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[PATCH 4.13 31/33] x86/debug: Handle warnings before the notifier chain, to fix KGDB crash




4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>

commit b8347c2196492f4e1cccde3d92fda1cc2cc7de7e upstream.

Commit:

  9a93848fe787 ("x86/debug: Implement __WARN() using UD0")

turned warnings into UD0, but the fixup code only runs after the
notify_die() chain. This is a problem, in particular, with kgdb,
which kicks in as if it was a BUG().

Fix this by running the fixup code before the notifier chain in
the invalid op handler path.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Tested-by: Ilya Dryomov <idryomov@xxxxxxxxx>
Acked-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Richard Weinberger <richard.weinberger@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20170724100428.19173-1-alexander.shishkin@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kernel/traps.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -221,9 +221,6 @@ do_trap_no_signal(struct task_struct *ts
 		if (fixup_exception(regs, trapnr))
 			return 0;
 
-		if (fixup_bug(regs, trapnr))
-			return 0;
-
 		tsk->thread.error_code = error_code;
 		tsk->thread.trap_nr = trapnr;
 		die(str, regs, error_code);
@@ -304,6 +301,13 @@ static void do_error_trap(struct pt_regs
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 
+	/*
+	 * WARN*()s end up here; fix them up before we call the
+	 * notifier chain.
+	 */
+	if (!user_mode(regs) && fixup_bug(regs, trapnr))
+		return;
+
 	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
 			NOTIFY_STOP) {
 		cond_local_irq_enable(regs);