[PATCH] staging: android: ashmem: Fix possible deadlock in ashmem_ioctl

[Joel Fernandes (Google)]

On Tue, Feb 27, 2018 at 10:59 PM, Yisheng Xie <xieyisheng1 at huawei.com> wrote:
> ashmem_mutex may create a chain of dependencies like:
>
> CPU0                                    CPU1
>  mmap syscall                           ioctl syscall
>  -> mmap_sem (acquired)                 -> ashmem_ioctl
>  -> ashmem_mmap                            -> ashmem_mutex (acquired)
>     -> ashmem_mutex (try to acquire)       -> copy_from_user
>                                               -> mmap_sem (try to acquire)
>
> There is a lock odering problem between mmap_sem and ashmem_mutex causing
> a lockdep splat[1] during a syzcaller test. This patch fixes the problem
> by move copy_from_user out of ashmem_mutex.
>
> [1] https://www.spinics.net/lists/kernel/msg2733200.html
>
> Fixes: ce8a3a9e76d0 (staging: android: ashmem: Fix a race condition in pin ioctls)
> Reported-by: syzbot+d7a918a7a8e1c952bc36 at syzkaller.appspotmail.com
> Signed-off-by: Yisheng Xie <xieyisheng1 at huawei.com>

Greg,
Could you take this patch for the stable trees? I do see it in staging
already. I couldn't find it in stable so wanted to bring it to your
attention. If you already aware of it, please ignore my note.

Thanks,
- Joel