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

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Message-ID: <56C86954.6030101@digikod.net>
Date:	Sat, 20 Feb 2016 14:25:40 +0100
From:	Mickaël Salaün <mic@...ikod.net>
To:	Al Viro <viro@...IV.linux.org.uk>,
	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	"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 20/02/2016 04:54, Al Viro wrote:
> 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.
> 

I can also trigger bugs with a bad inode pointer dereference in atime_needs_update: if (inode->i_flags & S_NOATIME)

I think the bug may be somewhere in the nd->depth handling (when its value is 0) in fs/namei.c:get_link(): struct saved *last = nd->stack + nd->depth - 1

Moreover, is it intentional that touch_atime() is called by get_link() even if the access (previously checked with security_file_open(), e.g. by do_last) is denied?


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