binder: check for overflow when alloc for security context

When allocating space in the target buffer for the security context,
make sure the extra_buffers_size doesn't overflow. This can only
happen if the given size is invalid, but an overflow can turn it
into a valid size. Fail the transaction if an overflow is detected.
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -3121,6 +3121,7 @@ static void binder_transaction(struct binder_proc *proc,
 
 	if (target_node && target_node->txn_security_ctx) {
 		u32 secid;
+		size_t added_size;
 
 		security_task_getsecid(proc->tsk, &secid);
 		ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
@@ -3130,7 +3131,15 @@ static void binder_transaction(struct binder_proc *proc,
 			return_error_line = __LINE__;
 			goto err_get_secctx_failed;
 		}
-		extra_buffers_size += ALIGN(secctx_sz, sizeof(u64));
+		added_size = ALIGN(secctx_sz, sizeof(u64));
+		extra_buffers_size += added_size;
+		if (extra_buffers_size < added_size) {
+			/* integer overflow of extra_buffers_size */
+			return_error = BR_FAILED_REPLY;
+			return_error_param = EINVAL;
+			return_error_line = __LINE__;
+			goto err_bad_extra_size;
+		}
 	}
 
 	trace_binder_transaction(reply, t, target_node);
@@ -3480,6 +3489,7 @@ static void binder_transaction(struct binder_proc *proc,
 	t->buffer->transaction = NULL;
 	binder_alloc_free_buf(&target_proc->alloc, t->buffer);
 err_binder_alloc_buf_failed:
+err_bad_extra_size:
 	if (secctx)
 		security_release_secctx(secctx, secctx_sz);
 err_get_secctx_failed: