ip: fix ip_mc_may_pull() return value

ip_mc_may_pull() must return 0 if there is a problem, not an errno.

syzbot reported :

BUG: KASAN: use-after-free in br_ip4_multicast_igmp3_report net/bridge/br_multicast.c:947 [inline]
BUG: KASAN: use-after-free in br_multicast_ipv4_rcv net/bridge/br_multicast.c:1631 [inline]
BUG: KASAN: use-after-free in br_multicast_rcv+0x3cd8/0x4440 net/bridge/br_multicast.c:1741
Read of size 4 at addr ffff88820a4084ee by task syz-executor.2/11183

CPU: 1 PID: 11183 Comm: syz-executor.2 Not tainted 5.0.0+ #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:131
 br_ip4_multicast_igmp3_report net/bridge/br_multicast.c:947 [inline]
 br_multicast_ipv4_rcv net/bridge/br_multicast.c:1631 [inline]
 br_multicast_rcv+0x3cd8/0x4440 net/bridge/br_multicast.c:1741
 br_handle_frame_finish+0xa3a/0x14c0 net/bridge/br_input.c:108
 br_nf_hook_thresh+0x2ec/0x380 net/bridge/br_netfilter_hooks.c:1005
 br_nf_pre_routing_finish+0x8e2/0x1750 net/bridge/br_netfilter_hooks.c:410
 NF_HOOK include/linux/netfilter.h:289 [inline]
 NF_HOOK include/linux/netfilter.h:283 [inline]
 br_nf_pre_routing+0x7e7/0x13a0 net/bridge/br_netfilter_hooks.c:506
 nf_hook_entry_hookfn include/linux/netfilter.h:119 [inline]
 nf_hook_slow+0xbf/0x1f0 net/netfilter/core.c:511
 nf_hook include/linux/netfilter.h:244 [inline]
 NF_HOOK include/linux/netfilter.h:287 [inline]
 br_handle_frame+0x95b/0x1450 net/bridge/br_input.c:305
 __netif_receive_skb_core+0xa96/0x3040 net/core/dev.c:4902
 __netif_receive_skb_one_core+0xa8/0x1a0 net/core/dev.c:4971
 __netif_receive_skb+0x2c/0x1c0 net/core/dev.c:5083
 netif_receive_skb_internal+0x117/0x660 net/core/dev.c:5186
 netif_receive_skb+0x6e/0x5a0 net/core/dev.c:5261

Fixes: ba5ea614 ("bridge: simplify ip_mc_check_igmp() and ipv6_mc_check_mld() calls")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/igmp.h

@@ -110,7 +110,7 @@ struct ip_mc_list {
 static inline int ip_mc_may_pull(struct sk_buff *skb, unsigned int len)
 {
 	if (skb_transport_offset(skb) + ip_transport_len(skb) < len)
-		return -EINVAL;
+		return 0;
 
 	return pskb_may_pull(skb, len);
 }