linux-kernel - Re: [PATCH] tun: don't zeroize sock->file on detach

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Date:	Tue, 21 Aug 2012 13:18:01 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	David Miller <davem@...emloft.net>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"rick.jones2@...com" <rick.jones2@...com>,
	"ycheng@...gle.com" <ycheng@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mikulas@...ax.karlin.mff.cuni.cz
Subject: Re: [PATCH] tun: don't zeroize sock->file on detach

On Tue, Aug 21, 2012 at 12:04 PM, Stanislav Kinsbursky
<skinsbursky@...allels.com> wrote:
> 10.08.2012 03:16, David Miller пишет:
>
>> From: Stanislav Kinsbursky <skinsbursky@...allels.com>
>> Date: Thu, 09 Aug 2012 16:50:40 +0400
>>
>>> This is a fix for bug, introduced in 3.4 kernel by commit
>>> 1ab5ecb90cb6a3df1476e052f76a6e8f6511cb3d, which, among other things,
>>> replaced
>>> simple sock_put() by sk_release_kernel(). Below is sequence, which leads
>>> to
>>> oops for non-persistent devices:
>>>
>>> tun_chr_close()
>>> tun_detach()                            <== tun->socket.file = NULL
>>> tun_free_netdev()
>>> sk_release_sock()
>>> sock_release(sock->file == NULL)
>>> iput(SOCK_INODE(sock))                  <== dereference on NULL pointer
>>>
>>> This patch just removes zeroing of socket's file from __tun_detach().
>>> sock_release() will do this.
>>>
>>> Cc: stable@...r.kernel.org
>>> Reported-by: Ruan Zhijie <ruanzhijie@...mail.com>
>>> Tested-by: Ruan Zhijie <ruanzhijie@...mail.com>
>>> Acked-by: Al Viro <viro@...IV.linux.org.uk>
>>> Acked-by: Eric Dumazet <edumazet@...gle.com>
>>> Acked-by: Yuchung Cheng <ycheng@...gle.com>
>>> Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>
>>
>>
>> Applied, thanks.
>>
>
> Hi, David.
> I found out, that this commit: b09e786bd1dd66418b69348cb110f3a64764626a
> was previous attempt to fix the problem.
> I believe this commit have to be dropped.

Have you tried testing with that commit reverted? AFAICT from reading
the code, if you revert b09e786bd1dd66418b69348cb110f3a64764626a then
the sockets_in_use count becomes incorrect, because sock_release()
will be calling this_cpu_sub() for each tun socket teardown when there
was no corresponding this_cpu_add() for the tun socket (because the
tun socket is not allocated with sock_alloc()).

Can you sketch in more detail why that commit should be dropped?

neal
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