[PATCH] refcount: Use CONFIG_BUG_ON_DATA_CORRUPTION for UAF-related errors

[Jann Horn]

Some refcount issues are not necessarily associated with memory corruption,
but REFCOUNT_ADD_UAF suggests that a UAF either just happened or is about
to happen.

REFCOUNT_SUB_UAF is also an indicator that reference counting is wrong, and
suggests (less strongly) that a UAF access might have happened recently.

In these cases, BUG() is appropriate if CONFIG_BUG_ON_DATA_CORRUPTION is
set.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
MAINTAINERS specifies no specific maintainer for lib/refcount.c, but it
does have an entry for include/linux/refcount.h, so I guess I should
route this patch based on that.

I decided to send this patch after wondering how exploitable it would be
to have a refcount_inc() call on an object which has reached refcount 0,
but is not yet freed because of something like an RCU grace period.
---
 lib/refcount.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/refcount.c b/lib/refcount.c
index a207a8f22b3c..c0f0dc5296eb 100644
--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -10,6 +10,15 @@
 
 #define REFCOUNT_WARN(str)	WARN_ONCE(1, "refcount_t: " str ".\n")
 
+#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
+#define REFCOUNT_CORRUPTION(str) ({		\
+	pr_err("refcount_t: " str ".\n");	\
+	BUG();					\
+})
+#else
+#define REFCOUNT_CORRUPTION(str) REFCOUNT_WARN(str)
+#endif
+
 void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
 {
 	refcount_set(r, REFCOUNT_SATURATED);
@@ -22,10 +31,10 @@ void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
 		REFCOUNT_WARN("saturated; leaking memory");
 		break;
 	case REFCOUNT_ADD_UAF:
-		REFCOUNT_WARN("addition on 0; use-after-free");
+		REFCOUNT_CORRUPTION("addition on 0; use-after-free");
 		break;
 	case REFCOUNT_SUB_UAF:
-		REFCOUNT_WARN("underflow; use-after-free");
+		REFCOUNT_CORRUPTION("underflow; use-after-free");
 		break;
 	case REFCOUNT_DEC_LEAK:
 		REFCOUNT_WARN("decrement hit 0; leaking memory");
@@ -84,7 +93,7 @@ bool refcount_dec_not_one(refcount_t *r)
 
 		new = val - 1;
 		if (new > val) {
-			WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
+			REFCOUNT_CORRUPTION("underflow; use-after-free");
 			return true;
 		}
 

---
base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
change-id: 20260812-refcount-bug-on-data-corruption-f1a566f771e6

Best regards,
--  
Jann Horn <jannh@xxxxxxxxxx>