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Subject: Re: [PATCH] pipe: fix potential use-after-free in pipe_read()
To:     Al Viro <viro@zeniv.linux.org.uk>
CC:     Matthew Wilcox <willy@infradead.org>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>
References: <20221117115323.1718-1-thunder.leizhen@huawei.com>
 <Y4Bh0fTf4vZ6/6Pc@ZenIV>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <3a38686c-040a-202c-2778-7670a2b3f087@huawei.com>
Date:   Fri, 25 Nov 2022 15:52:53 +0800
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On 2022/11/25 14:33, Al Viro wrote:
> On Thu, Nov 17, 2022 at 07:53:23PM +0800, Zhen Lei wrote:
>> Accessing buf->flags after pipe_buf_release(pipe, buf) is unsafe, because
>> the 'buf' memory maybe freed.
> 
> Huh?  What are you talking about?
>                         struct pipe_buffer *buf = &pipe->bufs[tail & mask];
> To free *buf you would need to free the entire damn array, which is
> obviously not going to be possible here; if you are talking about reuse
> of *buf - that's controlled by pipe->tail, and we do not assign it until
> later.
> 
> Fetching any fields of *buf is safe; what can get freed is buf->page, not
> buf itself.  So that buf->flags access is fine.

Right. Thank you for explaining clearly. Sorry, I misunderstood in the
course of learning.

> 
> .
> 

-- 
Regards,
  Zhen Lei