[PATCH] binder: fix async_free_space accounting for empty parcels

[Todd Kjos]

In 4.13, commit 74310e06be4d ("android: binder: Move buffer out of area shared with user space")
fixed a kernel structure visibility issue. As part of that patch,
sizeof(void *) was used as the buffer size for 0-length data payloads so
the driver could detect abusive clients sending 0-length asynchronous
transactions to a server by enforcing limits on async_free_size.

Unfortunately, on the "free" side, the accounting of async_free_space
did not add the sizeof(void *) back. The result was that up to 8-bytes of
async_free_space were leaked on every async transaction of 8-bytes or
less.  These small transactions are uncommon, so this accounting issue
has gone undetected for several years.

The fix is to use "buffer_size" (the allocated buffer size) instead of
"size" (the logical buffer size) when updating the async_free_space
during the free operation. These are the same except for this
corner case of asynchronous transactions with payloads < 8 bytes.

Fixes: 74310e06be4d ("android: binder: Move buffer out of area shared with user space")
Signed-off-by: Todd Kjos <tkjos@xxxxxxxxxx>
---
 drivers/android/binder_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 340515f54498..47bc74a8c7b6 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -671,7 +671,7 @@ static void binder_free_buf_locked(struct binder_alloc *alloc,
 	BUG_ON(buffer->user_data > alloc->buffer + alloc->buffer_size);
 
 	if (buffer->async_transaction) {
-		alloc->free_async_space += size + sizeof(struct binder_buffer);
+		alloc->free_async_space += buffer_size + sizeof(struct binder_buffer);
 
 		binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC_ASYNC,
 			     "%d: binder_free_buf size %zd async free %zd\n",
-- 
2.34.1.307.g9b7440fafd-goog