Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped
From: Oleg Nesterov <hidden>
Date: 2017-04-02 15:35:27
Also in:
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Possibly related (same subject, not in this thread)
- 2017-04-05 · Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped · Oleg Nesterov <oleg@redhat.com>
- 2017-04-03 · Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped · Eric W. Biederman <hidden>
- 2017-04-03 · Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped · Oleg Nesterov <oleg@redhat.com>
- 2017-04-02 · Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped · Eric W. Biederman <hidden>
On 04/01, Eric W. Biederman wrote:
quoted hunk ↗ jump to hunk
--- 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.