[UNIX] Remote DoS Vulnerability in Netfilter's Subsystem (tcp-option)

From: SecuriTeam (support_at_securiteam.com)
Date: 07/04/04

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Date: 4 Jul 2004 19:04:38 +0200

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      Remote DoS Vulnerability in Netfilter's Subsystem (tcp-option)
    ------------------------------------------------------------------------

    SUMMARY

    There is a remotely exploitable bug in all Linux kernel 2.6 series due to
    using incorrect variable type. Vulnerability is connected to netfilter
    subsystem and may cause DoS. It's disclosed only when using iptables with
    rules matching TCP options (i.e. --tcp-option). There is no difference
    what action is taking up by matching rule.

    Vulnerability was detected on i386 architecture. The other ones weren't
    tested but it seems to be vulnerable too.

    DETAILS

    Problem lies in tcp_find_option() function
    (net/ipv4/netfilter/ip_tables.c). There is local array `opt' defined as:
        char opt[60 - sizeof(struct tcphdr)];

    Which contains TCP options extracted from packet. Function mentioned above
    searches for specified option in this array.

    Options in TCP packet, with some exceptions, are organized in the
    following way:
            Octet no. Length Field
            -----------------------------
                    0 1 Opcode
                    1 1 Length of all option (N + 2)
                    2 N Params

    The function iterates over options in array:
        for (i = 0; i < optlen; ) {
                if (opt[i] == option) return !invert;
                if (opt[i] < 2) i++;
                else i += opt[i+1]?:1;
        }

    Moving counter by the option length.

    But, in case the `length' value is greater than 127, the value of this
    octet in `opt' is implicitly casted to char, which results in negative
    number and the loop counter moving back. In some cases it is possible that
    counter cycles through the contents of this array infinitely.

    Impact:
    After sending one suitably prepared TCP packet to victim host, kernel goes
    into infinite loop consuming all CPU resources, rendering the box
    unresponsive. Of course, there is no need to have a shell access to
    attacked host.

    Exploitation:
    Example of packet-of-death:
    0x0000: 4500 0030 1234 4000 ff06 e83f c0a8 0001
    0x0010: c0a8 0002 0400 1000 0000 0064 0000 0064
    0x0020: 7000 0fa0 dc6a 0000 0204 05b4 0101 04fd

    Fix:
    There is only need to change type of `opt' array from signed char to
    unsigned (or, better to u_int8_t) as it was defined in 2.4 kernels or
    prior to version 1.16 of net/ipv4/netfilter/ip_tables.c file.

    --- net/ipv4/netfilter/ip_tables.c.orig 2004-04-04 05:36:47.000000000
    +0200
    +++ net/ipv4/netfilter/ip_tables.c 2004-06-24 21:24:26.000000000 +0200
    @@ -1461,7 +1461,7 @@
                    int *hotdrop)
     {
            /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
    - char opt[60 - sizeof(struct tcphdr)];
    + u_int8_t opt[60 - sizeof(struct tcphdr)];
            unsigned int i;
     
            duprintf("tcp_match: finding option\n");

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:adwol@polsl.gliwice.pl> Adam
    Osuchowski (Computer Centre of The Silesian University of Technology) and
    Tomasz Dubinski.

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