Re: KASAN: invalid-access Write in enqueue_timer

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

Hi Catalin,

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

quoted

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.

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 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

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 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,

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 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?