[PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append_data

[Tadeusz Struk]

Syzbot found a kernel bug in the ipv6 stack:
LINK: https://syzkaller.appspot.com/bug?id=205d6f11d72329ab8d62a610c44c5e7e25415580
The reproducer triggers it by sending a crafted message via sendmmsg()
call, which triggers skb_over_panic, and crashes the kernel:

skbuff: skb_over_panic: text:ffffffff84647fb4 len:65575 put:65575
head:ffff888109ff0000 data:ffff888109ff0088 tail:0x100af end:0xfec0
dev:<NULL>

Add a check that prevents an invalid packet with MTU equall to the
fregment header size to eat up all the space for payload.

The reproducer can be found here:
LINK: https://syzkaller.appspot.com/text?tag=ReproC&x=1648c83fb00000

Cc: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Martin KaFai Lau <kafai@xxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: bpf@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx

Reported-by: syzbot+e223cf47ec8ae183f2a0@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
---
v2: Instead of updating the alloclen add a check that prevents
    an invalid packet with MTU equall to the fregment header size
    to eat up all the space for payload.
    Fix suggested by Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
---
 net/ipv6/ip6_output.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 4788f6b37053..6d45112322a0 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1649,6 +1649,16 @@ static int __ip6_append_data(struct sock *sk,
 			skb->protocol = htons(ETH_P_IPV6);
 			skb->ip_summed = csummode;
 			skb->csum = 0;
+
+			/*
+			 *	Check if there is still room for payload
+			 */
+			if (fragheaderlen >= mtu) {
+				err = -EMSGSIZE;
+				kfree_skb(skb);
+				goto error;
+			}
+
 			/* reserve for fragmentation and ipsec header */
 			skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
 				    dst_exthdrlen);
-- 
2.35.1