Re: fanotify use after free.

[Dave Jones <davej@redhat.com>]

On Tue, Jan 28, 2014 at 12:40:17AM +0100, Jan Kara wrote:
 > On Fri 24-01-14 08:26:45, Jiri Kosina wrote:
 > > On Fri, 24 Jan 2014, Jan Kara wrote:
 > > 
 > > >   Strange. I've installed systemd system (openSUSE 13.1) and it boots 
 > > > with the latest Linus' kernel just fine (and I have at least FANOTIFY 
 > > > and SLAB debugging set the same way as you). But it was only a KVM 
 > > > guest. I'll try tomorrow with a physical machine I guess.
 > > 
 > > FWIW the system I am reliably able to reproduce this on is opensuse 12.3 
 > > with this systemd version: 
 > > 
 > > Version     : 195
 > > Release     : 13.18.1
 >   Hum, still no luck with reproduction (either on physical machine or with
 > KVM). Anyway, I've looked at the code again and the previous patch had a
 > stupid bug (passing different pointer to fsnotify_destroy_event() than we
 > should have), plus also the merging function in fanotify was too
 > aggressive. Can you try the attached patch? It boots for me but that means
 > nothing since I cannot reproduce the issue... Thanks!

still not good I'm afraid. I still see corruption very early on in boot
and now it panics and locks up too.

Again, this happens so early that I can't grab it over usb-serial.
I stuck an mdelay(10000) in the slub corruption detector, and managed
to grab a photo of the first trace.

Trace:
? preempt_schedule
lock_acquire
? lockref_put_or_lock
_raw_spin_lock
? lockref_put_or_lock
dput
path_put
fanotify_free_event
fsnotify_destroy_event
fanotify_handle_event
? mntput
? path_openat
? handle_mm_fault
send_to_group
? fsnotify
fsnotify
do_sys_open
sys_open
RIP: lock_acquire

  2b:*	4d 8b 64 c6 08       	mov    0x8(%r14,%rax,8),%r12     <-- trapping instruction

R14 is 0x6b6b6b6b6b6b6c03, which looks like a use-after-free.

I also notice you mention SLAB above, but I've been using SLUB. I don't
know if the choice of allocator makes a difference in reproducability.

It's also worth noting that I have lockdep enabled, which may be perturbing things
to some degree.

	Dave