linux-kernel - Re: Found it! (was Re: [3.10] Oopses in kmem_cache_allocate() via prepare

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Date:	Mon, 2 Dec 2013 17:06:21 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Simon Kirby <sim@...tway.ca>, Ian Applegate <ia@...udflare.com>,
	Christoph Lameter <cl@...two.org>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...ionio.com>
Subject: Re: Found it! (was Re: [3.10] Oopses in kmem_cache_allocate() via
 prepare_creds())

On Mon, Dec 02, 2013 at 08:00:38AM -0800, Linus Torvalds wrote:
> On Sat, Nov 30, 2013 at 1:08 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I still don't see what could be wrong with the pipe_inode_info thing,
> > but the fact that it's been so consistent in your traces does make me
> > suspect it really is *that* particular slab.
> 
> I think I finally found it.
> 
> I've spent waaayy too much time looking at and thinking about that
> code without seeing anything wrong, but this morning I woke up and
> thought to myself "What if.."
> 
> And looking at the code again, I went "BINGO".
> 
> All our reference counting etc seems right, but we have one very
> subtle bug: on the freeing path, we have a pattern like this:
> 
>         spin_lock(&inode->i_lock);
>         if (!--pipe->files) {
>                 inode->i_pipe = NULL;
>                 kill = 1;
>         }
>         spin_unlock(&inode->i_lock);
>         __pipe_unlock(pipe);
>         if (kill)
>                 free_pipe_info(pipe);
> 
> which on the face of it is trying to be very careful in not accessing
> the pipe-info after it is released by having that "kill" flag, and
> doing the release last.
> 
> And it's complete garbage.
> 
> Why?
> 
> Because the thread that decrements "pipe->files" *without* releasing
> it, will very much access it after it has been dropped: that
> "__pipe_unlock(pipe)" happens *after* we've decremented the pipe
> reference count and dropped the inode lock. So another CPU can come in
> and free the structure concurrently with that __pipe_unlock(pipe).
> 
> This happens in two places, and we don't actually need or want the
> pipe lock for the pipe->files accesses (since pipe->files is protected
> by inode->i_lock, not the pipe lock), so the solution is to just do
> the __pipe_unlock() before the whole dance about the pipe->files
> reference count.
> 
> Patch appended. And no wonder nobody has ever seen it, because the
> race is unlikely as hell to ever happen. Simon, I assume it will be
> another few months before we can say "yeah, that fixed it", but I
> really think this is it. It explains all the symptoms, including
> "DEBUG_PAGEALLOC didn't catch it" (because the access happens just as
> it is released, and DEBUG_PAGEALLOC takes too long to actually free
> unmap the page etc).

Nice catch.  I'd probably add

/* we are holding inode->i_pipe->files */
static void drop_pipe(struct inode *inode, struct pipe_inode_info *pipe)
{
	int kill = 0;
        spin_lock(&inode->i_lock);
	WARN_ON(pipe != inode->i_pipe);
        if (!--pipe->files) {
                inode->i_pipe = NULL;
                kill = 1;
        }
        spin_unlock(&inode->i_lock);
        if (kill)
                free_pipe_info(pipe);
}

and use it in both places...
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