binder: Set end of SG buffer area properly.

In case the target node requests a security context, the
extra_buffers_size is increased with the size of the security context.
But, that size is not available for use by regular scatter-gather
buffers; make sure the ending of that buffer is marked correctly.
Acked-by: NTodd Kjos <tkjos@google.com>
Fixes: ec74136d ("binder: create node flag to request sender's security context")
Signed-off-by: NMartijn Coenen <maco@android.com>
Cc: stable@vger.kernel.org # 5.1+
Link: https://lore.kernel.org/r/20190709110923.220736-1-maco@android.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -3239,7 +3239,8 @@ static void binder_transaction(struct binder_proc *proc,
 	buffer_offset = off_start_offset;
 	off_end_offset = off_start_offset + tr->offsets_size;
 	sg_buf_offset = ALIGN(off_end_offset, sizeof(void *));
-	sg_buf_end_offset = sg_buf_offset + extra_buffers_size;
+	sg_buf_end_offset = sg_buf_offset + extra_buffers_size -
+		ALIGN(secctx_sz, sizeof(u64));
 	off_min = 0;
 	for (buffer_offset = off_start_offset; buffer_offset < off_end_offset;
 	     buffer_offset += sizeof(binder_size_t)) {