KASAN: invalid-access Write in enqueue_timer

[Jason A. Donenfeld]

On Tue, Feb 16, 2021 at 6:46 PM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
>
> Hi Catalin,
>
> On Tue, Feb 16, 2021 at 6:28 PM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > Adding Jason and Ard. It may be a use-after-free in the wireguard
> > driver.
>
> Thanks for sending this my way. Note: to my knowledge, Ard doesn't
> work on wireguard.
>
> > >  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?
>
> The allocation and deallocation backtrace is confusing
>
> > >  alloc_netdev_mqs+0x5c/0x3bc net/core/dev.c:10546
> > >  rtnl_create_link+0xc8/0x2b0 net/core/rtnetlink.c:3171
> > >  __rtnl_newlink+0x5bc/0x800 net/core/rtnetlink.c:3433
>
> This suggests it's part of the `ip link add wg0 type wireguard` nelink
> call, during it's allocation of the netdevice's private area. For
> this, the wg_device struct is used. It has no timer_list structures in
> it!
>
> Similarly,
>
> > >  netdev_freemem+0x18/0x2c net/core/dev.c:10500
> > >  netdev_release+0x30/0x44 net/core/net-sysfs.c:1828
> > >  device_release+0x34/0x90 drivers/base/core.c:1980
>
> That smells like `ip link del wg0 type wireguard`. But again,
> wg_device doesn't have any timer_lists in it.
>
> So what's happening here exactly? I'm not really sure yet...
>
> It'd be nice to have a reproducer.
>
>
> Jason


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

And one from ext4:

https://syzkaller.appspot.com/text?tag=CrashReport&x=17685330d00000

And from from ext4 with fddup:

https://syzkaller.appspot.com/text?tag=CrashReport&x=17685330d00000
https://syzkaller.appspot.com/text?tag=CrashReport&x=12d326e8d00000

It might not actually be a wireguard bug?