[PATCH] binder: fix BUG_ON found by selinux-testsuite

[Todd Kjos <tkjos@android.com>]

The selinux-testsuite found an issue resulting in a BUG_ON()
where a conditional relied on a size_t going negative when
checking the validity of a buffer offset.

Fixes: 7a67a39320df ("binder: add function to copy binder object from buffer")
Reported-by: Paul Moore <paul@paul-moore.com>
Tested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Todd Kjos <tkjos@google.com>
---
Please add to 5.1 (fixes problem introduced in 5.1-rc1)

 drivers/android/binder.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 8685882da64cd..4b9c7ca492e6d 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2057,7 +2057,8 @@ static size_t binder_get_object(struct binder_proc *proc,
 	size_t object_size = 0;
 
 	read_size = min_t(size_t, sizeof(*object), buffer->data_size - offset);
-	if (read_size < sizeof(*hdr) || !IS_ALIGNED(offset, sizeof(u32)))
+	if (offset > buffer->data_size || read_size < sizeof(*hdr) ||
+	    !IS_ALIGNED(offset, sizeof(u32)))
 		return 0;
 	binder_alloc_copy_from_buffer(&proc->alloc, object, buffer,
 				      offset, read_size);
-- 
2.21.0.225.g810b269d1ac-goog