netfilter: xt_TCPOPTSTRIP: fix possible off by one access

Fix a possible off by one access since optlen()
touches opt[offset+1] unsafely when i == tcp_hdrlen(skb) - 1.

This patch replaces tcp_hdrlen() by the local variable tcp_hdrlen
that stores the TCP header length, to save some cycles.

Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/netfilter/xt_TCPOPTSTRIP.c b/net/netfilter/xt_TCPOPTSTRIP.c
index b68fa191710f..625fa1d636a0 100644
--- a/net/netfilter/xt_TCPOPTSTRIP.c
+++ b/net/netfilter/xt_TCPOPTSTRIP.c
@@ -38,7 +38,7 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
         struct tcphdr *tcph;
         u_int16_t n, o;
         u_int8_t *opt;
-        int len;
+        int len, tcp_hdrlen;
 
         /* This is a fragment, no TCP header is available */
         if (par->fragoff != 0)
@@ -52,7 +52,9 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
                 return NF_DROP;
 
         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
-        if (tcph->doff * 4 > len)
+        tcp_hdrlen = tcph->doff * 4;
+
+        if (len < tcp_hdrlen)
                 return NF_DROP;
 
         opt  = (u_int8_t *)tcph;
@@ -61,10 +63,10 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
          * Walk through all TCP options - if we find some option to remove,
          * set all octets to %TCPOPT_NOP and adjust checksum.
          */
-        for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
+        for (i = sizeof(struct tcphdr); i < tcp_hdrlen - 1; i += optl) {
                 optl = optlen(opt, i);
 
-                if (i + optl > tcp_hdrlen(skb))
+                if (i + optl > tcp_hdrlen)
                         break;
 
                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))