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

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Date:   Thu, 22 Feb 2018 14:48:32 +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 v2] staging: android: ashmem: Fix lockdep issue during
 llseek

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.

Joel, did the original reporter say this patch solved their issue or
not?  For some reason I didn't think it worked properly...

thanks,

greg k-h