ }
> >+
> >+ kfree(data);
> >+ fdput(f);
> >+
> >+ if (!list_empty(&trusted_verity_root_digests))
> >+ dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
> >+
> >+ return 0;
> >+
> >+err:
> >+ kfree(data);
> >+
>
> Maybe add a comment that any load failure will invalidate the entire list?
ok
> Otherwise looks good! The only other thing I can think of is pondering more
> about more carefully failing closed. E.g. instead of just throwing away all
> the other hashes on a file load failure, maybe lock out future attempts to
> set it too? I'm not sure this is actually useful, though. :P it shouldn't be
> possible to corrupt the file, etc. But in the universe where things like
> DirtyCOW happens, I've gotten even more paranoid. ;)
Sure, we can do that
On Mon, May 16, 2022 at 11:17:44AM -0700, Matthias Kaehlcke wrote:
> On Fri, May 13, 2022 at 03:36:26PM -0700, Kees Cook wrote:
> >
> >
> > On May 4, 2022 12:54:18 PM PDT, Matthias Kaehlcke <[email protected]> wrote:
> > >Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> > >devices.
> > >
> > >This change adds the concept of trusted verity devices to LoadPin. LoadPin
> > >maintains a list of root digests of verity devices it considers trusted.
> > >Userspace can populate this list through an ioctl on the new LoadPin
> > >securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> > >a file with verity digests as parameter. Verity reads the digests from
> > >this file after confirming that the file is located on the pinned root.
> > >The list of trusted digests can only be set up once, which is typically
> > >done at boot time.
> > >
> > >When a kernel file is read LoadPin first checks (as usual) whether the file
> > >is located on the pinned root, if so the file can be loaded. Otherwise, if
> > >the verity extension is enabled, LoadPin determines whether the file is
> > >located on a verity backed device and whether the root digest of that
> >
> > I think this should be "... on an already trusted device ..."
>
> It's not entirely clear which part you want me to substitute. 'an already
> trusted device' makes me wonder whether you are thinking about reading the
> list of digests, and not the general case of reading a kernel file, which
> this paragraph intends to describe.
Sorry, I think I confused myself while reading what you'd written. I
think it's fine as is. I think I had skipped around in my mind thinking
about the trusted verity hashes file coming from the pinned root, but
you basically already said that. :) Nevermind!
> > >+static int read_trusted_verity_root_digests(unsigned int fd)
> > >+{
> > >+ struct fd f;
> > >+ void *data;
> >
> > Probably easier if this is u8 *?
>
> Maybe slightly, it would then require a cast when passing it to
> kernel_read_file()
Oh, good point. That is a kinda weird API.
>
> > >+ int rc;
> > >+ char *p, *d;
> > >+
> > >+ /* The list of trusted root digests can only be set up once */
> > >+ if (!list_empty(&trusted_verity_root_digests))
> > >+ return -EPERM;
> > >+
> > >+ f = fdget(fd);
> > >+ if (!f.file)
> > >+ return -EINVAL;
> > >+
> > >+ data = kzalloc(SZ_4K, GFP_KERNEL);
> > >+ if (!data) {
> > >+ rc = -ENOMEM;
> > >+ goto err;
> > >+ }
> > >+
> > >+ rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
> > >+ if (rc < 0)
> > >+ goto err;
So maybe, here, you could do:
p = data;
p[rc] '\0';
p = strim(p);
etc... (the void * -> char * cast in the assignment should be accepted
without warning?)
> > >+
> > >+ ((char *)data)[rc] = '\0';
> > >+
> > >+ p = strim(data);
> > >+ while ((d = strsep(&p, ",")) != NULL) {
> >
> > Maybe be flexible and add newline as a separator too?
>
> Sure, I can add that. I'd also be fine with just allowing a newline as
> separator, which seems a reasonable format for a sysfs file.
Yeah, that was my thinking too. And easier to parse for command line
tools, etc. Not a requirement at all, but might make testing easier,
etc.
--
Kees Cook
On Mon, May 16, 2022 at 08:44:37PM -0700, Kees Cook wrote:
> On Mon, May 16, 2022 at 11:17:44AM -0700, Matthias Kaehlcke wrote:
> > On Fri, May 13, 2022 at 03:36:26PM -0700, Kees Cook wrote:
> > >
> > >
> > > On May 4, 2022 12:54:18 PM PDT, Matthias Kaehlcke <[email protected]> wrote:
> > > >Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> > > >devices.
> > > >
> > > >This change adds the concept of trusted verity devices to LoadPin. LoadPin
> > > >maintains a list of root digests of verity devices it considers trusted.
> > > >Userspace can populate this list through an ioctl on the new LoadPin
> > > >securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> > > >a file with verity digests as parameter. Verity reads the digests from
> > > >this file after confirming that the file is located on the pinned root.
> > > >The list of trusted digests can only be set up once, which is typically
> > > >done at boot time.
> > > >
> > > >When a kernel file is read LoadPin first checks (as usual) whether the file
> > > >is located on the pinned root, if so the file can be loaded. Otherwise, if
> > > >the verity extension is enabled, LoadPin determines whether the file is
> > > >located on a verity backed device and whether the root digest of that
> > >
> > > I think this should be "... on an already trusted device ..."
> >
> > It's not entirely clear which part you want me to substitute. 'an already
> > trusted device' makes me wonder whether you are thinking about reading the
> > list of digests, and not the general case of reading a kernel file, which
> > this paragraph intends to describe.
>
> Sorry, I think I confused myself while reading what you'd written. I
> think it's fine as is. I think I had skipped around in my mind thinking
> about the trusted verity hashes file coming from the pinned root, but
> you basically already said that. :) Nevermind!
>
> > > >+static int read_trusted_verity_root_digests(unsigned int fd)
> > > >+{
> > > >+ struct fd f;
> > > >+ void *data;
> > >
> > > Probably easier if this is u8 *?
> >
> > Maybe slightly, it would then require a cast when passing it to
> > kernel_read_file()
>
> Oh, good point. That is a kinda weird API.
>
> >
> > > >+ int rc;
> > > >+ char *p, *d;
> > > >+
> > > >+ /* The list of trusted root digests can only be set up once */
> > > >+ if (!list_empty(&trusted_verity_root_digests))
> > > >+ return -EPERM;
> > > >+
> > > >+ f = fdget(fd);
> > > >+ if (!f.file)
> > > >+ return -EINVAL;
> > > >+
> > > >+ data = kzalloc(SZ_4K, GFP_KERNEL);
> > > >+ if (!data) {
> > > >+ rc = -ENOMEM;
> > > >+ goto err;
> > > >+ }
> > > >+
> > > >+ rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
> > > >+ if (rc < 0)
> > > >+ goto err;
>
> So maybe, here, you could do:
>
> p = data;
> p[rc] '\0';
> p = strim(p);
>
> etc... (the void * -> char * cast in the assignment should be accepted
> without warning?)
Yes, that would work, I'll change it accordingly, thanks!
> > > >+
> > > >+ ((char *)data)[rc] = '\0';
> > > >+
> > > >+ p = strim(data);
> > > >+ while ((d = strsep(&p, ",")) != NULL) {
> > >
> > > Maybe be flexible and add newline as a separator too?
> >
> > Sure, I can add that. I'd also be fine with just allowing a newline as
> > separator, which seems a reasonable format for a sysfs file.
>
> Yeah, that was my thinking too. And easier to parse for command line
> tools, etc. Not a requirement at all, but might make testing easier,
> etc.
Ok, I'll change it to use newline as the only separator.