binder: fix async space check for 0-sized buffers

commit 3091c21d3e9322428691ce0b7a0cfa9c0b239eeb upstream.

Move the padding of 0-sized buffers to an earlier stage to account for
this round up during the alloc->free_async_space check.

Fixes: 74310e06be ("android: binder: Move buffer out of area shared with user space")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-5-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 05088b886fea59cc827e5b5cedb66165cf532f72)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>

drivers/android/binder_alloc.c

@@ -429,6 +429,10 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
 				alloc->pid, extra_buffers_size);
 		return ERR_PTR(-EINVAL);
 	}
+
+	/* Pad 0-size buffers so they get assigned unique addresses */
+	size = max(size, sizeof(void *));
+
 	if (is_async &&
 	    alloc->free_async_space < size + sizeof(struct binder_buffer)) {
 		binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC,
@@ -437,9 +441,6 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
 		return ERR_PTR(-ENOSPC);
 	}
 
-	/* Pad 0-size buffers so they get assigned unique addresses */
-	size = max(size, sizeof(void *));
-
 	while (n) {
 		buffer = rb_entry(n, struct binder_buffer, rb_node);
 		BUG_ON(!buffer->free);