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>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 27241a767f17..c64aca611ac5 100644
--- 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..
          */