x86/mce: Convert threshold_bank.cpus from atomic_t to refcount_t

The refcount_t type and corresponding API should be used instead
of atomic_t when the variable is used as a reference counter. This
allows to avoid accidental refcounter overflows that might lead to
use-after-free situations.
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Yazen Ghannam <Yazen.Ghannam@amd.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1492695536-5947-1-git-send-email-elena.reshetova@intel.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/amd_nb.h

@@ -3,6 +3,7 @@
 
 #include <linux/ioport.h>
 #include <linux/pci.h>
+#include <linux/refcount.h>
 
 struct amd_nb_bus_dev_range {
 	u8 bus;
@@ -55,7 +56,7 @@ struct threshold_bank {
 	struct threshold_block	*blocks;
 
 	/* initialized to the number of CPUs on the node sharing this bank */
-	atomic_t		cpus;
+	refcount_t		cpus;
 };
 
 struct amd_northbridge {

arch/x86/kernel/cpu/mcheck/mce_amd.c

@@ -1202,7 +1202,7 @@ static int threshold_create_bank(unsigned int cpu, unsigned int bank)
 				goto out;
 
 			per_cpu(threshold_banks, cpu)[bank] = b;
-			atomic_inc(&b->cpus);
+			refcount_inc(&b->cpus);
 
 			err = __threshold_add_blocks(b);
 
@@ -1225,7 +1225,7 @@ static int threshold_create_bank(unsigned int cpu, unsigned int bank)
 	per_cpu(threshold_banks, cpu)[bank] = b;
 
 	if (is_shared_bank(bank)) {
-		atomic_set(&b->cpus, 1);
+		refcount_set(&b->cpus, 1);
 
 		/* nb is already initialized, see above */
 		if (nb) {
@@ -1289,7 +1289,7 @@ static void threshold_remove_bank(unsigned int cpu, int bank)
 		goto free_out;
 
 	if (is_shared_bank(bank)) {
-		if (!atomic_dec_and_test(&b->cpus)) {
+		if (!refcount_dec_and_test(&b->cpus)) {
 			__threshold_remove_blocks(b);
 			per_cpu(threshold_banks, cpu)[bank] = NULL;
 			return;