Re: [PATCH v2] staging: android: ashmem: Fix lockdep issue during llseek

[Greg Hackmann]

On 02/22/2018 01:02 PM, Joel Fernandes wrote:
> (reposting in plain text, sorry for the previous HTML email, I should
> have not posted from the Phone)
> 
> On Thu, Feb 22, 2018 at 5:48 AM, Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> On Fri, Feb 16, 2018 at 11:02:01AM -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
>>> causing a lockdep splat [2] during a syzcaller test, this patch fixes
>>> the issue by unlocking the mutex earlier. Functionally that's Ok since
>>> we don't need to protect vfs_llseek.
>>>
>>> [1] https://patchwork.kernel.org/patch/10185031/
>>> [2] https://lkml.org/lkml/2018/1/10/48
>>>
>>> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
>>> Cc: Arve Hjonnevag <arve@xxxxxxxxxxx>
>>> Cc: Greg Hackmann <ghackmann@xxxxxxxxxx>
>>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Reported-by: syzbot+8ec30bb7bf1a981a2012@xxxxxxxxxxxxxxxxxxxxxxxxx
>>> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
>>> ---
>>> Changes since first version:
>>> Don't relock after vfs call since its not needed. Only reason we lock is
>>> to protect races with asma->file.
>>> https://patchwork.kernel.org/patch/10185031/
>>
>> I'd like some acks from others before I take this patch.
> 
> GregH, Todd, could you provide Acks?

Acked-by: Greg Hackmann <ghackmann@xxxxxxxxxx>