[PATCH net] sctp: break out if skb_header_pointer returns NULL in sctp_rcv_ootb

[Xin Long <lucien.xin@gmail.com>]

We should always check if skb_header_pointer's return is NULL before
using it, otherwise it may cause null-ptr-deref, as syzbot reported:

  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: 0010:sctp_rcv_ootb net/sctp/input.c:705 [inline]
  RIP: 0010:sctp_rcv+0x1d84/0x3220 net/sctp/input.c:196
  Call Trace:
  <IRQ>
   sctp6_rcv+0x38/0x60 net/sctp/ipv6.c:1109
   ip6_protocol_deliver_rcu+0x2e9/0x1ca0 net/ipv6/ip6_input.c:422
   ip6_input_finish+0x62/0x170 net/ipv6/ip6_input.c:463
   NF_HOOK include/linux/netfilter.h:307 [inline]
   NF_HOOK include/linux/netfilter.h:301 [inline]
   ip6_input+0x9c/0xd0 net/ipv6/ip6_input.c:472
   dst_input include/net/dst.h:460 [inline]
   ip6_rcv_finish net/ipv6/ip6_input.c:76 [inline]
   NF_HOOK include/linux/netfilter.h:307 [inline]
   NF_HOOK include/linux/netfilter.h:301 [inline]
   ipv6_rcv+0x28c/0x3c0 net/ipv6/ip6_input.c:297

Fixes: 3acb50c18d8d ("sctp: delay as much as possible skb_linearize")
Reported-by: syzbot+581aff2ae6b860625116@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/sctp/input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 5ef86fdb1176..1f1786021d9c 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -702,7 +702,7 @@ static int sctp_rcv_ootb(struct sk_buff *skb)
 		ch = skb_header_pointer(skb, offset, sizeof(*ch), &_ch);
 
 		/* Break out if chunk length is less then minimal. */
-		if (ntohs(ch->length) < sizeof(_ch))
+		if (!ch || ntohs(ch->length) < sizeof(_ch))
 			break;
 
 		ch_end = offset + SCTP_PAD4(ntohs(ch->length));
-- 
2.27.0