Re: [PATCH ipsec] xfrm: policy: fix bydst hlist corruption on hash rebuild

[Steffen Klassert <steffen.klassert@secunet.com>]

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+0165480d4ef07360eeda@syzkaller.appspotmail.com
> Fixes: 1548bc4e0512 ("xfrm: policy: delete inexact policies from inexact list on hash rebuild")
> Signed-off-by: Florian Westphal <fw@strlen.de>

Applied, thanks Florian!