Re: KASAN: invalid-access Write in enqueue_timer

On Tue, Feb 16, 2021 at 7:01 PM Catalin Marinas [off-list ref] wrote:

On Tue, Feb 16, 2021 at 06:50:20PM +0100, Jason A. Donenfeld wrote:

quoted

On Tue, Feb 16, 2021 at 6:46 PM Jason A. Donenfeld [off-list ref] wrote:

quoted

On Tue, Feb 16, 2021 at 6:28 PM Catalin Marinas [off-list ref] wrote:

quoted

quoted

 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.

quoted

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

[...]

quoted

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