ipv6: anycast: insert aca into global hash under idev->lock

In the Linux kernel, the following vulnerability has been resolved:

ipv6: anycast: insert aca into global hash under idev->lock

syzbot reported a splat [1]: a slab-use-after-free in

ipv6_chk_acast_addr(), which walks the global inet6_acaddr_lst[] hash

under RCU and dereferences a struct ifacaddr6 that has already been

freed while still linked in the hash, so a later reader walks into a

dangling node.

In __ipv6_dev_ac_inc() the aca is allocated with refcount 1, then

aca_get() bumps it to 2 to keep it alive across the unlocked region.

It is published to idev->ac_list under idev->lock, but

ipv6_add_acaddr_hash() runs after write_unlock_bh(). A concurrent

teardown (ipv6_ac_destroy_dev() from addrconf_ifdown(), under RTNL)

can slip into that window:

CPU0 __ipv6_dev_ac_inc CPU1 ipv6_ac_destroy_dev (RTNL)

------------------------------ ------------------------------------

aca_alloc() refcnt 1

aca_get() refcnt 2

write_lock_bh(idev->lock)

add aca to ac_list

write_unlock_bh(idev->lock)

write_lock_bh(idev->lock)

pull aca off ac_list

write_unlock_bh(idev->lock)

ipv6_del_acaddr_hash(aca)

hlist_del_init_rcu() is a no-op,

aca is not in the hash yet

aca_put() refcnt 2->1

ipv6_add_acaddr_hash(aca)

aca now inserted into the hash

aca_put() refcnt 1->0

call_rcu(aca_free_rcu) -> kfree(aca)

The hash removal becomes a no-op because the insertion has not

happened yet, so once CPU0 inserts and drops the last reference, the

aca is freed while still linked in inet6_acaddr_lst[], and readers

dereference freed memory after the slab slot is reused.

This window opened once RTNL stopped serializing the join path against

device teardown. Move ipv6_add_acaddr_hash() inside the idev->lock

section so the ac_list and hash insertions are atomic with respect to

teardown: a racing remover now either misses the aca entirely or finds

it in both lists.

acaddr_hash_lock is now nested under idev->lock, which is acquired in

softirq context, so switch all acaddr_hash_lock sites to spin_lock_bh()

to avoid the irq lock inversion reported in [2].

[1] https://syzkaller.appspot.com/bug?extid=a01df04303c131efbf3a

[2] https://lore.kernel.org/netdev/[email protected]/