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

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Message-ID: <20180216163257.GA7752@kroah.com>
Date:   Fri, 16 Feb 2018 17:32:57 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     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 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?  :)

thanks,

greg k-h