x86/mce/AMD: Make sure banks were initialized before accessing them

["Rafał Miłecki" <zajec5@gmail.com>]

From: Rafał Miłecki <rafal@milecki.pl>

It may happen amd_threshold_interrupt() fires before banks get setup as
part of the threshold_init_device() call. Such a case requires a proper
check to avoid NULL pointer dereference.

This problem has been noticed on HP EliteBook 745 G5 (Ryzen 2500U) with
BIOS 1.03.01. It doesn't boot unless mce is disabled (e.g. with
"mce=off").

This fixes a regression introduced in kernel 4.10. It was the first
kernel referencing "threshold_banks" variable in the
amd_threshold_interrupt().

Reported-by: John Clemens <clemej@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201291
Fixes: 18807ddb7f88 ("x86/mce/AMD: Reset Threshold Limit after logging error")
Cc: stable@vger.kernel.org # v4.10+
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
---
 arch/x86/kernel/cpu/mcheck/mce_amd.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce_amd.c b/arch/x86/kernel/cpu/mcheck/mce_amd.c
index dd33c357548f..379f54afc266 100644
--- a/arch/x86/kernel/cpu/mcheck/mce_amd.c
+++ b/arch/x86/kernel/cpu/mcheck/mce_amd.c
@@ -935,12 +935,19 @@ static void amd_threshold_interrupt(void)
 {
 	struct threshold_block *first_block = NULL, *block = NULL, *tmp = NULL;
 	unsigned int bank, cpu = smp_processor_id();
+	struct threshold_bank **banks;
+
+	banks = per_cpu(threshold_banks, cpu);
+	if (!banks) {
+		pr_warn("Threshold interrupt fired before MCE setup\n");
+		return;
+	}
 
 	for (bank = 0; bank < mca_cfg.banks; ++bank) {
 		if (!(per_cpu(bank_map, cpu) & (1 << bank)))
 			continue;
 
-		first_block = per_cpu(threshold_banks, cpu)[bank]->blocks;
+		first_block = banks[bank]->blocks;
 		if (!first_block)
 			continue;