Re: [PATCH 1/6] signal: Remove the bogus sigkill_pending in ptrace_stop

From: Eric W. Biederman
Date: Fri Sep 24 2021 - 11:48:33 EST

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Thu, Sep 23, 2021 at 07:09:34PM -0500, Eric W. Biederman wrote:
>> The existence of sigkill_pending is a little silly as it is
>> functionally a duplicate of fatal_signal_pending that is used in
>> exactly one place.
> sigkill_pending() checks for &tsk->signal->shared_pending.signal but
> fatal_signal_pending() doesn't.

The extra test is unnecessary as all SIGKILL's visit complete_signal
immediately run the loop:

* Start a group exit and wake everybody up.
* This way we don't have other threads
* running and doing things after a slower
* thread has the fatal signal pending.
signal->flags = SIGNAL_GROUP_EXIT;
signal->group_exit_code = sig;
signal->group_stop_count = 0;
t = p;
do {
task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
sigaddset(&t->pending.signal, SIGKILL);
signal_wake_up(t, 1);
} while_each_thread(p, t);

Which sets SIGKILL in the task specific queue. Which means only the
non-shared queue needs to be tested. Further fatal_signal_pending would
be buggy if this was not the case.

>> Checking for pending fatal signals and returning early in ptrace_stop
>> is actively harmful. It casues the ptrace_stop called by
>> ptrace_signal to return early before setting current->exit_code.
>> Later when ptrace_signal reads the signal number from
>> current->exit_code is undefined, making it unpredictable what will
>> happen.
>> Instead rely on the fact that schedule will not sleep if there is a
>> pending signal that can awaken a task.
> This reasoning sound fine, but I can't see where it's happening.
> It looks like recalc_sigpending() is supposed to happen at the start
> of scheduling? I see it at the end of ptrace_stop(), though, so it looks
> like it's reasonable to skip checking shared_pending.
> (Does the scheduler deal with shared_pending directly?)

In the call of signal_pending_state from kernel/core/.c:__schedule().

ptrace_stop would actually be badly broken today if that was not the
case as several places enter into ptrace_event without testing signals

>> Removing the explict sigkill_pending test fixes fixes ptrace_signal
>> when ptrace_stop does not stop because current->exit_code is always
>> set to to signr.
>> Cc: stable@xxxxxxxxxxxxxxx
>> Fixes: 3d749b9e676b ("ptrace: simplify ptrace_stop()->sigkill_pending() path")
>> Fixes: 1a669c2f16d4 ("Add arch_ptrace_stop")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>> kernel/signal.c | 18 ++++--------------
>> 1 file changed, 4 insertions(+), 14 deletions(-)
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index 952741f6d0f9..9f2dc9cf3208 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2182,15 +2182,6 @@ static inline bool may_ptrace_stop(void)
>> return true;
>> }
>> -/*
>> - * Return non-zero if there is a SIGKILL that should be waking us up.
>> - * Called with the siglock held.
>> - */
>> -static bool sigkill_pending(struct task_struct *tsk)
>> -{
>> - return sigismember(&tsk->pending.signal, SIGKILL) ||
>> - sigismember(&tsk->signal->shared_pending.signal, SIGKILL);
>> -}
>> /*
>> * This must be called with current->sighand->siglock held.
>> @@ -2217,17 +2208,16 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
>> * calling arch_ptrace_stop, so we must release it now.
>> * To preserve proper semantics, we must do this before
>> * any signal bookkeeping like checking group_stop_count.
>> - * Meanwhile, a SIGKILL could come in before we retake the
>> - * siglock. That must prevent us from sleeping in TASK_TRACED.
>> - * So after regaining the lock, we must check for SIGKILL.
> Where is the sleep this comment is talking about?
> i.e. will recalc_sigpending() have been called before the above sleep
> would happen? I assume it's after ptrace_stop() returns... But I want to
> make sure the sleep isn't in ptrace_stop() itself somewhere I can't see.
> I *do* see freezable_schedule() called, and that dumps us into
> __schedule(), and I don't see a recalc before it checks
> signal_pending_state().
> Does a recalc need to happen in plce of the old sigkill_pending()
> call?

You read that correctly freezable_schedule is where ptrace_stop sleeps.

The call chain you are looking for looks something like:
set_tsk_thread_flag(t, TIF_SIGPENDING)

That is to say complete_signal sets TIF_SIGPENDING and
the per task siqueue SIGKILL entry.

Calling recalc_sigpending is only needed when a signal is removed from
the queues, not when a signal is added.