netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

commit 2638fd0f92d4397884fd991d8f4925cb3f081901 upstream.

Denys provided an awesome KASAN report pointing to an use
after free in xt_TCPMSS

I have provided three patches to fix this issue, either in xt_TCPMSS or
in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
impact.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[wt: adjust context]

Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 7011c71646f0266eb75c856bc49fea7b5030bd52..c656269c4cf072e565506a7860b042d4cfc6c85a 100644 (file)
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -68,7 +68,7 @@ tcpmss_mangle_packet(struct sk_buff *skb,
        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 
        /* Header cannot be larger than the packet */
-       if (tcplen < tcph->doff*4)
+       if (tcplen < tcph->doff*4 || tcph->doff*4 < sizeof(struct tcphdr))
                return -1;
 
        if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -117,6 +117,10 @@ tcpmss_mangle_packet(struct sk_buff *skb,
        if (tcplen > tcph->doff*4)
                return 0;
 
+       /* tcph->doff has 4 bits, do not wrap it to 0 */
+       if (tcph->doff >= 15)
+               return 0;
+
        /*
         * MSS Option not found ?! add it..
         */