Re: [PATCH] ashmem: Fix lockdep issue during llseek

[Al Viro]

On Thu, Jan 25, 2018 at 06:46:49PM -0800, Joel Fernandes wrote:
> ashmem_mutex create a chain of dependencies like so:
> 
> (1)
> mmap syscall ->
>   mmap_sem ->  (acquired)
>   ashmem_mmap
>   ashmem_mutex (try to acquire)
>   (block)
> 
> (2)
> llseek syscall ->
>   ashmem_llseek ->
>   ashmem_mutex ->  (acquired)
>   inode_lock ->
>   inode->i_rwsem (try to acquire)
>   (block)
> 
> (3)
> getdents ->
>   iterate_dir ->
>   inode_lock ->
>   inode->i_rwsem   (acquired)
>   copy_to_user ->
>   mmap_sem         (try to acquire)
> 
> There is a lock ordering created between mmap_sem and inode->i_rwsem
> during a syzcaller test, this patch fixes the issue by releasing the
> ashmem_mutex before the call to vfs_llseek, and reacquiring it after.

That looks odd.  If this approach works, what the hell do you need
ashmem_mutex for in ashmem_llseek() in the first place?  What is
it protecting there?