[Cosmic,1/1] fsnotify: Fix fsnotify_mark_connector race

From: Robert Kolchmeyer <rkolchmeyer@google.com>

BugLink: http://bugs.launchpad.net/bugs/1771075

fsnotify() acquires a reference to a fsnotify_mark_connector through
the SRCU-protected pointer to_tell->i_fsnotify_marks. However, it
appears that no precautions are taken in fsnotify_put_mark() to
ensure that fsnotify() drops its reference to this
fsnotify_mark_connector before assigning a value to its 'destroy_next'
field. This can result in fsnotify_put_mark() assigning a value
to a connector's 'destroy_next' field right before fsnotify() tries to
traverse the linked list referenced by the connector's 'list' field.
Since these two fields are members of the same union, this behavior
results in a kernel panic.

This issue is resolved by moving the connector's 'destroy_next' field
into the object pointer union. This should work since the object pointer
access is protected by both a spinlock and the value of the 'flags'
field, and the 'flags' field is cleared while holding the spinlock in
fsnotify_put_mark() before 'destroy_next' is updated. It shouldn't be
possible for another thread to accidentally read from the object pointer
after the 'destroy_next' field is updated.

The offending behavior here is extremely unlikely; since
fsnotify_put_mark() removes references to a connector (specifically,
it ensures that the connector is unreachable from the inode it was
formerly attached to) before updating its 'destroy_next' field, a
sizeable chunk of code in fsnotify_put_mark() has to execute in the
short window between when fsnotify() acquires the connector reference
and saves the value of its 'list' field. On the HEAD kernel, I've only
been able to reproduce this by inserting a udelay(1) in fsnotify().
However, I've been able to reproduce this issue without inserting a
udelay(1) anywhere on older unmodified release kernels, so I believe
it's worth fixing at HEAD.

References: https://bugzilla.kernel.org/show_bug.cgi?id=199437
Fixes: 08991e83b7286635167bab40927665a90fb00d81
CC: stable@vger.kernel.org
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>
(cherry picked from commit d90a10e2444ba5a351fa695917258ff4c5709fa5)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
---
 include/linux/fsnotify_backend.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

I referred to this same issue and fix in
https://bugs.launchpad.net/ubuntu/+source/linux-azure/+bug/1765564

On Wed, May 23, 2018 at 11:27 AM, Joseph Salisbury
<joseph.salisbury@canonical.com> wrote:
> From: Robert Kolchmeyer <rkolchmeyer@google.com>
>
> BugLink: http://bugs.launchpad.net/bugs/1771075
>
> fsnotify() acquires a reference to a fsnotify_mark_connector through
> the SRCU-protected pointer to_tell->i_fsnotify_marks. However, it
> appears that no precautions are taken in fsnotify_put_mark() to
> ensure that fsnotify() drops its reference to this
> fsnotify_mark_connector before assigning a value to its 'destroy_next'
> field. This can result in fsnotify_put_mark() assigning a value
> to a connector's 'destroy_next' field right before fsnotify() tries to
> traverse the linked list referenced by the connector's 'list' field.
> Since these two fields are members of the same union, this behavior
> results in a kernel panic.
>
> This issue is resolved by moving the connector's 'destroy_next' field
> into the object pointer union. This should work since the object pointer
> access is protected by both a spinlock and the value of the 'flags'
> field, and the 'flags' field is cleared while holding the spinlock in
> fsnotify_put_mark() before 'destroy_next' is updated. It shouldn't be
> possible for another thread to accidentally read from the object pointer
> after the 'destroy_next' field is updated.
>
> The offending behavior here is extremely unlikely; since
> fsnotify_put_mark() removes references to a connector (specifically,
> it ensures that the connector is unreachable from the inode it was
> formerly attached to) before updating its 'destroy_next' field, a
> sizeable chunk of code in fsnotify_put_mark() has to execute in the
> short window between when fsnotify() acquires the connector reference
> and saves the value of its 'list' field. On the HEAD kernel, I've only
> been able to reproduce this by inserting a udelay(1) in fsnotify().
> However, I've been able to reproduce this issue without inserting a
> udelay(1) anywhere on older unmodified release kernels, so I believe
> it's worth fixing at HEAD.
>
> References: https://bugzilla.kernel.org/show_bug.cgi?id=199437
> Fixes: 08991e83b7286635167bab40927665a90fb00d81
> CC: stable@vger.kernel.org
> Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> (cherry picked from commit d90a10e2444ba5a351fa695917258ff4c5709fa5)
> Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
> ---
>  include/linux/fsnotify_backend.h | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> index c6c6931..3504ac3 100644
> --- a/include/linux/fsnotify_backend.h
> +++ b/include/linux/fsnotify_backend.h
> @@ -216,12 +216,10 @@ struct fsnotify_mark_connector {
>         union { /* Object pointer [lock] */
>                 struct inode *inode;
>                 struct vfsmount *mnt;
> -       };
> -       union {
> -               struct hlist_head list;
>                 /* Used listing heads to free after srcu period expires */
>                 struct fsnotify_mark_connector *destroy_next;
>         };
> +       struct hlist_head list;
>  };
>
>  /*
> --
> 2.7.4
>
>
> --
> kernel-team mailing list
> kernel-team@lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team