LinuxLists.cc - BUG: GPF in non-whitelisted uaccess (non-canonical address?)

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Subject: Re: BUG: GPF in non-whitelisted uaccess (non-canonical address?)

Hey

On Wed, Nov 14, 2018 at 1:25 AM syzbot
<[email protected]> wrote:
> syzbot has found a reproducer for the following crash on:
>
> HEAD commit: ccda4af0f4b9 Linux 4.20-rc2
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13b4e77b400000
> kernel config: https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
> dashboard link: https://syzkaller.appspot.com/bug?extid=72473edc9bf4eb1c6556
> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1646a225400000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=108a6533400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: [email protected]
>
[...]
> BUG: GPF in non-whitelisted uaccess (non-canonical address?)

This uses sendpage(2) to feed data from a file into a uhid chardev.
The default behavior of the kernel is to create a temporary pipe, then
splice from the file into the pipe, and then splice again from the
pipe into uhid.

The kernel provides default implementations for splicing between files
and any other file. The default implementation of `.splice_write()`
uses kmap() to map the page from the pipe and then uses the
__kernel_write() (which uses .f_op->write()) to push the data into the
target file. The problem is, __kernel_write() sets the address-space
to KERNEL_DS `set_fs(get_ds())`, thus granting the UHID request access
to kernel memory.

I see several ways to fix that, the most simple solution is to simply
prevent splice/sendpage on uhid (by setting f_op.splice_write to a
dummy). Alternatively, we can implement a proper splice helper that
takes the page directly, rather than through the __kernel_write()
default implementation.

Thanks
David

2018-11-14 16:55:14

by Dmitry Vyukov

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Subject: Re: BUG: GPF in non-whitelisted uaccess (non-canonical address?)

On Wed, Nov 14, 2018 at 4:20 AM, David Herrmann <[email protected]> wrote:
> Hey
>
> On Wed, Nov 14, 2018 at 1:25 AM syzbot
> <[email protected]> wrote:
>> syzbot has found a reproducer for the following crash on:
>>
>> HEAD commit: ccda4af0f4b9 Linux 4.20-rc2
>> git tree: upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13b4e77b400000
>> kernel config: https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
>> dashboard link: https://syzkaller.appspot.com/bug?extid=72473edc9bf4eb1c6556
>> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1646a225400000
>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=108a6533400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: [email protected]
>>
> [...]
>> BUG: GPF in non-whitelisted uaccess (non-canonical address?)
>
> This uses sendpage(2) to feed data from a file into a uhid chardev.
> The default behavior of the kernel is to create a temporary pipe, then
> splice from the file into the pipe, and then splice again from the
> pipe into uhid.
>
> The kernel provides default implementations for splicing between files
> and any other file. The default implementation of `.splice_write()`
> uses kmap() to map the page from the pipe and then uses the
> __kernel_write() (which uses .f_op->write()) to push the data into the
> target file. The problem is, __kernel_write() sets the address-space
> to KERNEL_DS `set_fs(get_ds())`, thus granting the UHID request access
> to kernel memory.
>
> I see several ways to fix that, the most simple solution is to simply
> prevent splice/sendpage on uhid (by setting f_op.splice_write to a
> dummy). Alternatively, we can implement a proper splice helper that
> takes the page directly, rather than through the __kernel_write()
> default implementation.

also +dtor for uhid

2018-11-14 17:17:50

[permalink] [raw]

Subject: Re: BUG: GPF in non-whitelisted uaccess (non-canonical address?)

On Wed, Nov 14, 2018 at 08:52:46AM -0800, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> On Wed, Nov 14, 2018 at 4:20 AM, David Herrmann <[email protected]> wrote:
> > Hey
> >
> > On Wed, Nov 14, 2018 at 1:25 AM syzbot
> > <[email protected]> wrote:
> >> syzbot has found a reproducer for the following crash on:
> >>
> >> HEAD commit: ccda4af0f4b9 Linux 4.20-rc2
> >> git tree: upstream
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=13b4e77b400000
> >> kernel config: https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=72473edc9bf4eb1c6556
> >> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
> >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1646a225400000
> >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=108a6533400000
> >>
> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >> Reported-by: [email protected]
> >>
> > [...]
> >> BUG: GPF in non-whitelisted uaccess (non-canonical address?)
> >
> > This uses sendpage(2) to feed data from a file into a uhid chardev.
> > The default behavior of the kernel is to create a temporary pipe, then
> > splice from the file into the pipe, and then splice again from the
> > pipe into uhid.
> >
> > The kernel provides default implementations for splicing between files
> > and any other file. The default implementation of `.splice_write()`
> > uses kmap() to map the page from the pipe and then uses the
> > __kernel_write() (which uses .f_op->write()) to push the data into the
> > target file. The problem is, __kernel_write() sets the address-space
> > to KERNEL_DS `set_fs(get_ds())`, thus granting the UHID request access
> > to kernel memory.
> >
> > I see several ways to fix that, the most simple solution is to simply
> > prevent splice/sendpage on uhid (by setting f_op.splice_write to a
> > dummy). Alternatively, we can implement a proper splice helper that
> > takes the page directly, rather than through the __kernel_write()
> > default implementation.
>
> also +dtor for uhid
>

Well, the problem is that uhid_char_write() reads from a user pointer embedded
in the write() payload. (Which really is abusing write(), but I assume it
cannot be changed at this point...) Thus it's unsafe to be called under
KERNEL_DS. So it needs:

if (uaccess_kernel())
return -EACCES;

See sg_check_file_access(), called from sg_read() and sg_write(), for another
example of this in the kernel.

- Eric

2018-11-14 18:07:31

[permalink] [raw]

Subject: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS

From: Eric Biggers <[email protected]>

When a UHID_CREATE command is written to the uhid char device, a
copy_from_user() is done from a user pointer embedded in the command.
When the address limit is KERNEL_DS, e.g. as is the case during
sendfile(), this can read from kernel memory. Therefore, UHID_CREATE
must not be allowed in this case.

For consistency and to make sure all current and future uhid commands
are covered, apply the restriction to uhid_char_write() as a whole
rather than to UHID_CREATE specifically.

Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
helpers fault on kernel addresses"), allowing this bug to be found.

Reported-by: [email protected]
Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
Cc: <[email protected]> # v3.6+
Cc: Jann Horn <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
---
drivers/hid/uhid.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 3c55073136064..e94c5e248b56e 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
int ret;
size_t len;

+ if (uaccess_kernel()) { /* payload may contain a __user pointer */
+ pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
+ __func__, task_tgid_vnr(current), current->comm);
+ return -EACCES;
+ }
+
/* we need at least the "type" member of uhid_event */
if (count < sizeof(__u32))
return -EINVAL;
--
2.19.1.930.g4563a0d9d0-goog

2018-11-14 18:15:41

[permalink] [raw]

Subject: Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS

On Wed, Nov 14, 2018 at 10:03 AM Eric Biggers <[email protected]> wrote:
>
> From: Eric Biggers <[email protected]>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sendfile(), this can read from kernel memory. Therefore, UHID_CREATE
> must not be allowed in this case.

Hmm, instead of disallowing access, can we switch back to USER_DS
before trying to use the user pointer?

>
>
> For consistency and to make sure all current and future uhid commands
> are covered, apply the restriction to uhid_char_write() as a whole
> rather than to UHID_CREATE specifically.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: [email protected]
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <[email protected]> # v3.6+
> Cc: Jann Horn <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>
> ---
> drivers/hid/uhid.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..e94c5e248b56e 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> int ret;
> size_t len;
>
> + if (uaccess_kernel()) { /* payload may contain a __user pointer */
> + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
> + __func__, task_tgid_vnr(current), current->comm);
> + return -EACCES;
> + }
> +
> /* we need at least the "type" member of uhid_event */
> if (count < sizeof(__u32))
> return -EINVAL;
> --
> 2.19.1.930.g4563a0d9d0-goog
>

2018-11-14 18:20:10

by Jann Horn

[permalink] [raw]

Subject: Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS

+cc Andy

On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers <[email protected]> wrote:
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sendfile(), this can read from kernel memory. Therefore, UHID_CREATE
> must not be allowed in this case.
>
> For consistency and to make sure all current and future uhid commands
> are covered, apply the restriction to uhid_char_write() as a whole
> rather than to UHID_CREATE specifically.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: [email protected]

Wheeeee, it found something! :)

> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <[email protected]> # v3.6+
> Cc: Jann Horn <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>
> ---
> drivers/hid/uhid.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..e94c5e248b56e 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> int ret;
> size_t len;
>
> + if (uaccess_kernel()) { /* payload may contain a __user pointer */
> + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
> + __func__, task_tgid_vnr(current), current->comm);
> + return -EACCES;
> + }

If this file can conceivably be opened by a process that doesn't have
root privileges, this check should be something along the lines of
ib_safe_file_access() or sg_check_file_access().

Checking for uaccess_kernel() prevents the symptom that syzkaller
notices - a user being able to cause a kernel memory access -, but it
doesn't deal with the case where a user opens a file descriptor to
this device and tricks a more privileged process into writing into it
(e.g. by passing it to a suid binary as stdout or stderr).

Looking closer, I wonder whether this kind of behavior is limited to
the UHID_CREATE request, which has a comment on it saying "/*
Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly
kludge away from the code paths you're supposed to be using, that
would be nice...

2018-11-14 22:28:54

[permalink] [raw]

Subject: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

From: Eric Biggers <[email protected]>

When a UHID_CREATE command is written to the uhid char device, a
copy_from_user() is done from a user pointer embedded in the command.
When the address limit is KERNEL_DS, e.g. as is the case during
sys_sendfile(), this can read from kernel memory. Alternatively,
information can be leaked from a setuid binary that is tricked to write
to the file descriptor. Therefore, forbid UHID_CREATE in these cases.

No other commands in uhid_char_write() are affected by this bug and
UHID_CREATE is marked as "obsolete", so apply the restriction to
UHID_CREATE only rather than to uhid_char_write() entirely.

Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
helpers fault on kernel addresses"), allowing this bug to be found.

Reported-by: [email protected]
Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
Cc: <[email protected]> # v3.6+
Cc: Jann Horn <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
---
drivers/hid/uhid.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 3c55073136064..051639c09f728 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -12,6 +12,7 @@

#include <linux/atomic.h>
#include <linux/compat.h>
+#include <linux/cred.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/hid.h>
@@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,

switch (uhid->input_buf.type) {
case UHID_CREATE:
+ /*
+ * 'struct uhid_create_req' contains a __user pointer which is
+ * copied from, so it's unsafe to allow this with elevated
+ * privileges (e.g. from a setuid binary) or via kernel_write().
+ */
+ if (file->f_cred != current_cred() || uaccess_kernel()) {
+ pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
+ task_tgid_vnr(current), current->comm);
+ ret = -EACCES;
+ goto unlock;
+ }
ret = uhid_dev_create(uhid, &uhid->input_buf);
break;
case UHID_CREATE2:
--
2.19.1.930.g4563a0d9d0-goog

2018-11-14 22:29:43

[permalink] [raw]

Subject: Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS

On Wed, Nov 14, 2018 at 07:18:39PM +0100, 'Jann Horn' via syzkaller-bugs wrote:
> +cc Andy
>
> On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers <[email protected]> wrote:
> > When a UHID_CREATE command is written to the uhid char device, a
> > copy_from_user() is done from a user pointer embedded in the command.
> > When the address limit is KERNEL_DS, e.g. as is the case during
> > sendfile(), this can read from kernel memory. Therefore, UHID_CREATE
> > must not be allowed in this case.
> >
> > For consistency and to make sure all current and future uhid commands
> > are covered, apply the restriction to uhid_char_write() as a whole
> > rather than to UHID_CREATE specifically.
> >
> > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > helpers fault on kernel addresses"), allowing this bug to be found.
> >
> > Reported-by: [email protected]
>
> Wheeeee, it found something! :)
>
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <[email protected]> # v3.6+
> > Cc: Jann Horn <[email protected]>
> > Signed-off-by: Eric Biggers <[email protected]>
> > ---
> > drivers/hid/uhid.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > index 3c55073136064..e94c5e248b56e 100644
> > --- a/drivers/hid/uhid.c
> > +++ b/drivers/hid/uhid.c
> > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > int ret;
> > size_t len;
> >
> > + if (uaccess_kernel()) { /* payload may contain a __user pointer */
> > + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
> > + __func__, task_tgid_vnr(current), current->comm);
> > + return -EACCES;
> > + }
>
> If this file can conceivably be opened by a process that doesn't have
> root privileges, this check should be something along the lines of
> ib_safe_file_access() or sg_check_file_access().
>
> Checking for uaccess_kernel() prevents the symptom that syzkaller
> notices - a user being able to cause a kernel memory access -, but it
> doesn't deal with the case where a user opens a file descriptor to
> this device and tricks a more privileged process into writing into it
> (e.g. by passing it to a suid binary as stdout or stderr).
>

Yep, I'll do that.

> Looking closer, I wonder whether this kind of behavior is limited to
> the UHID_CREATE request, which has a comment on it saying "/*
> Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly
> kludge away from the code paths you're supposed to be using, that
> would be nice...
>

I wanted to be careful, but yes AFAICS it can be limited to UHID_CREATE only,
so I'll do that instead.

- Eric

2018-11-14 22:30:40

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
>
> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> >
> > From: Eric Biggers <[email protected]>
> >
> > When a UHID_CREATE command is written to the uhid char device, a
> > copy_from_user() is done from a user pointer embedded in the command.
> > When the address limit is KERNEL_DS, e.g. as is the case during
> > sys_sendfile(), this can read from kernel memory. Alternatively,
> > information can be leaked from a setuid binary that is tricked to write
> > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> >
> > No other commands in uhid_char_write() are affected by this bug and
> > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > UHID_CREATE only rather than to uhid_char_write() entirely.
> >
> > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > helpers fault on kernel addresses"), allowing this bug to be found.
> >
> > Reported-by: [email protected]
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <[email protected]> # v3.6+
> > Cc: Jann Horn <[email protected]>
> > Cc: Andy Lutomirski <[email protected]>
> > Signed-off-by: Eric Biggers <[email protected]>
>
> Reviewed-by: Jann Horn <[email protected]>
>
> > ---
> > drivers/hid/uhid.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > index 3c55073136064..051639c09f728 100644
> > --- a/drivers/hid/uhid.c
> > +++ b/drivers/hid/uhid.c
> > @@ -12,6 +12,7 @@
> >
> > #include <linux/atomic.h>
> > #include <linux/compat.h>
> > +#include <linux/cred.h>
> > #include <linux/device.h>
> > #include <linux/fs.h>
> > #include <linux/hid.h>
> > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> >
> > switch (uhid->input_buf.type) {
> > case UHID_CREATE:
> > + /*
> > + * 'struct uhid_create_req' contains a __user pointer which is
> > + * copied from, so it's unsafe to allow this with elevated
> > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > + */

uhid is a privileged interface so we would go from root to less
privileged (if at all). If non-privileged process can open uhid it can
construct virtual keyboard and inject whatever keystrokes it wants.

Also, instead of disallowing access, can we ensure that we switch back
to USER_DS before trying to load data from the user pointer?

> > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > + task_tgid_vnr(current), current->comm);
> > + ret = -EACCES;
> > + goto unlock;
> > + }
> > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > break;
> > case UHID_CREATE2:
> > --
> > 2.19.1.930.g4563a0d9d0-goog
> >

Thanks.

--
Dmitry

2018-11-14 22:36:00

by Jann Horn

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
>
> From: Eric Biggers <[email protected]>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: [email protected]
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <[email protected]> # v3.6+
> Cc: Jann Horn <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>

Reviewed-by: Jann Horn <[email protected]>

> ---
> drivers/hid/uhid.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..051639c09f728 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -12,6 +12,7 @@
>
> #include <linux/atomic.h>
> #include <linux/compat.h>
> +#include <linux/cred.h>
> #include <linux/device.h>
> #include <linux/fs.h>
> #include <linux/hid.h>
> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>
> switch (uhid->input_buf.type) {
> case UHID_CREATE:
> + /*
> + * 'struct uhid_create_req' contains a __user pointer which is
> + * copied from, so it's unsafe to allow this with elevated
> + * privileges (e.g. from a setuid binary) or via kernel_write().
> + */
> + if (file->f_cred != current_cred() || uaccess_kernel()) {
> + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> + task_tgid_vnr(current), current->comm);
> + ret = -EACCES;
> + goto unlock;
> + }
> ret = uhid_dev_create(uhid, &uhid->input_buf);
> break;
> case UHID_CREATE2:
> --
> 2.19.1.930.g4563a0d9d0-goog
>

2018-11-14 22:39:13

by Jann Horn

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <[email protected]> wrote:
> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
> > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> > > When a UHID_CREATE command is written to the uhid char device, a
> > > copy_from_user() is done from a user pointer embedded in the command.
> > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > information can be leaked from a setuid binary that is tricked to write
> > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > >
> > > No other commands in uhid_char_write() are affected by this bug and
> > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > UHID_CREATE only rather than to uhid_char_write() entirely.
[...]
> > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
[...]
> > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > >
> > > switch (uhid->input_buf.type) {
> > > case UHID_CREATE:
> > > + /*
> > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > + * copied from, so it's unsafe to allow this with elevated
> > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > + */
>
> uhid is a privileged interface so we would go from root to less
> privileged (if at all). If non-privileged process can open uhid it can
> construct virtual keyboard and inject whatever keystrokes it wants.
>
> Also, instead of disallowing access, can we ensure that we switch back
> to USER_DS before trying to load data from the user pointer?

Does that even make sense? You are using some deprecated legacy
interface; you interact with it by splicing a request from something
like a file or a pipe into the uhid device; but the request you're
splicing through contains a pointer into userspace memory? Do you know
of anyone who is actually doing that? If not, anyone who does want to
do this for some reason in the future can just go use UHID_CREATE2
instead.

> > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > + task_tgid_vnr(current), current->comm);
> > > + ret = -EACCES;
> > > + goto unlock;
> > > + }
> > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > break;
> > > case UHID_CREATE2:

2018-11-14 22:48:49

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Wed, Nov 14, 2018 at 2:38 PM Jann Horn <[email protected]> wrote:
>
> On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <[email protected]> wrote:
> > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
> > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > information can be leaked from a setuid binary that is tricked to write
> > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > >
> > > > No other commands in uhid_char_write() are affected by this bug and
> > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> [...]
> > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> [...]
> > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > >
> > > > switch (uhid->input_buf.type) {
> > > > case UHID_CREATE:
> > > > + /*
> > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > + * copied from, so it's unsafe to allow this with elevated
> > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > + */
> >
> > uhid is a privileged interface so we would go from root to less
> > privileged (if at all). If non-privileged process can open uhid it can
> > construct virtual keyboard and inject whatever keystrokes it wants.
> >
> > Also, instead of disallowing access, can we ensure that we switch back
> > to USER_DS before trying to load data from the user pointer?
>
> Does that even make sense? You are using some deprecated legacy
> interface; you interact with it by splicing a request from something
> like a file or a pipe into the uhid device; but the request you're
> splicing through contains a pointer into userspace memory? Do you know
> of anyone who is actually doing that? If not, anyone who does want to
> do this for some reason in the future can just go use UHID_CREATE2
> instead.

I do not know if anyone is still using UHID_CREATE with sendpage and
neither do you really. It is all about not breaking userspace without
good reason and here ensuring that we switch to USER_DS and then back
to whatever it was does not seem too hard.

>
> > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > + task_tgid_vnr(current), current->comm);
> > > > + ret = -EACCES;
> > > > + goto unlock;
> > > > + }
> > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > break;
> > > > case UHID_CREATE2:

2018-11-14 23:02:00

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

Hi Dmitry,

On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
> >
> > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> > >
> > > From: Eric Biggers <[email protected]>
> > >
> > > When a UHID_CREATE command is written to the uhid char device, a
> > > copy_from_user() is done from a user pointer embedded in the command.
> > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > information can be leaked from a setuid binary that is tricked to write
> > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > >
> > > No other commands in uhid_char_write() are affected by this bug and
> > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > >
> > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > helpers fault on kernel addresses"), allowing this bug to be found.
> > >
> > > Reported-by: [email protected]
> > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > Cc: <[email protected]> # v3.6+
> > > Cc: Jann Horn <[email protected]>
> > > Cc: Andy Lutomirski <[email protected]>
> > > Signed-off-by: Eric Biggers <[email protected]>
> >
> > Reviewed-by: Jann Horn <[email protected]>
> >
> > > ---
> > > drivers/hid/uhid.c | 12 ++++++++++++
> > > 1 file changed, 12 insertions(+)
> > >
> > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > index 3c55073136064..051639c09f728 100644
> > > --- a/drivers/hid/uhid.c
> > > +++ b/drivers/hid/uhid.c
> > > @@ -12,6 +12,7 @@
> > >
> > > #include <linux/atomic.h>
> > > #include <linux/compat.h>
> > > +#include <linux/cred.h>
> > > #include <linux/device.h>
> > > #include <linux/fs.h>
> > > #include <linux/hid.h>
> > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > >
> > > switch (uhid->input_buf.type) {
> > > case UHID_CREATE:
> > > + /*
> > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > + * copied from, so it's unsafe to allow this with elevated
> > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > + */
>
> uhid is a privileged interface so we would go from root to less
> privileged (if at all). If non-privileged process can open uhid it can
> construct virtual keyboard and inject whatever keystrokes it wants.
>
> Also, instead of disallowing access, can we ensure that we switch back
> to USER_DS before trying to load data from the user pointer?
>

Actually uhid doesn't have any capability checks, so it's up to userspace to
assign permissions to the device node. I think it's best not to make
assumptions about how the interface will be used and to be consistent with how
other ->write() methods in the kernel handle the misfeature where a __user
pointer in the write() or read() payload is dereferenced. Temporarily switching
to USER_DS would only avoid one of the two problems.

Do you think the proposed restrictions would actually break anything?

- Eric

> > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > + task_tgid_vnr(current), current->comm);
> > > + ret = -EACCES;
> > > + goto unlock;
> > > + }
> > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > break;
> > > case UHID_CREATE2:
> > > --

2018-11-14 23:22:03

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Wed, Nov 14, 2018 at 3:00 PM Eric Biggers <[email protected]> wrote:
>
> Hi Dmitry,
>
> On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
> > >
> > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> > > >
> > > > From: Eric Biggers <[email protected]>
> > > >
> > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > information can be leaked from a setuid binary that is tricked to write
> > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > >
> > > > No other commands in uhid_char_write() are affected by this bug and
> > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > > >
> > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > > helpers fault on kernel addresses"), allowing this bug to be found.
> > > >
> > > > Reported-by: [email protected]
> > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > > Cc: <[email protected]> # v3.6+
> > > > Cc: Jann Horn <[email protected]>
> > > > Cc: Andy Lutomirski <[email protected]>
> > > > Signed-off-by: Eric Biggers <[email protected]>
> > >
> > > Reviewed-by: Jann Horn <[email protected]>
> > >
> > > > ---
> > > > drivers/hid/uhid.c | 12 ++++++++++++
> > > > 1 file changed, 12 insertions(+)
> > > >
> > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > > index 3c55073136064..051639c09f728 100644
> > > > --- a/drivers/hid/uhid.c
> > > > +++ b/drivers/hid/uhid.c
> > > > @@ -12,6 +12,7 @@
> > > >
> > > > #include <linux/atomic.h>
> > > > #include <linux/compat.h>
> > > > +#include <linux/cred.h>
> > > > #include <linux/device.h>
> > > > #include <linux/fs.h>
> > > > #include <linux/hid.h>
> > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > >
> > > > switch (uhid->input_buf.type) {
> > > > case UHID_CREATE:
> > > > + /*
> > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > + * copied from, so it's unsafe to allow this with elevated
> > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > + */
> >
> > uhid is a privileged interface so we would go from root to less
> > privileged (if at all). If non-privileged process can open uhid it can
> > construct virtual keyboard and inject whatever keystrokes it wants.
> >
> > Also, instead of disallowing access, can we ensure that we switch back
> > to USER_DS before trying to load data from the user pointer?
> >
>
> Actually uhid doesn't have any capability checks, so it's up to userspace to
> assign permissions to the device node.

Yes. There are quite a few such instances where kernel does not bother
implementing superfluous checks and instead relies on userspace to
provide sane environment. IIRC nobody in the kernel enforces root
filesystem not be accessible to ordinary users, it is done with
generic permission checks.

> I think it's best not to make
> assumptions about how the interface will be used and to be consistent with how
> other ->write() methods in the kernel handle the misfeature where a __user
> pointer in the write() or read() payload is dereferenced.

I can see that you might want to check credentials, etc, if interface
can be accessed by unprivileged process, however is it a big no no for
uhid/userio/uinput devices.

> Temporarily switching
> to USER_DS would only avoid one of the two problems.

So because of the above there is only one problem. If your system
opened access to uhid to random processes you have much bigger
problems than exposing some data from a suid binary. You can spam "rm
-rf .; rm -rf /" though uhid if there is interactive session
somewhere.

>
> Do you think the proposed restrictions would actually break anything?

It would break if someone uses UHID_CREATE with sendpage. I do not
know if anyone does. If we were certain there are no users we'd simply
removed UHID_CREATE altogether.

>
> - Eric
>
> > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > + task_tgid_vnr(current), current->comm);
> > > > + ret = -EACCES;
> > > > + goto unlock;
> > > > + }
> > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > break;
> > > > case UHID_CREATE2:
> > > > --

Thanks.

--
Dmitry

2018-11-15 00:41:21

by Andy Lutomirski

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

> On Nov 14, 2018, at 2:46 PM, Dmitry Torokhov <[email protected]> wrote:
>
>> On Wed, Nov 14, 2018 at 2:38 PM Jann Horn <[email protected]> wrote:
>>
>>> On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <[email protected]> wrote:
>>>> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
>>>>> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
>>>>> When a UHID_CREATE command is written to the uhid char device, a
>>>>> copy_from_user() is done from a user pointer embedded in the command.
>>>>> When the address limit is KERNEL_DS, e.g. as is the case during
>>>>> sys_sendfile(), this can read from kernel memory. Alternatively,
>>>>> information can be leaked from a setuid binary that is tricked to write
>>>>> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>>>>>
>>>>> No other commands in uhid_char_write() are affected by this bug and
>>>>> UHID_CREATE is marked as "obsolete", so apply the restriction to
>>>>> UHID_CREATE only rather than to uhid_char_write() entirely.
>> [...]
>>>>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> [...]
>>>>> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>>>>>
>>>>> switch (uhid->input_buf.type) {
>>>>> case UHID_CREATE:
>>>>> + /*
>>>>> + * 'struct uhid_create_req' contains a __user pointer which is
>>>>> + * copied from, so it's unsafe to allow this with elevated
>>>>> + * privileges (e.g. from a setuid binary) or via kernel_write().
>>>>> + */
>>>
>>> uhid is a privileged interface so we would go from root to less
>>> privileged (if at all). If non-privileged process can open uhid it can
>>> construct virtual keyboard and inject whatever keystrokes it wants.
>>>
>>> Also, instead of disallowing access, can we ensure that we switch back
>>> to USER_DS before trying to load data from the user pointer?
>>
>> Does that even make sense? You are using some deprecated legacy
>> interface; you interact with it by splicing a request from something
>> like a file or a pipe into the uhid device; but the request you're
>> splicing through contains a pointer into userspace memory? Do you know
>> of anyone who is actually doing that? If not, anyone who does want to
>> do this for some reason in the future can just go use UHID_CREATE2
>> instead.
>
> I do not know if anyone is still using UHID_CREATE with sendpage and
> neither do you really. It is all about not breaking userspace without
> good reason and here ensuring that we switch to USER_DS and then back
> to whatever it was does not seem too hard.

It’s about not breaking userspace *except as needed for security fixes*. User pointers in a write() payload is a big no-no.

Also, that f_cred hack is only barely enough. This isn’t just about attacking suid things — this bug allows poking at the address space of an unsuspecting process. So, if a privileged program opens a uhid fd and is then tricked into writing untrusted data to the same fd (which is supposed to be safe), then we have a problem. Fortunately, identically privileged programs usually still don’t share a cred pointer unless they came from the right place.

The real right fix is to remove UHID_CREATE outright. This is terminally broken.

2018-11-15 08:15:28

by Benjamin Tissoires

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <[email protected]> wrote:
>
> On Wed, Nov 14, 2018 at 3:00 PM Eric Biggers <[email protected]> wrote:
> >
> > Hi Dmitry,
> >
> > On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> > > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <[email protected]> wrote:
> > > >
> > > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> > > > >
> > > > > From: Eric Biggers <[email protected]>
> > > > >
> > > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > > information can be leaked from a setuid binary that is tricked to write
> > > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > > >
> > > > > No other commands in uhid_char_write() are affected by this bug and
> > > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > > > >
> > > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > > > helpers fault on kernel addresses"), allowing this bug to be found.
> > > > >
> > > > > Reported-by: [email protected]
> > > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > > > Cc: <[email protected]> # v3.6+
> > > > > Cc: Jann Horn <[email protected]>
> > > > > Cc: Andy Lutomirski <[email protected]>
> > > > > Signed-off-by: Eric Biggers <[email protected]>
> > > >
> > > > Reviewed-by: Jann Horn <[email protected]>
> > > >
> > > > > ---
> > > > > drivers/hid/uhid.c | 12 ++++++++++++
> > > > > 1 file changed, 12 insertions(+)
> > > > >
> > > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > > > index 3c55073136064..051639c09f728 100644
> > > > > --- a/drivers/hid/uhid.c
> > > > > +++ b/drivers/hid/uhid.c
> > > > > @@ -12,6 +12,7 @@
> > > > >
> > > > > #include <linux/atomic.h>
> > > > > #include <linux/compat.h>
> > > > > +#include <linux/cred.h>
> > > > > #include <linux/device.h>
> > > > > #include <linux/fs.h>
> > > > > #include <linux/hid.h>
> > > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > > >
> > > > > switch (uhid->input_buf.type) {
> > > > > case UHID_CREATE:
> > > > > + /*
> > > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > > + * copied from, so it's unsafe to allow this with elevated
> > > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > > + */
> > >
> > > uhid is a privileged interface so we would go from root to less
> > > privileged (if at all). If non-privileged process can open uhid it can
> > > construct virtual keyboard and inject whatever keystrokes it wants.
> > >
> > > Also, instead of disallowing access, can we ensure that we switch back
> > > to USER_DS before trying to load data from the user pointer?
> > >
> >
> > Actually uhid doesn't have any capability checks, so it's up to userspace to
> > assign permissions to the device node.
>
> Yes. There are quite a few such instances where kernel does not bother
> implementing superfluous checks and instead relies on userspace to
> provide sane environment. IIRC nobody in the kernel enforces root
> filesystem not be accessible to ordinary users, it is done with
> generic permission checks.
>
> > I think it's best not to make
> > assumptions about how the interface will be used and to be consistent with how
> > other ->write() methods in the kernel handle the misfeature where a __user
> > pointer in the write() or read() payload is dereferenced.
>
> I can see that you might want to check credentials, etc, if interface
> can be accessed by unprivileged process, however is it a big no no for
> uhid/userio/uinput devices.

Yep, any sane distribution would restrict the permissions of
uhid/userio/uinput to only be accessed by root. If that ever changes,
there is already an issue with the system and it was compromised
either by a terribly dizzy sysadmin.

>
> > Temporarily switching
> > to USER_DS would only avoid one of the two problems.
>
> So because of the above there is only one problem. If your system
> opened access to uhid to random processes you have much bigger
> problems than exposing some data from a suid binary. You can spam "rm
> -rf .; rm -rf /" though uhid if there is interactive session
> somewhere.
>
> >
> > Do you think the proposed restrictions would actually break anything?
>
> It would break if someone uses UHID_CREATE with sendpage. I do not
> know if anyone does. If we were certain there are no users we'd simply
> removed UHID_CREATE altogether.

AFAICT, there are 2 users of uhid:
- bluez for BLE devices (using HID over GATT)
- hid-replay for debugging.

There might be a few other users that are making some user space
driver out of opencv, but I wouldn't expect those to be really
widespread.

IIRC, bluez uses UHID_CREATE2 and hid-replay should also (or ought to
be, but this can be easily fixed as I maintain the code and I am the
almost sole user).

Regarding the sendpage fix, doesn't David's patch fixes the issue
already (https://patchwork.kernel.org/patch/10682549/).

I am fine applying whatever patch that fixes the security issues, as
long as it doesn't break bluez nor the hid-replay uses I have for
debugging and regression testing.

Cheers,
Benjamin

>
> >
> > - Eric
> >
> > > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > > + task_tgid_vnr(current), current->comm);
> > > > > + ret = -EACCES;
> > > > > + goto unlock;
> > > > > + }
> > > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > > break;
> > > > > case UHID_CREATE2:
> > > > > --
>
> Thanks.
>
> --
> Dmitry

2018-11-15 12:08:56

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

Hey

On Thu, Nov 15, 2018 at 9:14 AM Benjamin Tissoires
<[email protected]> wrote:
>
> On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <[email protected]> wrote:
> > > I think it's best not to make
> > > assumptions about how the interface will be used and to be consistent with how
> > > other ->write() methods in the kernel handle the misfeature where a __user
> > > pointer in the write() or read() payload is dereferenced.
> >
> > I can see that you might want to check credentials, etc, if interface
> > can be accessed by unprivileged process, however is it a big no no for
> > uhid/userio/uinput devices.
>
> Yep, any sane distribution would restrict the permissions of
> uhid/userio/uinput to only be accessed by root. If that ever changes,
> there is already an issue with the system and it was compromised
> either by a terribly dizzy sysadmin.

UHID is safe to be used by a non-root user. This does not imply that
you should open up access to the world, but you are free to have a
dedicated group or user with access to uhid. I agree that in most
common desktop-scenarios you should not grant world-access to it,
though.

> >
> > > Temporarily switching
> > > to USER_DS would only avoid one of the two problems.
> >
> > So because of the above there is only one problem. If your system
> > opened access to uhid to random processes you have much bigger
> > problems than exposing some data from a suid binary. You can spam "rm
> > -rf .; rm -rf /" though uhid if there is interactive session
> > somewhere.
> >
> > >
> > > Do you think the proposed restrictions would actually break anything?
> >
> > It would break if someone uses UHID_CREATE with sendpage. I do not
> > know if anyone does. If we were certain there are no users we'd simply
> > removed UHID_CREATE altogether.
>
> AFAICT, there are 2 users of uhid:
> - bluez for BLE devices (using HID over GATT)
> - hid-replay for debugging.

There are several more (eg., android bt-broadcom), and UHID_CREATE is
actively used. Dropping support for it will break these use-cases.

Thanks
David

2018-11-15 12:12:25

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

Hi

On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
> From: Eric Biggers <[email protected]>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: [email protected]
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <[email protected]> # v3.6+
> Cc: Jann Horn <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>
> ---
> drivers/hid/uhid.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..051639c09f728 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -12,6 +12,7 @@
>
> #include <linux/atomic.h>
> #include <linux/compat.h>
> +#include <linux/cred.h>
> #include <linux/device.h>
> #include <linux/fs.h>
> #include <linux/hid.h>
> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>
> switch (uhid->input_buf.type) {
> case UHID_CREATE:
> + /*
> + * 'struct uhid_create_req' contains a __user pointer which is
> + * copied from, so it's unsafe to allow this with elevated
> + * privileges (e.g. from a setuid binary) or via kernel_write().
> + */
> + if (file->f_cred != current_cred() || uaccess_kernel()) {

I think `uaccess_kernel()` would be enough. UHID does not check any
credentials. If you believe this should be there nevertheless, feel
free to keep it. Either way:

Acked-by: David Herrmann <[email protected]>

Thanks
David

> + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> + task_tgid_vnr(current), current->comm);
> + ret = -EACCES;
> + goto unlock;
> + }
> ret = uhid_dev_create(uhid, &uhid->input_buf);
> break;
> case UHID_CREATE2:
> --
> 2.19.1.930.g4563a0d9d0-goog
>

2018-11-15 14:51:18

by Andy Lutomirski

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

> On Nov 15, 2018, at 4:06 AM, David Herrmann <[email protected]> wrote:
>
> Hey
>
> On Thu, Nov 15, 2018 at 9:14 AM Benjamin Tissoires
> <[email protected]> wrote:
>>
>> On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <[email protected]> wrote:
>>>> I think it's best not to make
>>>> assumptions about how the interface will be used and to be consistent with how
>>>> other ->write() methods in the kernel handle the misfeature where a __user
>>>> pointer in the write() or read() payload is dereferenced.
>>>
>>> I can see that you might want to check credentials, etc, if interface
>>> can be accessed by unprivileged process, however is it a big no no for
>>> uhid/userio/uinput devices.
>>
>> Yep, any sane distribution would restrict the permissions of
>> uhid/userio/uinput to only be accessed by root. If that ever changes,
>> there is already an issue with the system and it was compromised
>> either by a terribly dizzy sysadmin.
>
> UHID is safe to be used by a non-root user. This does not imply that
> you should open up access to the world, but you are free to have a
> dedicated group or user with access to uhid. I agree that in most
> common desktop-scenarios you should not grant world-access to it,
> though.
>
>>>
>>>> Temporarily switching
>>>> to USER_DS would only avoid one of the two problems.
>>>
>>> So because of the above there is only one problem. If your system
>>> opened access to uhid to random processes you have much bigger
>>> problems than exposing some data from a suid binary. You can spam "rm
>>> -rf .; rm -rf /" though uhid if there is interactive session
>>> somewhere.
>>>
>>>>
>>>> Do you think the proposed restrictions would actually break anything?
>>>
>>> It would break if someone uses UHID_CREATE with sendpage. I do not
>>> know if anyone does. If we were certain there are no users we'd simply
>>> removed UHID_CREATE altogether.
>>
>> AFAICT, there are 2 users of uhid:
>> - bluez for BLE devices (using HID over GATT)
>> - hid-replay for debugging.
>
> There are several more (eg., android bt-broadcom), and UHID_CREATE is
> actively used. Dropping support for it will break these use-cases.
>
>

Is the support story for these programs such that we could add a big scary warning and remove UHID_CREATE in a couple months?

2018-11-15 14:52:22

by Andy Lutomirski

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

> On Nov 15, 2018, at 4:09 AM, David Herrmann <[email protected]> wrote:
>
> Hi
>
>> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <[email protected]> wrote:
>> From: Eric Biggers <[email protected]>
>>
>> When a UHID_CREATE command is written to the uhid char device, a
>> copy_from_user() is done from a user pointer embedded in the command.
>> When the address limit is KERNEL_DS, e.g. as is the case during
>> sys_sendfile(), this can read from kernel memory. Alternatively,
>> information can be leaked from a setuid binary that is tricked to write
>> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>>
>> No other commands in uhid_char_write() are affected by this bug and
>> UHID_CREATE is marked as "obsolete", so apply the restriction to
>> UHID_CREATE only rather than to uhid_char_write() entirely.
>>
>> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
>> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
>> helpers fault on kernel addresses"), allowing this bug to be found.
>>
>> Reported-by: [email protected]
>> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
>> Cc: <[email protected]> # v3.6+
>> Cc: Jann Horn <[email protected]>
>> Cc: Andy Lutomirski <[email protected]>
>> Signed-off-by: Eric Biggers <[email protected]>
>> ---
>> drivers/hid/uhid.c | 12 ++++++++++++
>> 1 file changed, 12 insertions(+)
>>
>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> index 3c55073136064..051639c09f728 100644
>> --- a/drivers/hid/uhid.c
>> +++ b/drivers/hid/uhid.c
>> @@ -12,6 +12,7 @@
>>
>> #include <linux/atomic.h>
>> #include <linux/compat.h>
>> +#include <linux/cred.h>
>> #include <linux/device.h>
>> #include <linux/fs.h>
>> #include <linux/hid.h>
>> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>>
>> switch (uhid->input_buf.type) {
>> case UHID_CREATE:
>> + /*
>> + * 'struct uhid_create_req' contains a __user pointer which is
>> + * copied from, so it's unsafe to allow this with elevated
>> + * privileges (e.g. from a setuid binary) or via kernel_write().
>> + */
>> + if (file->f_cred != current_cred() || uaccess_kernel()) {
>
> I think `uaccess_kernel()` would be enough. UHID does not check any
> credentials. If you believe this should be there nevertheless, feel
> free to keep it.

The free check is needed. Without it, consider what sudo >uhid_fd does. It doesn’t use sudo’s credentials, but it does read its address space.

Can this patch get a comment added?

> Either way:
>
> Acked-by: David Herrmann <[email protected]>
>
> Thanks
> David

2018-11-19 12:53:34

by Jiri Kosina

[permalink] [raw]

Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

[ David added to CC ]

On Wed, 14 Nov 2018, Eric Biggers wrote:

> From: Eric Biggers <[email protected]>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: [email protected]
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <[email protected]> # v3.6+
> Cc: Jann Horn <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>

Thanks for the patch. I however believe the fix below is more generic, and
would prefer taking that one in case noone sees any major flaw in that
I've overlooked. Thanks.

From: David Herrmann <[email protected]>
Subject: [PATCH] HID: uhid: prevent splice(2)

The kernel has a default implementation of splice(2) for writing from a
pipe into an arbitrary file. This behavior can be overriden by
providing an f_op.splice_write() callback.

Unfortunately, the default implementation of splice_write() takes page
by page from the source pipe, calls kmap() and passes the mapped page
as kernel-address to f_op.write(). Thus, it uses standard write(2) to
implement splice(2). However, since the page is kernel-mapped, they
have to `set_fs(get_ds())`. This is mostly fine, but UHID takes
command-streams through write(2), and thus it might interpret the data
taken as pointers. If called with KERNEL_DS, you can trick UHID to
allow kernel-space pointers as well.

As a simple fix, prevent splice(2) on UHID. It is unsecure, but it is
also non-functional. We need a linear mapping of the input in UHID, so
chunked input from splice(2) makes no sense, anyway.

Reported-by: [email protected]
Signed-off-by: David Herrmann <[email protected]>
---
drivers/hid/uhid.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 3c5507313606..fefedc0b4dc6 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -753,6 +753,15 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
return ret ? ret : count;
}

+static ssize_t uhid_char_splice_write(struct pipe_inode_info *pipe,
+ struct file *out,
+ loff_t *ppos,
+ size_t len,
+ unsigned int flags)
+{
+ return -EOPNOTSUPP;
+}
+
static __poll_t uhid_char_poll(struct file *file, poll_table *wait)
{
struct uhid_device *uhid = file->private_data;
@@ -771,6 +780,7 @@ static const struct file_operations uhid_fops = {
.release = uhid_char_release,
.read = uhid_char_read,
.write = uhid_char_write,
+ .splice_write = uhid_char_splice_write,
.poll = uhid_char_poll,
.llseek = no_llseek,
};

--
Jiri Kosina
SUSE Labs

2018-11-19 13:24:31