[PATCH v2] staging: android: ion: check for kref overflow

[Daniel Rosenberg <drosen@google.com>]

Userspace can cause the kref to handles to increment
arbitrarily high. Ensure it does not overflow.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
---
v2: Fixed patch corruption :(

This patch is against 4.4. It does not apply to master due to a large
rework of ion in 4.12 which removed the affected functions altogther.
4c23cbff073f3b9b ("staging: android: ion: Remove import interface")

It applies from 3.18 to 4.11, although with a trivial conflict resolution
for the later branches.
 drivers/staging/android/ion/ion.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
index 374f840f31a48..47cb163da9a07 100644
--- a/drivers/staging/android/ion/ion.c
+++ b/drivers/staging/android/ion/ion.c
@@ -15,6 +15,7 @@
  *
  */
 
+#include <linux/atomic.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/file.h>
@@ -387,6 +388,16 @@ static void ion_handle_get(struct ion_handle *handle)
 	kref_get(&handle->ref);
 }
 
+/* Must hold the client lock */
+static struct ion_handle *ion_handle_get_check_overflow(
+					struct ion_handle *handle)
+{
+	if (atomic_read(&handle->ref.refcount) + 1 == 0)
+		return ERR_PTR(-EOVERFLOW);
+	ion_handle_get(handle);
+	return handle;
+}
+
 static int ion_handle_put_nolock(struct ion_handle *handle)
 {
 	int ret;
@@ -433,9 +444,9 @@ static struct ion_handle *ion_handle_get_by_id_nolock(struct ion_client *client,
 
 	handle = idr_find(&client->idr, id);
 	if (handle)
-		ion_handle_get(handle);
+		return ion_handle_get_check_overflow(handle);
 
-	return handle ? handle : ERR_PTR(-EINVAL);
+	return ERR_PTR(-EINVAL);
 }
 
 struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
@@ -1202,7 +1213,7 @@ struct ion_handle *ion_import_dma_buf(struct ion_client *client, int fd)
 	/* if a handle exists for this buffer just take a reference to it */
 	handle = ion_handle_lookup(client, buffer);
 	if (!IS_ERR(handle)) {
-		ion_handle_get(handle);
+		handle = ion_handle_get_check_overflow(handle);
 		mutex_unlock(&client->lock);
 		goto end;
 	}
-- 
2.18.0.865.gffc8e1a3cd6-goog