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

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Date:   Fri, 16 Feb 2018 09:37:59 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Todd Kjos <tkjos@...gle.com>,
        Arve Hjonnevag <arve@...roid.com>,
        Greg Hackmann <ghackmann@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] staging: ashmem: Fix lockdep issue during llseek

On Fri, Feb 16, 2018 at 8:32 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Mon, Feb 12, 2018 at 05:01:25PM -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>
>> ---
>>  drivers/staging/android/ashmem.c | 15 +++++++--------
>>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> Please always properly version your patches, and put what changed below
> the --- line, so I have a hint as to which patch to apply.
> Documentation/SubmittingPatches has the full details of how to do this.
>
> Can you resend me the "latest" version of this patch, so I have a chance
> of getting it right?  :)

Sorry about that :) Fixing now, and will resend. This version you're
replying to is the latest version which is the second version (v2).

- Joel