On Mon, Dec 20, 2021 at 11:02 AM Todd Kjos tkjos@google.com wrote:

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@google.com

I forgot to CC stable. This applies to all stable branches back to 4.14. Cc: stable@vger.kernel.org # 4.14+

---

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