linux-kernel - Re: fs: NULL deref in atime_needs

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Date:	Tue, 1 Mar 2016 09:59:27 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Ian Kent <raven@...maw.net>,
	Mickaël Salaün <mic@...ikod.net>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: fs: NULL deref in atime_needs_update

On Mon, Feb 29, 2016 at 7:19 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Mon, Feb 29, 2016 at 5:19 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> On Mon, Feb 29, 2016 at 04:54:54PM +0100, Dmitry Vyukov wrote:
>>
>>> Regardless of whether reordering is wrong or not, do we see how it can
>>> fix the WARNINGs/oopses? Because it does seem to. I've tried to revert
>>> just this part:
>>>
>>> -               *inode = d_backing_inode(dentry);
>>>                 negative = d_is_negative(dentry);
>>> +               *inode = d_backing_inode(dentry);
>>>
>>> And got:
>>>
>>> [  976.609688] WARNING: CPU: 0 PID: 12126 at fs/namei.c:1587
>>> lookup_fast+0x3fa/0x450()
>>> [  976.626768] WARNING: CPU: 0 PID: 12126 at fs/namei.c:3123
>>> path_openat+0x12bc/0x1520()
>>>
>>> in 15 minutes.
>>
>> dentry going from negative to positive                  lookup_fast()
>>                                                         fetch NULL ->d_inode
>> store non-NULL ->d_inode
>> store new ->d_flags
>>                                                         fetch new ->d_flags
>>                                                         check ->d_seq
>> bump ->d_seq by 2
>>
>> Change the order of fetches and you'll get rid of that scenario.
>>
>>> In particular, applying this on top the previous patch will be
>>> inconclusive, because I already don't see the warnings.
>>
>> Apply it with that reordering reversed, please.
>
>
> OK, just wanted to make sure that we keep track of the situation.
> Restarted testing with combined patch. Here is it just in case:
> https://gist.githubusercontent.com/dvyukov/67fe363d5ce2e2b06c71/raw/4d1b6c23f8dff7e0f8e2e3cab7e50208fddb0570/gistfile1.txt

No warnings/crashes in 15 hours on 3 VMs!