signal: Remove the bogus sigkill_pending in ptrace_stop

commit 7d613f9f72ec8f90ddefcae038fdae5adb8404b3 upstream.

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.

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.

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@vger.kernel.org
Fixes: 3d749b9e676b ("ptrace: simplify ptrace_stop()->sigkill_pending() path")
Fixes: 1a669c2f16d4 ("Add arch_ptrace_stop")
Link: https://lkml.kernel.org/r/87pmsyx29t.fsf@disp2133
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/signal.c b/kernel/signal.c
index 487bf4f5dadf476171c081fd8938a88d22993423..cdf314be13731477cc5a6cf8a5b193fb80311703 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2169,15 +2169,6 @@ 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.
@@ -2204,17 +2195,16 @@ 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.
                 */
                spin_unlock_irq(&current->sighand->siglock);
                arch_ptrace_stop(exit_code, info);
                spin_lock_irq(&current->sighand->siglock);
-               if (sigkill_pending(current))
-                       return;
        }
 
+       /*
+        * schedule() will not sleep if there is a pending signal that
+        * can awaken the task.
+        */
        set_special_state(TASK_TRACED);
 
        /*