On Tue, Jul 02, 2019 at 12:46:00PM +0200, Florian Westphal wrote:
> syzbot reported following spat:
>
> BUG: KASAN: use-after-free in __write_once_size include/linux/compiler.h:221
> BUG: KASAN: use-after-free in hlist_del_rcu include/linux/rculist.h:455
> BUG: KASAN: use-after-free in xfrm_hash_rebuild+0xa0d/0x1000
> net/xfrm/xfrm_policy.c:1318
> Write of size 8 at addr ffff888095e79c00 by task kworker/1:3/8066
> Workqueue: events xfrm_hash_rebuild
> Call Trace:
> __write_once_size include/linux/compiler.h:221 [inline]
> hlist_del_rcu include/linux/rculist.h:455 [inline]
> xfrm_hash_rebuild+0xa0d/0x1000 net/xfrm/xfrm_policy.c:1318
> process_one_work+0x814/0x1130 kernel/workqueue.c:2269
> Allocated by task 8064:
> __kmalloc+0x23c/0x310 mm/slab.c:3669
> kzalloc include/linux/slab.h:742 [inline]
> xfrm_hash_alloc+0x38/0xe0 net/xfrm/xfrm_hash.c:21
> xfrm_policy_init net/xfrm/xfrm_policy.c:4036 [inline]
> xfrm_net_init+0x269/0xd60 net/xfrm/xfrm_policy.c:4120
> ops_init+0x336/0x420 net/core/net_namespace.c:130
> setup_net+0x212/0x690 net/core/net_namespace.c:316
>
> The faulting address is the address of the old chain head,
> free'd by xfrm_hash_resize().
>
> In xfrm_hash_rehash(), chain heads get re-initialized without
> any hlist_del_rcu:
>
> for (i = hmask; i >= 0; i--)
> INIT_HLIST_HEAD(odst + i);
>
> Then, hlist_del_rcu() gets called on the about to-be-reinserted policy
> when iterating the per-net list of policies.
>
> hlist_del_rcu() will then make chain->first be nonzero again:
>
> static inline void __hlist_del(struct hlist_node *n)
> {
> struct hlist_node *next = n->next; // address of next element in list
> struct hlist_node **pprev = n->pprev;// location of previous elem, this
> // can point at chain->first
> WRITE_ONCE(*pprev, next); // chain->first points to next elem
> if (next)
> next->pprev = pprev;
>
> Then, when we walk chainlist to find insertion point, we may find a
> non-empty list even though we're supposedly reinserting the first
> policy to an empty chain.
>
> To fix this first unlink all exact and inexact policies instead of
> zeroing the list heads.
>
> Add the commands equivalent to the syzbot reproducer to xfrm_policy.sh,
> without fix KASAN catches the corruption as it happens, SLUB poisoning
> detects it a bit later.
>
> Reported-by: syzbot+0165480d4ef07360e...@syzkaller.appspotmail.com
> Fixes: 1548bc4e0512 ("xfrm: policy: delete inexact policies from inexact list
> on hash rebuild")
> Signed-off-by: Florian Westphal <f...@strlen.de>
Applied, thanks Florian!
