[tip:x86/urgent] x86: Call fixup_exception() before notify_die() in math_error()

[tip-bot for Siarhei Liakh <tipbot@zytor.com>]

Commit-ID:  3ae6295ccb7cf6d344908209701badbbbb503e40
Gitweb:     https://git.kernel.org/tip/3ae6295ccb7cf6d344908209701badbbbb503e40
Author:     Siarhei Liakh <Siarhei.Liakh@concurrent-rt.com>
AuthorDate: Thu, 14 Jun 2018 19:36:07 +0000
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 20 Jun 2018 11:44:56 +0200

x86: Call fixup_exception() before notify_die() in math_error()

fpu__drop() has an explicit fwait which under some conditions can trigger a
fixable FPU exception while in kernel. Thus, we should attempt to fixup the
exception first, and only call notify_die() if the fixup failed just like
in do_general_protection(). The original call sequence incorrectly triggers
KDB entry on debug kernels under particular FPU-intensive workloads.

Andy noted, that this makes the whole conditional irq enable thing even
more inconsistent, but fixing that it outside the scope of this.

Signed-off-by: Siarhei Liakh <siarhei.liakh@concurrent-rt.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Borislav  Petkov" <bpetkov@suse.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/DM5PR11MB201156F1CAB2592B07C79A03B17D0@DM5PR11MB2011.namprd11.prod.outlook.com

---
 arch/x86/kernel/traps.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 03f3d7695dac..162a31d80ad5 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -834,16 +834,18 @@ static void math_error(struct pt_regs *regs, int error_code, int trapnr)
 	char *str = (trapnr == X86_TRAP_MF) ? "fpu exception" :
 						"simd exception";
 
-	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, SIGFPE) == NOTIFY_STOP)
-		return;
 	cond_local_irq_enable(regs);
 
 	if (!user_mode(regs)) {
-		if (!fixup_exception(regs, trapnr)) {
-			task->thread.error_code = error_code;
-			task->thread.trap_nr = trapnr;
+		if (fixup_exception(regs, trapnr))
+			return;
+
+		task->thread.error_code = error_code;
+		task->thread.trap_nr = trapnr;
+
+		if (notify_die(DIE_TRAP, str, regs, error_code,
+					trapnr, SIGFPE) != NOTIFY_STOP)
 			die(str, regs, error_code);
-		}
 		return;
 	}