From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: [PATCH 4.9 042/207] signal: Remove the bogus sigkill_pending in ptrace_stop
Date:   Wed, 24 Nov 2021 12:55:13 +0100
Message-Id: <20211124115705.297261639@linuxfoundation.org>
In-Reply-To: <20211124115703.941380739@linuxfoundation.org>
References: <20211124115703.941380739@linuxfoundation.org>
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From: Eric W. Biederman <ebiederm@xmission.com>

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>
---
 kernel/signal.c |   17 ++---------------
 1 file changed, 2 insertions(+), 15 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1824,16 +1824,6 @@ static inline int may_ptrace_stop(void)
 }
 
 /*
- * Return non-zero if there is a SIGKILL that should be waking us up.
- * Called with the siglock held.
- */
-static int 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.
  *
  * This should be the path for all ptrace stops.
@@ -1858,15 +1848,10 @@ static void ptrace_stop(int exit_code, i
 		 * 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;
 	}
 
 	/*
@@ -1875,6 +1860,8 @@ static void ptrace_stop(int exit_code, i
 	 * Also, transition to TRACED and updates to ->jobctl should be
 	 * atomic with respect to siglock and should be done after the arch
 	 * hook as siglock is released and regrabbed across it.
+	 * schedule() will not sleep if there is a pending signal that
+	 * can awaken the task.
 	 */
 	set_current_state(TASK_TRACED);