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

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>

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 27241a767f17b4b27d24095a31e5e9a2d3e29ce4..c64aca611ac5c5f81ad7c925652bbb90554763ac 100644 (file)
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -104,7 +104,7 @@ tcpmss_mangle_packet(struct sk_buff *skb,
        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
        tcp_hdrlen = tcph->doff * 4;
 
-       if (len < tcp_hdrlen)
+       if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
                return -1;
 
        if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -152,6 +152,10 @@ tcpmss_mangle_packet(struct sk_buff *skb,
        if (len > tcp_hdrlen)
                return 0;
 
+       /* tcph->doff has 4 bits, do not wrap it to 0 */
+       if (tcp_hdrlen >= 15 * 4)
+               return 0;
+
        /*
         * MSS Option not found ?! add it..
         */