[PATCH v2] kref: Avoid null pointer dereference after WARN

[Kees Cook <keescook@chromium.org>]

From: Daniel Micay <danielmicay@gmail.com>

The WARN_ON() checking for a NULL release pointer was (sensibly)
removed in commit ec48c940da6c ("kref: remove WARN_ON for NULL release
functions") since it offered no protection at all about calling a NULL
release pointer. However, it should instead be a BUG() since continuing
with a NULL release pointer will lead to a NULL pointer execution
anyway. Systems with an incorrectly set mmap_min_addr and no PXN/SMEP
protection would be left open to executing userspace memory.

The kref_put() case is extracted from PaX, and Kees Cook noted it should
be extended to the other two cases.

Comparison of my build with WARN, with nothing, and with BUG:

   text            data    bss         dec       hex     filename
  11300251        5586597 13955072    30841920  1d69c40 vmlinux.warn
  11298136        5586597 13955072    30839805  1d693fd vmlinux.none
  11300062        5586629 13955072    30841763  1d69ba3 vmlinux.bug

Signed-off-by: Daniel Micay <danielmicay@gmail.com>
[kees: clarify commit log, refreshed diff, moved into if statement]
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/kref.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 29220724bf1c..651a12d2425f 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -67,6 +67,7 @@ static inline void kref_get(struct kref *kref)
 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
 {
 	if (refcount_dec_and_test(&kref->refcount)) {
+		BUG_ON(release == NULL);
 		release(kref);
 		return 1;
 	}
@@ -78,6 +79,7 @@ static inline int kref_put_mutex(struct kref *kref,
 				 struct mutex *lock)
 {
 	if (refcount_dec_and_mutex_lock(&kref->refcount, lock)) {
+		BUG_ON(release == NULL);
 		release(kref);
 		return 1;
 	}
@@ -89,6 +91,7 @@ static inline int kref_put_lock(struct kref *kref,
 				spinlock_t *lock)
 {
 	if (refcount_dec_and_lock(&kref->refcount, lock)) {
+		BUG_ON(release == NULL);
 		release(kref);
 		return 1;
 	}
-- 
2.7.4


-- 
Kees Cook
Pixel Security