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Subject[PATCH 4.9 006/101] x86: Call fixup_exception() before notify_die() in math_error()
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

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

From: Siarhei Liakh <Siarhei.Liakh@concurrent-rt.com>

commit 3ae6295ccb7cf6d344908209701badbbbb503e40 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -799,16 +799,18 @@ static void math_error(struct pt_regs *r
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;
}


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