linux-kernel - [PATCH] x86: Call fixup_exception() before notify_die() in math

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Date:   Thu, 14 Jun 2018 19:36:07 +0000
From:   Siarhei Liakh <Siarhei.Liakh@...current-rt.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Borislav Petkov" <bpetkov@...e.de>
Subject: [PATCH] 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. This issue had been privately observed, fixed, and tested 
on 4.9.98, while this patch brings the fix to the upstream.

Signed-off-by: Siarhei Liakh <siarhei.liakh@...current-rt.com>
---

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a535dd6..68d77a3 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -835,16 +835,18 @@ 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;
 	}