linux-kernel - [PATCH] signal: Remove the bogus sigkill_pending in ptrace

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Date:   Sun, 05 Sep 2021 15:17:37 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>
Subject: [PATCH] signal: Remove the bogus sigkill_pending in ptrace_stop

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@...r.kernel.org
Fixes: 3d749b9e676b ("ptrace: simplify ptrace_stop()->sigkill_pending() path")
Fixes: 1a669c2f16d4 ("Add arch_ptrace_stop")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---

Linus if you would like this as a pull request or after the merge window
closes please let me know.  This change just fixes a very old and nasty
piece of confusion in the code and is trivially correct so I thought you
might appreciate it directly.

 kernel/signal.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index a3229add4455..b024447b3ff3 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2122,15 +2122,6 @@ 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.
@@ -2157,17 +2148,16 @@ 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);

 	/*
-- 
2.20.1