net: ipv6: fix skb_over_panic in __ip6_append_data

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>

Update the check that prevents an invalid packet with MTU equal
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
Reported-by: syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20220310232538.1044947-1-tadeusz.struk@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 4788f6b370539d16b9af6027d6cd028cdd9ace46..194832663d856a771fa52a0d6d09dffb7ff7cff6 100644 (file)
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1476,8 +1476,8 @@ static int __ip6_append_data(struct sock *sk,
                      sizeof(struct frag_hdr) : 0) +
                     rt->rt6i_nfheader_len;
 
-       if (mtu < fragheaderlen ||
-           ((mtu - fragheaderlen) & ~7) + fragheaderlen < sizeof(struct frag_hdr))
+       if (mtu <= fragheaderlen ||
+           ((mtu - fragheaderlen) & ~7) + fragheaderlen <= sizeof(struct frag_hdr))
                goto emsgsize;
 
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen -