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

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Message-ID: <8242bd71-4c84-73d4-81a1-01471024c338@google.com>
Date:   Tue, 27 Feb 2018 10:16:00 -0800
From:   Greg Hackmann <ghackmann@...gle.com>
To:     Joel Fernandes <joelaf@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Todd Kjos <tkjos@...gle.com>,
        Arve Hjonnevag <arve@...roid.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] staging: android: ashmem: Fix lockdep issue during
 llseek

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@...uxfoundation.org> 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@...gle.com>
>>> Cc: Arve Hjonnevag <arve@...roid.com>
>>> Cc: Greg Hackmann <ghackmann@...gle.com>
>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> Cc: stable@...r.kernel.org
>>> Reported-by: syzbot+8ec30bb7bf1a981a2012@...kaller.appspotmail.com
>>> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
>>> ---
>>> 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@...gle.com>