linux-kernel - Re: fs: gpf in simple

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Message-ID: <531A7CFD.9030603@oracle.com>
Date:	Fri, 07 Mar 2014 21:14:21 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Jan Kara <jack@...e.cz>
CC:	Al Viro <viro@...IV.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: fs: gpf in simple_setattr

On 03/06/2014 11:02 AM, Sasha Levin wrote:
> On 03/05/2014 07:45 AM, Jan Kara wrote:
>> On Tue 04-03-14 19:00:32, Sasha Levin wrote:
>>> On 03/03/2014 04:40 PM, Jan Kara wrote:
>>>> On Sat 01-03-14 15:05:21, Sasha Levin wrote:
>>>>>> ping again?
>>>>>>
>>>>>> I've been working on it, but don't see an obvious issue.
>>>>>>
>>>>>> It does look like an access to invalid memory easily doable from
>>>>>> userspace, so it should probably get fixed soon...
>>>>    Hum, can you maybe dump the name in dentry passed to simple_setattr()? Or
>>>> maybe even the whole path using dentry_path() (but not sure if that will
>>>> be workable on half-torn-down fs)? Maybe it will give us a hint at which
>>>> filesystem to look...
>>>
>>> It's just garbage, this is why I'm having a hard time making any progress with
>>> this bug.
>>    OK, but that is strange because we hold a reference to the dentry so
>> noone should free it. So dentry->d_name should be valid... Is the rest of
>> the dentry also garbage? E.g. does dentry->d_inode still point to the inode
>> we call __mark_inode_dirty() on? Is dentry->d_sb == dentry->d_inode->i_sb?
>> Also if the inode isn't completely garbage, we can maybe infer something
>> from inode->i_op - that should point to some statically allocated
>> operations struct so we should be able to guess fs type from that.
>
> It's actually pretty tricky. This issue being a race makes catching it at the right time
> difficult.
>
> I've tried catching it in simple_setattr() before calling mark_inode_dirty() by testing
> for the poison values inside inode, but they seem to be perfectly fine there and still
> show up as bad within mark_inode_dirty().
>
> Then I tried trapping it inside mark_inode_dirty(), but at that point I usually get garbage
> inside inode, and have no way to go back to dentry.
>
> Right now I'm just trying to dump everything that goes through simple_setattr() in hopes that
> I could easily figure out what went wrong by looking at the log, but that just stops the bug
> from reproducing.

I've tried the following code in simple_setattr() right before the call to mark_inode_dirty():

         p = dentry_path(dentry, pth, 200);
         printk(KERN_ERR "doh: %s %s\n", p, inode->i_sb->s_type->name);

but it seems that while 'p' ends up being "/", inode->i_sb is garbage and we can't pull out anything
about the file system.

Any way I could get anything useful any other way?


Thanks,
Sasha

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