linux-kernel - Re: [3.10] Oopses in kmem_cache_allocate() via prepare

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Date:	Sat, 30 Nov 2013 13:04:25 -0800
From:	Simon Kirby <sim@...tway.ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ian Applegate <ia@...udflare.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Lameter <cl@...two.org>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...ionio.com>
Subject: Re: [3.10] Oopses in kmem_cache_allocate() via prepare_creds()

On Sat, Nov 30, 2013 at 09:25:33AM -0800, Linus Torvalds wrote:

> On Sat, Nov 30, 2013 at 1:43 AM, Simon Kirby <sim@...tway.ca> wrote:
>
> > I turned on kmalloc-192 tracing to find what else is using it: struct
> > nfs_fh, struct bio, and struct cred. Poking around those, struct bio has
> > bi_cnt, but it is way down in the struct. struct cred has "usage", but it
> > comes first. Hmm. Nevertheless, I set:
> >
> > CONFIG_DEBUG_MUTEXES=y
> > CONFIG_DEBUG_LIST=y
> > CONFIG_DEBUG_CREDENTIALS=y
> >
> > And tried:
> >
> > diff --git a/include/linux/cred.h b/include/linux/cred.h
> > index 04421e8..2646fe9 100644
> > --- a/include/linux/cred.h
> > +++ b/include/linux/cred.h
> > @@ -205,7 +205,9 @@ static inline void validate_process_creds(void)
> >   */
> >  static inline struct cred *get_new_cred(struct cred *cred)
> >  {
> > -       atomic_inc(&cred->usage);
> > +       if (atomic_inc_return(&cred->usage) == 0x6c) {
> > +               WARN_ON(cred->uid == 0x6b);
> 
> Oh, damn, I thought you had found it, and got very excited and already
> wrote a long email about things I wanted you to try. And then I
> started looking closer...
> 
> That test is wrong. Both of those fields are 32-bit, so testing them
> against 0x6b/0x6c is bogus: you're just catching real cases. The
> reason it catches omreport is presumably because omreport runs as some
> special user that happens to have uid 107 (on my machine that happens
> to be qemu). And having a usage count of 108 isn't particularly
> strange either - creds get a lot of re-use.
> 
> So close. It *might* still be one of those cases, but it doesn't
> really sound very likely. "bi_cnt" is deep inside the struct bio, and
> "usage" is at offset 0, not offset 4. And the ns_fh isn't very
> interesting.

*head smack* Too much 8-bit AVR coding...

Makes sense now: uid=107(nagios) gid=109(nagios) groups=109(nagios)

Well, the chances of atomic_inc intentionally incrementing 0x6b6b6b6b are
probably pretty rare. I'll try that.

Simon-
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