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

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Message-ID: <20180126031346.GW13338@ZenIV.linux.org.uk>
Date:   Fri, 26 Jan 2018 03:13:46 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Todd Kjos <tkjos@...gle.com>,
        Arve Hjonnevag <arve@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] ashmem: Fix lockdep issue during llseek

On Thu, Jan 25, 2018 at 06:46:49PM -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
> during a syzcaller test, this patch fixes the issue by releasing the
> ashmem_mutex before the call to vfs_llseek, and reacquiring it after.

That looks odd.  If this approach works, what the hell do you need
ashmem_mutex for in ashmem_llseek() in the first place?  What is
it protecting there?