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

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Message-ID: <20160220035442.GE17997@ZenIV.linux.org.uk>
Date:	Sat, 20 Feb 2016 03:54:43 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	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>
Subject: Re: fs: NULL deref in atime_needs_update

On Sat, Feb 20, 2016 at 03:21:27AM +0000, Al Viro wrote:
> On Fri, Feb 19, 2016 at 08:32:10PM +0100, Dmitry Vyukov wrote:
> > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
> 
> NULL inode->i_sb, by the look of the offset, but I really don't understand
> where the hell is that code doing (or how is that instruction going to
> generate dereferencing of 0x50, for that matter).

BTW, Mickaël's trace *does* make sense and it's definitely NULL inode->i_sb
(inode itself - in %rsi, inode->i_sb - in %rdx, offset of s_flags is 0x50,
the line in question is
        if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))

What I don't understand is what could possibly have NULL ->i_sb in *any*
instance of struct inode.