netfilter: nat: can't use dst_hold on noref dst

The dst entry might already have a zero refcount, waiting on rcu list
to be free'd.  Using dst_hold() transitions its reference count to 1, and
next dst release will try to free it again -- resulting in a double free:

  WARNING: CPU: 1 PID: 0 at include/net/dst.h:239 nf_xfrm_me_harder+0xe7/0x130 [nf_nat]
  RIP: 0010:nf_xfrm_me_harder+0xe7/0x130 [nf_nat]
  Code: 48 8b 5c 24 60 65 48 33 1c 25 28 00 00 00 75 53 48 83 c4 68 5b 5d 41 5c c3 85 c0 74 0d 8d 48 01 f0 0f b1 0a 74 86 85 c0 75 f3 <0f> 0b e9 7b ff ff ff 29 c6 31 d2 b9 20 00 48 00 4c 89 e7 e8 31 27
  Call Trace:
  nf_nat_ipv4_out+0x78/0x90 [nf_nat_ipv4]
  nf_hook_slow+0x36/0xd0
  ip_output+0x9f/0xd0
  ip_forward+0x328/0x440
  ip_rcv+0x8a/0xb0

Use dst_hold_safe instead and bail out if we cannot take a reference.

Fixes: a4c2fd7f7891 ("net: remove DST_NOCACHE flag")
Reported-by: Martin Zaharinov <micron10@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/netfilter/nf_nat_core.c b/net/netfilter/nf_nat_core.c
index e2b196054dfc6dbe2eeedbee76a8ab342187e297..2268b10a9dcf844b8ce9d8d43c6b32e943553496 100644 (file)
--- a/net/netfilter/nf_nat_core.c
+++ b/net/netfilter/nf_nat_core.c
@@ -117,7 +117,8 @@ int nf_xfrm_me_harder(struct net *net, struct sk_buff *skb, unsigned int family)
        dst = skb_dst(skb);
        if (dst->xfrm)
                dst = ((struct xfrm_dst *)dst)->route;
-       dst_hold(dst);
+       if (!dst_hold_safe(dst))
+               return -EHOSTUNREACH;
 
        if (sk && !net_eq(net, sock_net(sk)))
                sk = NULL;