[PATCH net] netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment()

br_ip6_fragment() gets prevhdr, a pointer into the skb head, from
ip6_find_1stfragopt(), then calls skb_checksum_help().  For a cloned skb
skb_checksum_help() reallocates the head via pskb_expand_head(), leaving
prevhdr dangling.  It is later dereferenced in ip6_frag_next(), causing a
use-after-free write.

Re-find prevhdr after skb_checksum_help() so it points into the current
head.

  BUG: KASAN: slab-use-after-free in ip6_frag_next (net/ipv6/ip6_output.c:857)
  Write of size 1 at addr ffff888013ff5016 by task exploit/141
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   ip6_frag_next (net/ipv6/ip6_output.c:857)
   br_ip6_fragment (net/ipv6/netfilter.c:212)
   nf_ct_bridge_post (net/bridge/netfilter/nf_conntrack_bridge.c:407)
   nf_hook_slow (net/netfilter/core.c:619)
   br_forward_finish (net/bridge/br_forward.c:66)
   __br_forward (net/bridge/br_forward.c:115)
   maybe_deliver (net/bridge/br_forward.c:191)
   br_flood (net/bridge/br_forward.c:245)
   br_handle_frame_finish (net/bridge/br_input.c:229)
   br_handle_frame (net/bridge/br_input.c:442)
   ...
   packet_sendmsg (net/packet/af_packet.c:3114)
   ...
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 764dd163ac92 ("netfilter: nf_conntrack_bridge: add support for IPv6")
Reported-by: [email protected]
Signed-off-by: Xiang Mei (Microsoft) <[email protected]>
---
 net/ipv6/netfilter.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/netfilter.c b/net/ipv6/netfilter.c
index 6d80f85e55fa..547879da9532 100644
--- a/net/ipv6/netfilter.c
+++ b/net/ipv6/netfilter.c
@@ -147,6 +147,10 @@ int br_ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	    (err = skb_checksum_help(skb)))
 		goto blackhole;
 
+	err = ip6_find_1stfragopt(skb, &prevhdr);
+	if (err < 0)
+		goto blackhole;
+
 	hroom = LL_RESERVED_SPACE(skb->dev);
 	if (skb_has_frag_list(skb)) {
 		unsigned int first_len = skb_pagelen(skb);
-- 
2.43.0