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    Subject[PATCH 3.16 117/136] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff
    3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Dumazet <edumazet@google.com>

    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>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    net/netfilter/xt_TCPMSS.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    --- 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) {
    @@ -156,6 +156,10 @@ 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..
    */
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