On 04/01, Eric W. Biederman wrote:
>
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1052,6 +1052,7 @@ static int de_thread(struct task_struct *tsk)
> struct signal_struct *sig = tsk->signal;
> struct sighand_struct *oldsighand = tsk->sighand;
> spinlock_t *lock = &oldsighand->siglock;
> + bool may_hang;
>
> if (thread_group_empty(tsk))
> goto no_thread_group;
> @@ -1069,9 +1070,10 @@ static int de_thread(struct task_struct *tsk)
> return -EAGAIN;
> }
>
> + may_hang = atomic_read(&oldsighand->count) != 1;
> sig->group_exit_task = tsk;
> - sig->notify_count = zap_other_threads(tsk);
> - if (!thread_group_leader(tsk))
> + sig->notify_count = zap_other_threads(tsk, may_hang ? 1 : -1);
Eric, this is amazing. So with this patch exec does different things depening
on whether sighand is shared with another CLONE_SIGHAND task or not. To me
this doesn't look sane in any case.
And of course you do realize that it doesn't solve the problem entirely? If I
modify my test-case a little bit
int xxx(void *arg)
{
for (;;)
pause();
}
void *thread(void *arg)
{
ptrace(PTRACE_TRACEME, 0,0,0);
return NULL;
}
int main(void)
{
int pid = fork();
if (!pid) {
pthread_t pt;
char stack[16 * 1024];
clone(xxx, stack + 16*1024, CLONE_SIGHAND|CLONE_VM,
NULL);
pthread_create(&pt, NULL, thread, NULL);
pthread_join(pt, NULL);
execlp("echo", "echo", "passed", NULL);
}
sleep(1);
// or anything else which needs ->cred_guard_mutex,
// say open(/proc/$pid/mem)
ptrace(PTRACE_ATTACH, pid, 0,0);
kill(pid, SIGCONT);
return 0;
}
it should deadlock the same way?
So what is the point to make the, well imo insane, patch if it doesn't solve
the problem?
And btw zap_other_threads(may_hang == 0) is racy. Either you need tasklist or
exit_notify() should set tsk->exit_state under siglock, otherwise zap() can
return the wrong count.
Finally. This patch creates the nice security hole. Let me modify my test-case
again:
void *thread(void *arg)
{
ptrace(PTRACE_TRACEME, 0,0,0);
return NULL;
}
int main(void)
{
int pid = fork();
if (!pid) {
pthread_t pt;
pthread_create(&pt, NULL, thread, NULL);
pthread_join(pt, NULL);
execlp(path-to-setuid-binary, args);
}
sleep(1);
// Now we can send the signals to setiuid app
kill(pid+1, ANYSIGNAL);
return 0;
}
I see another email from your with another proposal. I disagree, will reply
soon.
Oleg.
