Re: [PATCH v3 08/11] ptrace: Admit ptrace_stop can generate spuriuos SIGTRAPs

Eric W. Biederman
2022-05-05T16:59:49Z

Oleg Nesterov <oleg@redhat.com> writes:

> On 05/04, Eric W. Biederman wrote:
>>
>> -static int ptrace_stop(int exit_code, int why, int clear_code,
>> - unsigned long message, kernel_siginfo_t *info)
>> +static int ptrace_stop(int exit_code, int why, unsigned long message,
>> + kernel_siginfo_t *info)
>> __releases(&current->sighand->siglock)
>> __acquires(&current->sighand->siglock)
>> {
>> @@ -2259,54 +2259,33 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
>>
>> spin_unlock_irq(&current->sighand->siglock);
>> read_lock(&tasklist_lock);
>> - if (likely(current->ptrace)) {
>> - /*
>> - * Notify parents of the stop.
>> - *
>> - * While ptraced, there are two parents - the ptracer and
>> - * the real_parent of the group_leader. The ptracer should
>> - * know about every stop while the real parent is only
>> - * interested in the completion of group stop. The states
>> - * for the two don't interact with each other. Notify
>> - * separately unless they're gonna be duplicates.
>> - */
>> + /*
>> + * Notify parents of the stop.
>> + *
>> + * While ptraced, there are two parents - the ptracer and
>> + * the real_parent of the group_leader. The ptracer should
>> + * know about every stop while the real parent is only
>> + * interested in the completion of group stop. The states
>> + * for the two don't interact with each other. Notify
>> + * separately unless they're gonna be duplicates.
>> + */
>> + if (current->ptrace)
>> do_notify_parent_cldstop(current, true, why);
>> - if (gstop_done && ptrace_reparented(current))
>> - do_notify_parent_cldstop(current, false, why);
>> + if (gstop_done && (!current->ptrace || ptrace_reparented(current)))
>> + do_notify_parent_cldstop(current, false, why);
>>
>> - /*
>> - * Don't want to allow preemption here, because
>> - * sys_ptrace() needs this task to be inactive.
>> - *
>> - * XXX: implement read_unlock_no_resched().
>> - */
>> - preempt_disable();
>> - read_unlock(&tasklist_lock);
>> - cgroup_enter_frozen();
>> - preempt_enable_no_resched();
>> - freezable_schedule();
>> - cgroup_leave_frozen(true);
>> - } else {
>> - /*
>> - * By the time we got the lock, our tracer went away.
>> - * Don't drop the lock yet, another tracer may come.
>> - *
>> - * If @gstop_done, the ptracer went away between group stop
>> - * completion and here. During detach, it would have set
>> - * JOBCTL_STOP_PENDING on us and we'll re-enter
>> - * TASK_STOPPED in do_signal_stop() on return, so notifying
>> - * the real parent of the group stop completion is enough.
>> - */
>> - if (gstop_done)
>> - do_notify_parent_cldstop(current, false, why);
>> -
>> - /* tasklist protects us from ptrace_freeze_traced() */
>> - __set_current_state(TASK_RUNNING);
>> - read_code = false;
>> - if (clear_code)
>> - exit_code = 0;
>> - read_unlock(&tasklist_lock);
>> - }
>> + /*
>> + * Don't want to allow preemption here, because
>> + * sys_ptrace() needs this task to be inactive.
>> + *
>> + * XXX: implement read_unlock_no_resched().
>> + */
>> + preempt_disable();
>> + read_unlock(&tasklist_lock);
>> + cgroup_enter_frozen();
>> + preempt_enable_no_resched();
>> + freezable_schedule();
>
> I must have missed something.
>
> So the tracee calls ptrace_notify() but debugger goes away before the
> ptrace_notify() takes siglock. After that the no longer traced task
> will sleep in TASK_TRACED ?
>
> Looks like ptrace_stop() needs to check current->ptrace before it does
> set_special_state(TASK_TRACED) with siglock held? Then we can rely on
> ptrace_unlink() which will wake the tracee up even if debugger exits.
>
> No?

Hmm. If the debugger goes away when siglock is dropped and reaquired at
the top of ptrace_stop, that would appear to set the debugged process up
to sleep indefinitely.

I was thinking of the SIGKILL case which is handled.

Thank you for catching that.

Eric