Re: [3.10] Oopses in kmem_cache_allocate() via prepare_creds()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Aug 19, 2013 at 02:16:36PM -0700, Linus Torvalds wrote:
> On Mon, Aug 19, 2013 at 1:29 PM, Christoph Lameter <cl@gentwo.org> wrote:
> > On Mon, 19 Aug 2013, Simon Kirby wrote:
> >
> >>    [... ]  The
> >> alloc/free traces are always the same -- always alloc_pipe_info and
> >> free_pipe_info. This is seen on 3.10 and (now) 3.11-rc4:
> >>
> >> Object ffff880090f19e78: 6b 6b 6b 6b 6c 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkklkkkkkkkkkkk
> >
> > This looks like an increment after free in the second 32 bit value of the
> > structure. First 32 bit value's poison is unchanged.
> 
> Ugh. If that is "struct pipe_inode_info" and I read it right, that's
> the "wait_lock" spinlock that is part of the mutex.
> 
> Doing a "spin_lock()" could indeed cause an increment operation. But
> it still sounds like a very odd case. And even for some wild pointer
> I'd then expect the spin_unlock to also happen, and to then increment
> the next byte (or word) too. More importantly, for a mutex, I'd expect
> the *other* fields to be corrupted too (the "waiter" field etc). That
> is, unless we're still spinning waiting for the mutex, but with that
> value we shouldn't, as far as I can see.

Point...  I would probably start with CONFIG_DEBUG_MUTEXES and see if
it screams about mutex magic, etc. having been buggered.  FWIW, pipe is
neither a FIFO nor an internal per-task one - it's a usual pipe(2) one,
which should've excluded most of the weird codepaths...