KASAN: invalid-access Write in enqueue_timer

[Dmitry Vyukov]

On Tue, Feb 16, 2021 at 7:15 PM Dmitry Vyukov <dvyukov at google.com> wrote:
> > On Tue, Feb 16, 2021 at 06:50:20PM +0100, Jason A. Donenfeld wrote:
> > > On Tue, Feb 16, 2021 at 6:46 PM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
> > > > On Tue, Feb 16, 2021 at 6:28 PM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > > > > >  hlist_add_head include/linux/list.h:883 [inline]
> > > > > >  enqueue_timer+0x18/0xc0 kernel/time/timer.c:581
> > > > > >  mod_timer+0x14/0x20 kernel/time/timer.c:1106
> > > > > >  mod_peer_timer drivers/net/wireguard/timers.c:37 [inline]
> > > > > >  wg_timers_any_authenticated_packet_traversal+0x68/0x90 drivers/net/wireguard/timers.c:215
> > > >
> > > > The line of hlist_add_head that it's hitting is:
> > > >
> > > > static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
> > > > {
> > > >        struct hlist_node *first = h->first;
> > > >        WRITE_ONCE(n->next, first);
> > > >        if (first)
> > > >
> > > > So that means it's the dereferencing of h that's a problem. That comes from:
> > > >
> > > > static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
> > > >                          unsigned int idx, unsigned long bucket_expiry)
> > > > {
> > > >
> > > >        hlist_add_head(&timer->entry, base->vectors + idx);
> > > >
> > > > That means it concerns base->vectors + idx, not the timer_list object
> > > > that wireguard manages. That's confusing. Could that imply that the
> > > > bug is in freeing a previous timer without removing it from the timer
> > > > lists, so that it winds up being in base->vectors?
> >
> > Good point, it's indeed likely that the timer list is messed up already,
> > just an unlucky encounter in the wireguard code.
> >
> > > Digging around on syzkaller, it looks like there's a similar bug on
> > > jbd2, concerning iptunnels's allocation:
> > >
> > > https://syzkaller.appspot.com/text?tag=CrashReport&x=13afb19cd00000
> > [...]
> > > It might not actually be a wireguard bug?
> >
> > I wonder whether syzbot reported similar issues with
> > CONFIG_KASAN_SW_TAGS. It shouldn't be that different from the HW_TAGS
> > but at least we can rule out qemu bugs with the MTE emulation.
>
> +Eric

I've seen some similar reports on other syzkaller instances. They all
have similar alloc/free stacks, but different access stacks.
This does not seem to be wireguard nor arm/mte related. It seems that
something released the device prematurely, and then some innocent code
gets a use-after-free.