UPSTREAM: binder: fix incorrect calculation for num_valid

commit 16981742717b04644a41052570fb502682a315d2 upstream.

For BINDER_TYPE_PTR and BINDER_TYPE_FDA transactions, the
num_valid local was calculated incorrectly causing the
range check in binder_validate_ptr() to miss out-of-bounds
offsets.

Fixes: bde4a19fc04f ("binder: use userspace pointer as base of buffer space")
Change-Id: Ida77db13d8e5b726f0b14513f55c2b30277338cd
Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191213202531.55010-1-tkjos@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 145988638
Signed-off-by: Todd Kjos <tkjos@google.com>

drivers/android/binder.c

@@ -3428,7 +3428,7 @@ static void binder_transaction(struct binder_proc *proc,
 			binder_size_t parent_offset;
 			struct binder_fd_array_object *fda =
 				to_binder_fd_array_object(hdr);
-			size_t num_valid = (buffer_offset - off_start_offset) *
+			size_t num_valid = (buffer_offset - off_start_offset) /
 						sizeof(binder_size_t);
 			struct binder_buffer_object *parent =
 				binder_validate_ptr(target_proc, t->buffer,
@@ -3502,7 +3502,7 @@ static void binder_transaction(struct binder_proc *proc,
 				t->buffer->user_data + sg_buf_offset;
 			sg_buf_offset += ALIGN(bp->length, sizeof(u64));
 
-			num_valid = (buffer_offset - off_start_offset) *
+			num_valid = (buffer_offset - off_start_offset) /
 					sizeof(binder_size_t);
 			ret = binder_fixup_parent(t, thread, bp,
 						  off_start_offset,