LinuxLists.cc - [PATCH v5 1/2] proc: pass file instead of inode to proc_mem

2024-06-05 16:50:37

Subject: [PATCH v5 1/2] proc: pass file instead of inode to proc_mem_open

The file struct is required in proc_mem_open() so its
f_mode can be checked when deciding whether to allow or
deny /proc/*/mem open requests via the new read/write
and foll_force restriction mechanism.

Thus instead of directly passing the inode to the fun,
we pass the file and get the inode inside it.

Cc: Jann Horn <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jeff Xu <[email protected]>
Signed-off-by: Adrian Ratiu <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
---
Changes in v5:
* Fixed task_nommu.c 0day build error
* Added Reviewed-by tag by Kees C. (thanks!)
* Rebased on next-20240605
---
fs/proc/base.c | 6 +++---
fs/proc/internal.h | 2 +-
fs/proc/task_mmu.c | 6 +++---
fs/proc/task_nommu.c | 2 +-
4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 72a1acd03675c..4c607089f66ed 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -794,9 +794,9 @@ static const struct file_operations proc_single_file_operations = {
};

-struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
+struct mm_struct *proc_mem_open(struct file *file, unsigned int mode)
{
- struct task_struct *task = get_proc_task(inode);
+ struct task_struct *task = get_proc_task(file->f_inode);
struct mm_struct *mm = ERR_PTR(-ESRCH);

if (task) {
@@ -816,7 +816,7 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)

static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
{
- struct mm_struct *mm = proc_mem_open(inode, mode);
+ struct mm_struct *mm = proc_mem_open(file, mode);

if (IS_ERR(mm))
return PTR_ERR(mm);
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index a71ac5379584a..d38b2eea40d12 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -295,7 +295,7 @@ struct proc_maps_private {
#endif
} __randomize_layout;

-struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode);
+struct mm_struct *proc_mem_open(struct file *file, unsigned int mode);

extern const struct file_operations proc_pid_maps_operations;
extern const struct file_operations proc_pid_numa_maps_operations;
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f8d35f993fe50..fe3b2182b0aae 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -210,7 +210,7 @@ static int proc_maps_open(struct inode *inode, struct file *file,
return -ENOMEM;

priv->inode = inode;
- priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
+ priv->mm = proc_mem_open(file, PTRACE_MODE_READ);
if (IS_ERR(priv->mm)) {
int err = PTR_ERR(priv->mm);

@@ -1030,7 +1030,7 @@ static int smaps_rollup_open(struct inode *inode, struct file *file)
goto out_free;

priv->inode = inode;
- priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
+ priv->mm = proc_mem_open(file, PTRACE_MODE_READ);
if (IS_ERR(priv->mm)) {
ret = PTR_ERR(priv->mm);

@@ -1754,7 +1754,7 @@ static int pagemap_open(struct inode *inode, struct file *file)
{
struct mm_struct *mm;

- mm = proc_mem_open(inode, PTRACE_MODE_READ);
+ mm = proc_mem_open(file, PTRACE_MODE_READ);
if (IS_ERR(mm))
return PTR_ERR(mm);
file->private_data = mm;
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index bce6745330003..a8ab182a4ed14 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -259,7 +259,7 @@ static int maps_open(struct inode *inode, struct file *file,
return -ENOMEM;

priv->inode = inode;
- priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
+ priv->mm = proc_mem_open(file, PTRACE_MODE_READ);
if (IS_ERR(priv->mm)) {
int err = PTR_ERR(priv->mm);

--
2.30.2

2024-06-05 16:50:55

by Adrian Ratiu

[permalink] [raw]

Subject: [PATCH v5 2/2] proc: restrict /proc/pid/mem

Prior to v2.6.39 write access to /proc/<pid>/mem was restricted,
after which it got allowed in commit 198214a7ee50 ("proc: enable
writing to /proc/pid/mem"). Famous last words from that patch:
"no longer a security hazard". :)

Afterwards exploits started causing drama like [1]. The exploits
using /proc/*/mem can be rather sophisticated like [2] which
installed an arbitrary payload from noexec storage into a running
process then exec'd it, which itself could include an ELF loader
to run arbitrary code off noexec storage.

One of the well-known problems with /proc/*/mem writes is they
ignore page permissions via FOLL_FORCE, as opposed to writes via
process_vm_writev which respect page permissions. These writes can
also be used to bypass mode bits.

To harden against these types of attacks, distrbutions might want
to restrict /proc/pid/mem accesses, either entirely or partially,
for eg. to restrict FOLL_FORCE usage.

Known valid use-cases which still need these accesses are:

* Debuggers which also have ptrace permissions, so they can access
memory anyway via PTRACE_POKEDATA & co. Some debuggers like GDB
are designed to write /proc/pid/mem for basic functionality.

* Container supervisors using the seccomp notifier to intercept
syscalls and rewrite memory of calling processes by passing
around /proc/pid/mem file descriptors.

There might be more, that's why these params default to disabled.

Regarding other mechanisms which can block these accesses:

* seccomp filters can be used to block mmap/mprotect calls with W|X
perms, but they often can't block open calls as daemons want to
read/write their runtime state and seccomp filters cannot check
file paths, so plain write calls can't be easily blocked.

* Since the mem file is part of the dynamic /proc/<pid>/ space, we
can't run chmod once at boot to restrict it (and trying to react
to every process and run chmod doesn't scale, and the kernel no
longer allows chmod on any of these paths).

* SELinux could be used with a rule to cover all /proc/*/mem files,
but even then having multiple ways to deny an attack is useful in
case one layer fails.

Thus we introduce four kernel parameters to restrict /proc/*/mem
access: open-read, open-write, write and foll_force. All these can
be independently set to the following values:

all => restrict all access unconditionally.
ptracer => restrict all access except for ptracer processes.

If left unset, the existing behaviour is preserved, i.e. access
is governed by basic file permissions.

Examples which can be passed by bootloaders:

proc_mem.restrict_foll_force=all
proc_mem.restrict_open_write=ptracer
proc_mem.restrict_open_read=ptracer
proc_mem.restrict_write=all

These knobs can also be enabled via Kconfig like for eg:

CONFIG_PROC_MEM_RESTRICT_WRITE_PTRACE_DEFAULT=y
CONFIG_PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT=y

Each distribution needs to decide what restrictions to apply,
depending on its use-cases. Embedded systems might want to do
more, while general-purpouse distros might want a more relaxed
policy, because for e.g. foll_force=all and write=all both break
break GDB, so it might be a bit excessive.

Based on an initial patch by Mike Frysinger <[email protected]>.

Link: https://lwn.net/Articles/476947/ [1]
Link: https://issues.chromium.org/issues/40089045 [2]
Cc: Guenter Roeck <[email protected]>
Cc: Doug Anderson <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jeff Xu <[email protected]>
Co-developed-by: Mike Frysinger <[email protected]>
Signed-off-by: Mike Frysinger <[email protected]>
Signed-off-by: Adrian Ratiu <[email protected]>
---
Changes in v5:
* Fixed 2 typos by Randy D. (thanks!)
* Fixed mm_access() resource leak
* Rebased on next-20240605
---
.../admin-guide/kernel-parameters.txt | 38 +++++
fs/proc/base.c | 130 +++++++++++++++++-
security/Kconfig | 68 +++++++++
3 files changed, 235 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f4f2b0ab61fae..035ed61a3e4e3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4814,6 +4814,44 @@
printk.time= Show timing data prefixed to each printk message line
Format: <bool> (1/Y/y=enable, 0/N/n=disable)

+ proc_mem.restrict_foll_force= [KNL]
+ Format: {all | ptracer}
+ Restricts the use of the FOLL_FORCE flag for /proc/*/mem access.
+ If restricted, the FOLL_FORCE flag will not be added to vm accesses.
+ Can be one of:
+ - 'all' restricts all access unconditionally.
+ - 'ptracer' allows access only for ptracer processes.
+ If not specified, FOLL_FORCE is always used.
+
+ proc_mem.restrict_open_read= [KNL]
+ Format: {all | ptracer}
+ Allows restricting read access to /proc/*/mem files during open().
+ Depending on restriction level, open for reads return -EACCES.
+ Can be one of:
+ - 'all' restricts all access unconditionally.
+ - 'ptracer' allows access only for ptracer processes.
+ If not specified, then basic file permissions continue to apply.
+
+ proc_mem.restrict_open_write= [KNL]
+ Format: {all | ptracer}
+ Allows restricting write access to /proc/*/mem files during open().
+ Depending on restriction level, open for writes return -EACCES.
+ Can be one of:
+ - 'all' restricts all access unconditionally.
+ - 'ptracer' allows access only for ptracer processes.
+ If not specified, then basic file permissions continue to apply.
+
+ proc_mem.restrict_write= [KNL]
+ Format: {all | ptracer}
+ Allows restricting write access to /proc/*/mem after the files
+ have been opened, during the actual write calls. This is useful for
+ systems which can't block writes earlier during open().
+ Depending on restriction level, writes will return -EACCES.
+ Can be one of:
+ - 'all' restricts all access unconditionally.
+ - 'ptracer' allows access only for ptracer processes.
+ If not specified, then basic file permissions continue to apply.
+
processor.max_cstate= [HW,ACPI]
Limit processor to maximum C-state
max_cstate=9 overrides any DMI blacklist limit.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 4c607089f66ed..3f33c579cb65c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -152,6 +152,30 @@ struct pid_entry {
NULL, &proc_pid_attr_operations, \
{ .lsmid = LSMID })

+#define DEFINE_EARLY_PROC_MEM_RESTRICT(CFG, name) \
+DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_PROC_MEM_RESTRICT_##CFG##_DEFAULT, \
+ proc_mem_restrict_##name##_all); \
+DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_PROC_MEM_RESTRICT_##CFG##_PTRACE_DEFAULT, \
+ proc_mem_restrict_##name##_ptracer); \
+ \
+static int __init early_proc_mem_restrict_##name(char *buf) \
+{ \
+ if (!buf) \
+ return -EINVAL; \
+ \
+ if (strcmp(buf, "all") == 0) \
+ static_key_slow_inc(&proc_mem_restrict_##name##_all.key); \
+ else if (strcmp(buf, "ptracer") == 0) \
+ static_key_slow_inc(&proc_mem_restrict_##name##_ptracer.key); \
+ return 0; \
+} \
+early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)
+
+DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_READ, open_read);
+DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_WRITE, open_write);
+DEFINE_EARLY_PROC_MEM_RESTRICT(WRITE, write);
+DEFINE_EARLY_PROC_MEM_RESTRICT(FOLL_FORCE, foll_force);
+
/*
* Count the number of hardlinks for the pid_entry table, excluding the .
* and .. links.
@@ -794,12 +818,56 @@ static const struct file_operations proc_single_file_operations = {
};

+static int __mem_open_access_permitted(struct file *file, struct task_struct *task)
+{
+ bool is_ptracer;
+
+ rcu_read_lock();
+ is_ptracer = current == ptrace_parent(task);
+ rcu_read_unlock();
+
+ if (file->f_mode & FMODE_WRITE) {
+ /* Deny if writes are unconditionally disabled via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT,
+ &proc_mem_restrict_open_write_all))
+ return -EACCES;
+
+ /* Deny if writes are allowed only for ptracers via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT,
+ &proc_mem_restrict_open_write_ptracer) &&
+ !is_ptracer)
+ return -EACCES;
+ }
+
+ if (file->f_mode & FMODE_READ) {
+ /* Deny if reads are unconditionally disabled via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_DEFAULT,
+ &proc_mem_restrict_open_read_all))
+ return -EACCES;
+
+ /* Deny if reads are allowed only for ptracers via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_PTRACE_DEFAULT,
+ &proc_mem_restrict_open_read_ptracer) &&
+ !is_ptracer)
+ return -EACCES;
+ }
+
+ return 0; /* R/W are not restricted */
+}
+
struct mm_struct *proc_mem_open(struct file *file, unsigned int mode)
{
struct task_struct *task = get_proc_task(file->f_inode);
struct mm_struct *mm = ERR_PTR(-ESRCH);
+ int ret;

if (task) {
+ ret = __mem_open_access_permitted(file, task);
+ if (ret) {
+ put_task_struct(task);
+ return ERR_PTR(ret);
+ }
+
mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
put_task_struct(task);

@@ -835,6 +903,62 @@ static int mem_open(struct inode *inode, struct file *file)
return ret;
}

+static bool __mem_rw_current_is_ptracer(struct file *file)
+{
+ struct inode *inode = file_inode(file);
+ struct task_struct *task = get_proc_task(inode);
+ struct mm_struct *mm = NULL;
+ int is_ptracer = false, has_mm_access = false;
+
+ if (task) {
+ rcu_read_lock();
+ is_ptracer = current == ptrace_parent(task);
+ rcu_read_unlock();
+
+ mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
+ if (mm && file->private_data == mm) {
+ has_mm_access = true;
+ mmput(mm);
+ }
+
+ put_task_struct(task);
+ }
+
+ return is_ptracer && has_mm_access;
+}
+
+static unsigned int __mem_rw_get_foll_force_flag(struct file *file)
+{
+ /* Deny if FOLL_FORCE is disabled via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_FOLL_FORCE_DEFAULT,
+ &proc_mem_restrict_foll_force_all))
+ return 0;
+
+ /* Deny if FOLL_FORCE is allowed only for ptracers via param */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT,
+ &proc_mem_restrict_foll_force_ptracer) &&
+ !__mem_rw_current_is_ptracer(file))
+ return 0;
+
+ return FOLL_FORCE;
+}
+
+static bool __mem_rw_block_writes(struct file *file)
+{
+ /* Block if writes are disabled via param proc_mem.restrict_write=all */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_WRITE_DEFAULT,
+ &proc_mem_restrict_write_all))
+ return true;
+
+ /* Block with an exception only for ptracers */
+ if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_WRITE_PTRACE_DEFAULT,
+ &proc_mem_restrict_write_ptracer) &&
+ !__mem_rw_current_is_ptracer(file))
+ return true;
+
+ return false;
+}
+
static ssize_t mem_rw(struct file *file, char __user *buf,
size_t count, loff_t *ppos, int write)
{
@@ -847,6 +971,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
if (!mm)
return 0;

+ if (write && __mem_rw_block_writes(file))
+ return -EACCES;
+
page = (char *)__get_free_page(GFP_KERNEL);
if (!page)
return -ENOMEM;
@@ -855,7 +982,8 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
if (!mmget_not_zero(mm))
goto free;

- flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
+ flags = (write ? FOLL_WRITE : 0);
+ flags |= __mem_rw_get_foll_force_flag(file);

while (count > 0) {
size_t this_len = min_t(size_t, count, PAGE_SIZE);
diff --git a/security/Kconfig b/security/Kconfig
index 412e76f1575d0..873f7c048a0b3 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -183,6 +183,74 @@ config STATIC_USERMODEHELPER_PATH
If you wish for all usermode helper programs to be disabled,
specify an empty string here (i.e. "").

+menu "Procfs mem restriction options"
+
+config PROC_MEM_RESTRICT_FOLL_FORCE_DEFAULT
+ bool "Restrict all FOLL_FORCE flag usage"
+ default n
+ help
+ Restrict all FOLL_FORCE usage during /proc/*/mem RW.
+ Debuggers like GDB require using FOLL_FORCE for basic
+ functionality.
+
+config PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT
+ bool "Restrict FOLL_FORCE usage except for ptracers"
+ default n
+ help
+ Restrict FOLL_FORCE usage during /proc/*/mem RW, except
+ for ptracer processes. Debuggers like GDB require using
+ FOLL_FORCE for basic functionality.
+
+config PROC_MEM_RESTRICT_OPEN_READ_DEFAULT
+ bool "Restrict all open() read access"
+ default n
+ help
+ Restrict all open() read access to /proc/*/mem files.
+ Use with caution: this can break init systems, debuggers,
+ container supervisors and other tasks using /proc/*/mem.
+
+config PROC_MEM_RESTRICT_OPEN_READ_PTRACE_DEFAULT
+ bool "Restrict open() for reads except for ptracers"
+ default n
+ help
+ Restrict open() read access except for ptracer processes.
+ Use with caution: this can break init systems, debuggers,
+ container supervisors and other non-ptrace capable tasks
+ using /proc/*/mem.
+
+config PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT
+ bool "Restrict all open() write access"
+ default n
+ help
+ Restrict all open() write access to /proc/*/mem files.
+ Debuggers like GDB and some container supervisors tasks
+ require opening as RW and may break.
+
+config PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT
+ bool "Restrict open() for writes except for ptracers"
+ default n
+ help
+ Restrict open() write access except for ptracer processes,
+ usually debuggers.
+
+config PROC_MEM_RESTRICT_WRITE_DEFAULT
+ bool "Restrict all write() calls"
+ default n
+ help
+ Restrict all /proc/*/mem direct write calls.
+ Open calls with RW modes are still allowed, this blocks
+ just the write() calls.
+
+config PROC_MEM_RESTRICT_WRITE_PTRACE_DEFAULT
+ bool "Restrict write() calls except for ptracers"
+ default n
+ help
+ Restrict /proc/*/mem direct write calls except for ptracer processes.
+ Open calls with RW modes are still allowed, this blocks just
+ the write() calls.
+
+endmenu
+
source "security/selinux/Kconfig"
source "security/smack/Kconfig"
source "security/tomoyo/Kconfig"
--
2.30.2

2024-06-06 17:45:57

by Kees Cook

[permalink] [raw]

Subject: Re: [PATCH v5 2/2] proc: restrict /proc/pid/mem

On Wed, Jun 05, 2024 at 07:49:31PM +0300, Adrian Ratiu wrote:
> + proc_mem.restrict_foll_force= [KNL]
> + Format: {all | ptracer}
> + Restricts the use of the FOLL_FORCE flag for /proc/*/mem access.
> + If restricted, the FOLL_FORCE flag will not be added to vm accesses.
> + Can be one of:
> + - 'all' restricts all access unconditionally.
> + - 'ptracer' allows access only for ptracer processes.
> + If not specified, FOLL_FORCE is always used.

It dawns on me that we likely need an "off" setting for these in case it
was CONFIG-enabled...

> +static int __init early_proc_mem_restrict_##name(char *buf) \
> +{ \
> + if (!buf) \
> + return -EINVAL; \
> + \
> + if (strcmp(buf, "all") == 0) \
> + static_key_slow_inc(&proc_mem_restrict_##name##_all.key); \
> + else if (strcmp(buf, "ptracer") == 0) \
> + static_key_slow_inc(&proc_mem_restrict_##name##_ptracer.key); \
> + return 0; \
> +} \
> +early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)

Why slow_inc here instead of the normal static_key_enable/disable?

And we should report misparsing too, so perhaps:

static int __init early_proc_mem_restrict_##name(char *buf) \
{ \
if (!buf) \
return -EINVAL; \
\
if (strcmp(buf, "all") == 0) { \
static_key_enable(&proc_mem_restrict_##name##_all.key); \
static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
} else if (strcmp(buf, "ptracer") == 0) { \
static_key_disable(&proc_mem_restrict_##name##_all.key); \
static_key_enable(&proc_mem_restrict_##name##_ptracer.key); \
} else if (strcmp(buf, "off") == 0) { \
static_key_disable(&proc_mem_restrict_##name##_all.key); \
static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
} else \
pr_warn("%s: ignoring unknown option '%s'\n", \
"proc_mem.restrict_" #name, buf); \
return 0; \
} \
early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)

> +static int __mem_open_access_permitted(struct file *file, struct task_struct *task)
> +{
> + bool is_ptracer;
> +
> + rcu_read_lock();
> + is_ptracer = current == ptrace_parent(task);
> + rcu_read_unlock();
> +
> + if (file->f_mode & FMODE_WRITE) {
> + /* Deny if writes are unconditionally disabled via param */
> + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT,
> + &proc_mem_restrict_open_write_all))
> + return -EACCES;
> +
> + /* Deny if writes are allowed only for ptracers via param */
> + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT,
> + &proc_mem_restrict_open_write_ptracer) &&
> + !is_ptracer)
> + return -EACCES;
> + }
> +
> + if (file->f_mode & FMODE_READ) {
> + /* Deny if reads are unconditionally disabled via param */
> + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_DEFAULT,
> + &proc_mem_restrict_open_read_all))
> + return -EACCES;
> +
> + /* Deny if reads are allowed only for ptracers via param */
> + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_PTRACE_DEFAULT,
> + &proc_mem_restrict_open_read_ptracer) &&
> + !is_ptracer)
> + return -EACCES;
> + }
> +
> + return 0; /* R/W are not restricted */
> +}

Given how deeply some of these behaviors may be in userspace, it might
be more friendly to report the new restrictions with a pr_notice() so
problems can be more easily tracked down. For example:

static void report_mem_rw_rejection(const char *action, struct task_struct *task)
{
pr_warn_ratelimited("Denied %s of /proc/%d/mem (%s) by pid %d (%s)\n",
action, task_pid_nr(task), task->comm,
task_pid_nr(current), current->comm);
}

...

if (file->f_mode & FMODE_WRITE) {
/* Deny if writes are unconditionally disabled via param */
if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT,
&proc_mem_restrict_open_write_all)) {
report_mem_rw_reject("all open-for-write");
return -EACCES;
}

/* Deny if writes are allowed only for ptracers via param */
if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT,
&proc_mem_restrict_open_write_ptracer) &&
!is_ptracer)
report_mem_rw_reject("non-ptracer open-for-write");
return -EACCES;
}

etc

> +static bool __mem_rw_current_is_ptracer(struct file *file)
> +{
> + struct inode *inode = file_inode(file);
> + struct task_struct *task = get_proc_task(inode);
> + struct mm_struct *mm = NULL;
> + int is_ptracer = false, has_mm_access = false;
> +
> + if (task) {
> + rcu_read_lock();
> + is_ptracer = current == ptrace_parent(task);
> + rcu_read_unlock();
> +
> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
> + if (mm && file->private_data == mm) {
> + has_mm_access = true;
> + mmput(mm);
> + }
> +
> + put_task_struct(task);
> + }
> +
> + return is_ptracer && has_mm_access;
> +}

Thanks; this looks right to me now!

> +menu "Procfs mem restriction options"
> +
> +config PROC_MEM_RESTRICT_FOLL_FORCE_DEFAULT
> + bool "Restrict all FOLL_FORCE flag usage"
> + default n
> + help
> + Restrict all FOLL_FORCE usage during /proc/*/mem RW.
> + Debuggers like GDB require using FOLL_FORCE for basic
> + functionality.
> +
> +config PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT
> + bool "Restrict FOLL_FORCE usage except for ptracers"
> + default n
> + help
> + Restrict FOLL_FORCE usage during /proc/*/mem RW, except
> + for ptracer processes. Debuggers like GDB require using
> + FOLL_FORCE for basic functionality.

Can we adjust the Kconfigs to match the bootparam arguments? i.e.
instead of two for each mode, how about one with 3 settings ("all",
"ptrace", or "off")

choice
prompt "Restrict /proc/pid/mem FOLL_FORCE usage"
default PROC_MEM_RESTRICT_FOLL_FORCE_OFF
help
Reading and writing of /proc/pid/mem bypasses memory permission
checks due to the internal use of the FOLL_FORCE flag. This can be
used by attackers to manipulate process memory contents that
would have been otherwise protected. However, debuggers, like GDB,
use this to set breakpoints, etc. To force debuggers to fall back
to PEEK/POKE, see PROC_MEM_RESTRICT_OPEN_WRITE_ALL.

config PROC_MEM_RESTRICT_FOLL_FORCE_OFF
bool "Do not restrict FOLL_FORCE usage with /proc/pid/mem (regular)"
help
Regular behavior: continue to use the FOLL_FORCE flag for
/proc/pid/mem access.

config PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE
bool "Only allow ptracers to use FOLL_FORCE with /proc/pid/mem (safer)"
help
Only use the FOLL_FORCE flag for /proc/pid/mem access when the
current task is the active ptracer of the target task. (Safer,
least disruptive to most usage patterns.)

config PROC_MEM_RESTRICT_FOLL_FORCE_ALL
bool "Do not use FOLL_FORCE with /proc/pid/mem (safest)"
help
Remove the FOLL_FORCE flag for all /proc/pid/mem accesses.
(Safest, but may be disruptive to some usage patterns.)
endchoice

Then the static_keys can be defined like this mess (I couldn't find a
cleaner way to do it):

#define DEFINE_STATIC_KEY_PROC_MEM_ALL(name) \
DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_##name##_all); \
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_ptracer);
#define DEFINE_STATIC_KEY_PROC_MEM_PTRACE(name) \
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_all); \
DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_##name##_ptracer);
#define DEFINE_STATIC_KEY_PROC_MEM_OFF(name) \
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_all); \
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_ptracer);

#define DEFINE_STATIC_KEY_PROC_MEM_0(level, name)
#define DEFINE_STATIC_KEY_PROC_MEM_1(level, name) \
DEFINE_STATIC_KEY_PROC_MEM_##level(name)

#define _DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name) \
DEFINE_STATIC_KEY_PROC_MEM_##enabled(level, name)

#define DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name) \
_DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name)

#define DEFINE_STATIC_KEY_PROC_MEM(CFG, name) \
DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_ALL), ALL, name)
DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_PTRACE), PTRACE, name)
DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_OFF), OFF, name)

#define DEFINE_EARLY_PROC_MEM_RESTRICT(CFG, name) \
DEFINE_STATIC_KEY_PROC_MEM(CFG, name) \
static int __init early_proc_mem_restrict_##name(char *buf) \
{ \
if (!buf) \
return -EINVAL; \
\
if (strcmp(buf, "all") == 0) { \
static_key_enable(&proc_mem_restrict_##name##_all.key); \
static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
} else if (strcmp(buf, "ptracer") == 0) { \
static_key_disable(&proc_mem_restrict_##name##_all.key); \
static_key_enable(&proc_mem_restrict_##name##_ptracer.key); \
} else if (strcmp(buf, "off") == 0) { \
static_key_disable(&proc_mem_restrict_##name##_all.key); \
static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
} else \
pr_warn("%s: ignoring unknown option '%s'\n", \
"proc_mem.restrict_" #name, buf); \
return 0; \
} \
early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)

DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_READ, open_read);
DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_WRITE, open_write);
DEFINE_EARLY_PROC_MEM_RESTRICT(WRITE, write);
DEFINE_EARLY_PROC_MEM_RESTRICT(FOLL_FORCE, foll_force);

--
Kees Cook

2024-06-07 10:39:18

by Adrian Ratiu

[permalink] [raw]

Subject: Re: [PATCH v5 2/2] proc: restrict /proc/pid/mem

2024-06-12 18:14:00

by Adrian Ratiu

[permalink] [raw]

Subject: Re: [PATCH v5 2/2] proc: restrict /proc/pid/mem

On Thursday, June 06, 2024 20:45 EEST, Kees Cook <[email protected]> wrote:

> On Wed, Jun 05, 2024 at 07:49:31PM +0300, Adrian Ratiu wrote:
> > + proc_mem.restrict_foll_force= [KNL]
> > + Format: {all | ptracer}
> > + Restricts the use of the FOLL_FORCE flag for /proc/*/mem access.
> > + If restricted, the FOLL_FORCE flag will not be added to vm accesses.
> > + Can be one of:
> > + - 'all' restricts all access unconditionally.
> > + - 'ptracer' allows access only for ptracer processes.
> > + If not specified, FOLL_FORCE is always used.
>
> It dawns on me that we likely need an "off" setting for these in case it
> was CONFIG-enabled...
>
> > +static int __init early_proc_mem_restrict_##name(char *buf) \
> > +{ \
> > + if (!buf) \
> > + return -EINVAL; \
> > + \
> > + if (strcmp(buf, "all") == 0) \
> > + static_key_slow_inc(&proc_mem_restrict_##name##_all.key); \
> > + else if (strcmp(buf, "ptracer") == 0) \
> > + static_key_slow_inc(&proc_mem_restrict_##name##_ptracer.key); \
> > + return 0; \
> > +} \
> > +early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)
>
> Why slow_inc here instead of the normal static_key_enable/disable?
>
> And we should report misparsing too, so perhaps:
>
> static int __init early_proc_mem_restrict_##name(char *buf) \
> { \
> if (!buf) \
> return -EINVAL; \
> \
> if (strcmp(buf, "all") == 0) { \
> static_key_enable(&proc_mem_restrict_##name##_all.key); \
> static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
> } else if (strcmp(buf, "ptracer") == 0) { \
> static_key_disable(&proc_mem_restrict_##name##_all.key); \
> static_key_enable(&proc_mem_restrict_##name##_ptracer.key); \
> } else if (strcmp(buf, "off") == 0) { \
> static_key_disable(&proc_mem_restrict_##name##_all.key); \
> static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
> } else \
> pr_warn("%s: ignoring unknown option '%s'\n", \
> "proc_mem.restrict_" #name, buf); \
> return 0; \
> } \
> early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)
>
> > +static int __mem_open_access_permitted(struct file *file, struct task_struct *task)
> > +{
> > + bool is_ptracer;
> > +
> > + rcu_read_lock();
> > + is_ptracer = current == ptrace_parent(task);
> > + rcu_read_unlock();
> > +
> > + if (file->f_mode & FMODE_WRITE) {
> > + /* Deny if writes are unconditionally disabled via param */
> > + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT,
> > + &proc_mem_restrict_open_write_all))
> > + return -EACCES;
> > +
> > + /* Deny if writes are allowed only for ptracers via param */
> > + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT,
> > + &proc_mem_restrict_open_write_ptracer) &&
> > + !is_ptracer)
> > + return -EACCES;
> > + }
> > +
> > + if (file->f_mode & FMODE_READ) {
> > + /* Deny if reads are unconditionally disabled via param */
> > + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_DEFAULT,
> > + &proc_mem_restrict_open_read_all))
> > + return -EACCES;
> > +
> > + /* Deny if reads are allowed only for ptracers via param */
> > + if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_PTRACE_DEFAULT,
> > + &proc_mem_restrict_open_read_ptracer) &&
> > + !is_ptracer)
> > + return -EACCES;
> > + }
> > +
> > + return 0; /* R/W are not restricted */
> > +}
>
> Given how deeply some of these behaviors may be in userspace, it might
> be more friendly to report the new restrictions with a pr_notice() so
> problems can be more easily tracked down. For example:
>
> static void report_mem_rw_rejection(const char *action, struct task_struct *task)
> {
> pr_warn_ratelimited("Denied %s of /proc/%d/mem (%s) by pid %d (%s)\n",
> action, task_pid_nr(task), task->comm,
> task_pid_nr(current), current->comm);
> }
>
> ...
>
> if (file->f_mode & FMODE_WRITE) {
> /* Deny if writes are unconditionally disabled via param */
> if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_DEFAULT,
> &proc_mem_restrict_open_write_all)) {
> report_mem_rw_reject("all open-for-write");
> return -EACCES;
> }
>
> /* Deny if writes are allowed only for ptracers via param */
> if (static_branch_maybe(CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_PTRACE_DEFAULT,
> &proc_mem_restrict_open_write_ptracer) &&
> !is_ptracer)
> report_mem_rw_reject("non-ptracer open-for-write");
> return -EACCES;
> }
>
> etc
>
> > +static bool __mem_rw_current_is_ptracer(struct file *file)
> > +{
> > + struct inode *inode = file_inode(file);
> > + struct task_struct *task = get_proc_task(inode);
> > + struct mm_struct *mm = NULL;
> > + int is_ptracer = false, has_mm_access = false;
> > +
> > + if (task) {
> > + rcu_read_lock();
> > + is_ptracer = current == ptrace_parent(task);
> > + rcu_read_unlock();
> > +
> > + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
> > + if (mm && file->private_data == mm) {
> > + has_mm_access = true;
> > + mmput(mm);
> > + }
> > +
> > + put_task_struct(task);
> > + }
> > +
> > + return is_ptracer && has_mm_access;
> > +}
>
> Thanks; this looks right to me now!
>
> > +menu "Procfs mem restriction options"
> > +
> > +config PROC_MEM_RESTRICT_FOLL_FORCE_DEFAULT
> > + bool "Restrict all FOLL_FORCE flag usage"
> > + default n
> > + help
> > + Restrict all FOLL_FORCE usage during /proc/*/mem RW.
> > + Debuggers like GDB require using FOLL_FORCE for basic
> > + functionality.
> > +
> > +config PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT
> > + bool "Restrict FOLL_FORCE usage except for ptracers"
> > + default n
> > + help
> > + Restrict FOLL_FORCE usage during /proc/*/mem RW, except
> > + for ptracer processes. Debuggers like GDB require using
> > + FOLL_FORCE for basic functionality.
>
> Can we adjust the Kconfigs to match the bootparam arguments? i.e.
> instead of two for each mode, how about one with 3 settings ("all",
> "ptrace", or "off")
>
> choice
> prompt "Restrict /proc/pid/mem FOLL_FORCE usage"
> default PROC_MEM_RESTRICT_FOLL_FORCE_OFF
> help
> Reading and writing of /proc/pid/mem bypasses memory permission
> checks due to the internal use of the FOLL_FORCE flag. This can be
> used by attackers to manipulate process memory contents that
> would have been otherwise protected. However, debuggers, like GDB,
> use this to set breakpoints, etc. To force debuggers to fall back
> to PEEK/POKE, see PROC_MEM_RESTRICT_OPEN_WRITE_ALL.
>
> config PROC_MEM_RESTRICT_FOLL_FORCE_OFF
> bool "Do not restrict FOLL_FORCE usage with /proc/pid/mem (regular)"
> help
> Regular behavior: continue to use the FOLL_FORCE flag for
> /proc/pid/mem access.
>
> config PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE
> bool "Only allow ptracers to use FOLL_FORCE with /proc/pid/mem (safer)"
> help
> Only use the FOLL_FORCE flag for /proc/pid/mem access when the
> current task is the active ptracer of the target task. (Safer,
> least disruptive to most usage patterns.)
>
> config PROC_MEM_RESTRICT_FOLL_FORCE_ALL
> bool "Do not use FOLL_FORCE with /proc/pid/mem (safest)"
> help
> Remove the FOLL_FORCE flag for all /proc/pid/mem accesses.
> (Safest, but may be disruptive to some usage patterns.)
> endchoice
>
> Then the static_keys can be defined like this mess (I couldn't find a
> cleaner way to do it):
>
> #define DEFINE_STATIC_KEY_PROC_MEM_ALL(name) \
> DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_##name##_all); \
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_ptracer);
> #define DEFINE_STATIC_KEY_PROC_MEM_PTRACE(name) \
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_all); \
> DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_##name##_ptracer);
> #define DEFINE_STATIC_KEY_PROC_MEM_OFF(name) \
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_all); \
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_##name##_ptracer);
>
> #define DEFINE_STATIC_KEY_PROC_MEM_0(level, name)
> #define DEFINE_STATIC_KEY_PROC_MEM_1(level, name) \
> DEFINE_STATIC_KEY_PROC_MEM_##level(name)
>
> #define _DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name) \
> DEFINE_STATIC_KEY_PROC_MEM_##enabled(level, name)
>
> #define DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name) \
> _DEFINE_STATIC_KEY_PROC_MEM_PICK(enabled, level, name)
>
> #define DEFINE_STATIC_KEY_PROC_MEM(CFG, name) \
> DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_ALL), ALL, name)
> DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_PTRACE), PTRACE, name)
> DEFINE_STATIC_KEY_PROC_MEM_PICK(IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_##CFG##_OFF), OFF, name)
>
> #define DEFINE_EARLY_PROC_MEM_RESTRICT(CFG, name) \
> DEFINE_STATIC_KEY_PROC_MEM(CFG, name) \
> static int __init early_proc_mem_restrict_##name(char *buf) \
> { \
> if (!buf) \
> return -EINVAL; \
> \
> if (strcmp(buf, "all") == 0) { \
> static_key_enable(&proc_mem_restrict_##name##_all.key); \
> static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
> } else if (strcmp(buf, "ptracer") == 0) { \
> static_key_disable(&proc_mem_restrict_##name##_all.key); \
> static_key_enable(&proc_mem_restrict_##name##_ptracer.key); \
> } else if (strcmp(buf, "off") == 0) { \
> static_key_disable(&proc_mem_restrict_##name##_all.key); \
> static_key_disable(&proc_mem_restrict_##name##_ptracer.key); \
> } else \
> pr_warn("%s: ignoring unknown option '%s'\n", \
> "proc_mem.restrict_" #name, buf); \
> return 0; \
> } \
> early_param("proc_mem.restrict_" #name, early_proc_mem_restrict_##name)
>
> DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_READ, open_read);
> DEFINE_EARLY_PROC_MEM_RESTRICT(OPEN_WRITE, open_write);
> DEFINE_EARLY_PROC_MEM_RESTRICT(WRITE, write);
> DEFINE_EARLY_PROC_MEM_RESTRICT(FOLL_FORCE, foll_force);

Hello again,

I tried very hard to make the above work these past few days and gave up.
Couldn't find a way to get it to compile.
Tried to also debug the compiler preprocess output and my head hurts. :)

Would macros like the following be acceptable?
I know it's more verbose but also much easier to understand and it works.

#if IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_ALL)
DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_open_read_all);
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_ptracer);
#elif IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_PTRACE)
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_all);
DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_open_read_ptracer);
#else
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_all);
DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_ptracer);
#endif

2024-06-12 18:24:04

by Kees Cook

[permalink] [raw]

Subject: Re: [PATCH v5 2/2] proc: restrict /proc/pid/mem

On Wed, Jun 12, 2024 at 07:13:41PM +0100, Adrian Ratiu wrote:
> Would macros like the following be acceptable?
> I know it's more verbose but also much easier to understand and it works.
>
> #if IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_ALL)
> DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_open_read_all);
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_ptracer);
> #elif IS_ENABLED(CONFIG_PROC_MEM_RESTRICT_OPEN_READ_PTRACE)
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_all);
> DEFINE_STATIC_KEY_TRUE_RO(proc_mem_restrict_open_read_ptracer);
> #else
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_all);
> DEFINE_STATIC_KEY_FALSE_RO(proc_mem_restrict_open_read_ptracer);
> #endif

Yeah, that'd be fine by me. I was a little concerned I was
over-generalizing those macros. :P

--
Kees Cook