linux-kernel - [tip:x86/urgent] x86/debug: Handle warnings before the notifier chain, to fix KGDB crash

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Date:   Thu, 9 Nov 2017 22:33:57 -0800
From:   tip-bot for Alexander Shishkin <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, mingo@...nel.org, daniel.thompson@...aro.org,
        peterz@...radead.org, idryomov@...il.com,
        linux-kernel@...r.kernel.org, arjan@...ux.intel.com, bp@...en8.de,
        alexander.shishkin@...ux.intel.com, torvalds@...ux-foundation.org,
        richard.weinberger@...il.com, tglx@...utronix.de,
        jason.wessel@...driver.com
Subject: [tip:x86/urgent] x86/debug: Handle warnings before the notifier
 chain, to fix KGDB crash

Commit-ID:  a8d6c1bd62ffefb075c9d3570f07659e2a36ecb3
Gitweb:     https://git.kernel.org/tip/a8d6c1bd62ffefb075c9d3570f07659e2a36ecb3
Author:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
AuthorDate: Mon, 24 Jul 2017 13:04:28 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 10 Nov 2017 07:16:23 +0100

x86/debug: Handle warnings before the notifier chain, to fix KGDB crash

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@...ux.intel.com>
Tested-by: Ilya Dryomov <idryomov@...il.com>
Acked-by: Daniel Thompson <daniel.thompson@...aro.org>
Cc: Jason Wessel <jason.wessel@...driver.com>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Richard Weinberger <richard.weinberger@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: <stable@...r.kernel.org> # v4.12+
Link: http://lkml.kernel.org/r/20170724100428.19173-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/traps.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 67db4f4..5a6b8f8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -209,9 +209,6 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
 		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);
@@ -292,6 +289,13 @@ static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
 
 	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);