refcount: Report UAF for refcount_sub_and_test(0) when counter==0

When a reference counter is at zero and refcount_sub_and_test() is invoked
to subtract zero, the function accepts this request without any warning and
returns true. This behavior does not seem ideal because the counter being
already at zero indicates a use-after-free. Furthermore, returning true by
refcount_sub_and_test() in this case potentially results in a double-free
done by its caller.

Modify the underlying function __refcount_sub_and_test() to warn about this
case as a use-after-free and have it return false to avoid the potential
double-free.

Signed-off-by: Petr Pavlu <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

drivers/misc/lkdtm/refcount.c

···
 	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)) {
···
 	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

···
 	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;