[PATCH 5/5] fs: convert userfaultfd_ctx.refcount from atomic_t to refcount_t

[Elena Reshetova]

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.

Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
---
 fs/userfaultfd.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 3c421d0..06c334a 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -28,6 +28,7 @@
 #include <linux/ioctl.h>
 #include <linux/security.h>
 #include <linux/hugetlb.h>
+#include <linux/refcount.h>
 
 static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
 
@@ -52,7 +53,7 @@ struct userfaultfd_ctx {
 	/* a refile sequence protected by fault_pending_wqh lock */
 	struct seqcount refile_seq;
 	/* pseudo fd refcounting */
-	atomic_t refcount;
+	refcount_t refcount;
 	/* userfaultfd syscall flags */
 	unsigned int flags;
 	/* features requested from the userspace */
@@ -142,7 +143,7 @@ static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
  */
 static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
 {
-	if (!atomic_inc_not_zero(&ctx->refcount))
+	if (!refcount_inc_not_zero(&ctx->refcount))
 		BUG();
 }
 
@@ -156,7 +157,7 @@ static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
  */
 static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
 {
-	if (atomic_dec_and_test(&ctx->refcount)) {
+	if (refcount_dec_and_test(&ctx->refcount)) {
 		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
 		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
 		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
@@ -607,7 +608,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
 			return -ENOMEM;
 		}
 
-		atomic_set(&ctx->refcount, 1);
+		refcount_set(&ctx->refcount, 1);
 		ctx->flags = octx->flags;
 		ctx->state = UFFD_STATE_RUNNING;
 		ctx->features = octx->features;
@@ -1840,7 +1841,7 @@ static struct file *userfaultfd_file_create(int flags)
 	if (!ctx)
 		goto out;
 
-	atomic_set(&ctx->refcount, 1);
+	refcount_set(&ctx->refcount, 1);
 	ctx->flags = flags;
 	ctx->features = 0;
 	ctx->state = UFFD_STATE_WAIT_API;
-- 
2.7.4