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

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Message-ID: <20160229161954.GX17997@ZenIV.linux.org.uk>
Date:	Mon, 29 Feb 2016 16:19:54 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dmitry Vyukov <dvyukov@...gle.com>
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 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.