Merge branch 'for-next/kspp' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git

drivers/misc/lkdtm/refcount.c

@@ -182,6 +182,21 @@ static void lkdtm_REFCOUNT_SUB_AND_TEST_NEGATIVE(void)
 	check_negative(&neg, 3);
 }
 
+/*
+ * A refcount_sub_and_test() by zero when the counter is at zero should act like
+ * refcount_sub_and_test() above when going negative.
+ */
+static void lkdtm_REFCOUNT_SUB_AND_TEST_ZERO(void)
+{
+	refcount_t neg = REFCOUNT_INIT(0);
+
+	pr_info("attempting bad refcount_sub_and_test() at zero\n");
+	if (refcount_sub_and_test(0, &neg))
+		pr_warn("Weird: refcount_sub_and_test() reported zero\n");
+
+	check_negative(&neg, 0);
+}
+
 static void check_from_zero(refcount_t *ref)
 {
 	switch (refcount_read(ref)) {
@@ -400,6 +415,7 @@ static struct crashtype crashtypes[] = {
 	CRASHTYPE(REFCOUNT_DEC_NEGATIVE),
 	CRASHTYPE(REFCOUNT_DEC_AND_TEST_NEGATIVE),
 	CRASHTYPE(REFCOUNT_SUB_AND_TEST_NEGATIVE),
+	CRASHTYPE(REFCOUNT_SUB_AND_TEST_ZERO),
 	CRASHTYPE(REFCOUNT_INC_ZERO),
 	CRASHTYPE(REFCOUNT_ADD_ZERO),
 	CRASHTYPE(REFCOUNT_INC_SATURATED),

include/linux/refcount.h

@@ -266,12 +266,12 @@ bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
 	if (oldp)
 		*oldp = old;
 
-	if (old == i) {
+	if (old > 0 && old == i) {
 		smp_acquire__after_ctrl_dep();
 		return true;
 	}
 
-	if (unlikely(old < 0 || old - i < 0))
+	if (unlikely(old <= 0 || old - i < 0))
 		refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
 
 	return false;