Re: KASAN: invalid-access Write in enqueue_timer

[Catalin Marinas]

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@xxxxxxxxx> wrote:
> > On Tue, Feb 16, 2021 at 6:28 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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.

-- 
Catalin