Remote DoS vulnerability in Linux kernel 2.6.x

Type securityvulns
Reporter Securityvulns
Modified 2004-06-30T00:00:00


1. Overview

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.

2. 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 throught the contents of this array infinitely.

3. Impact

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

4. 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

5. 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 kernel 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");

6. Credits

Vulnerability was discovered, identified and fixed by Adam Osuchowski and Tomasz Dubinski.


Adam Osuchowski,

Silesian University of Technology, Computer Centre Gliwice, Poland