[PATCH] x86/MCE/AMD: Always give panic severity for UC errors in kernel context

[Yazen Ghannam]

From: Yazen Ghannam <yazen.ghannam@xxxxxxx>

[Upstream commit d65dfc81bb3894fdb68cbc74bbf5fb48d2354071]

The AMD severity grading function was introduced in kernel 4.1. The
current logic can possibly give MCE_AR_SEVERITY for uncorrectable
errors in kernel context. The system may then get stuck in a loop as
memory_failure() will try to handle the bad kernel memory and find it
busy.

Return MCE_PANIC_SEVERITY for all UC errors IN_KERNEL context on AMD
systems.

After:

  b2f9d678e28c ("x86/mce: Check for faults tagged in EXTABLE_CLASS_FAULT exception table entries")

was accepted in v4.6, this issue was masked because of the tail-end attempt
at kernel mode recovery in the #MC handler.

However, uncorrectable errors IN_KERNEL context should always be considered
unrecoverable and cause a panic.

Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 4.1.x, 4.4.x
Fixes: bf80bbd7dcf5 (x86/mce: Add an AMD severities-grading function)
---
Same as
  d65dfc81bb38 ("x86/MCE/AMD: Always give panic severity for UC errors in kernel context")
but fixed up to apply to v4.1 and v4.4 stable branches.

 arch/x86/kernel/cpu/mcheck/mce-severity.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce-severity.c b/arch/x86/kernel/cpu/mcheck/mce-severity.c
index 9c682c222071..a9287f0f06f2 100644
--- a/arch/x86/kernel/cpu/mcheck/mce-severity.c
+++ b/arch/x86/kernel/cpu/mcheck/mce-severity.c
@@ -200,6 +200,9 @@ static int mce_severity_amd(struct mce *m, int tolerant, char **msg, bool is_exc
 
 	if (m->status & MCI_STATUS_UC) {
 
+		if (ctx == IN_KERNEL)
+			return MCE_PANIC_SEVERITY;
+
 		/*
 		 * On older systems where overflow_recov flag is not present, we
 		 * should simply panic if an error overflow occurs. If
@@ -207,10 +210,6 @@ static int mce_severity_amd(struct mce *m, int tolerant, char **msg, bool is_exc
 		 * to at least kill process to prolong system operation.
 		 */
 		if (mce_flags.overflow_recov) {
-			/* software can try to contain */
-			if (!(m->mcgstatus & MCG_STATUS_RIPV) && (ctx == IN_KERNEL))
-				return MCE_PANIC_SEVERITY;
-
 			/* kill current process */
 			return MCE_AR_SEVERITY;
 		} else {
-- 
2.14.1