[PATCH v2] x86/MCE/AMD: Always give PANIC severity for UC errors IN_KERNEL context

[Yazen Ghannam]

From: Yazen Ghannam <yazen.ghannam@xxxxxxx>

The AMD severity grading function was introduced in v4.1 and has remained
logically unchanged with the exception of a separate SMCA severity grading
function for SMCA systems. 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.

Fixes: bf80bbd7dcf5 (x86/mce: Add an AMD severities-grading function)

Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
[ This needs to be reworked to apply to v4.1 and v4.4 stable branches.]
Cc: <stable@xxxxxxxxxxxxxxx> # 4.9.x
---
Link:
https://lkml.kernel.org/r/1505830031-9630-1-git-send-email-Yazen.Ghannam@xxxxxxx

v1->v2:
* Update commit message.

 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 2773c8547f69..f5518706baa6 100644
--- a/arch/x86/kernel/cpu/mcheck/mce-severity.c
+++ b/arch/x86/kernel/cpu/mcheck/mce-severity.c
@@ -245,6 +245,9 @@ 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
@@ -255,10 +258,6 @@ static int mce_severity_amd(struct mce *m, int tolerant, char **msg, bool is_exc
 			if (mce_flags.smca)
 				return mce_severity_amd_smca(m, ctx);
 
-			/* 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 {
-- 
2.7.4