[PATCH] samples/seccomp: Zero out datastructures based on seccomp_notif_sizes

From: Sargun Dhillon <sargun@sargun.me>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Sargun Dhillon <sargun@sargun.me>,
	Tycho Andersen <tycho@tycho.ws>,
	Kees Cook <keescook@chromium.org>
Subject: [PATCH] samples/seccomp: Zero out datastructures based on seccomp_notif_sizes
Date: Mon, 30 Dec 2019 12:35:03 -0800	[thread overview]
Message-ID: <20191230203503.4925-1-sargun@sargun.me> (raw)

The sizes by which seccomp_notif and seccomp_notif_resp are allocated are
based on the SECCOMP_GET_NOTIF_SIZES ioctl. This allows for graceful
extension of these datastructures. If userspace zeroes out the
datastructure based on its version, and it is lagging behind the kernel's
version, it will end up sending trailing garbage. On the other hand,
if it is ahead of the kernel version, it will write extra zero space,
and potentially cause corruption.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Tycho Andersen <tycho@tycho.ws>
Cc: Kees Cook <keescook@chromium.org>
---
 samples/seccomp/user-trap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
index 3e31ec0cf4a5..20291ec6489f 100644
--- a/samples/seccomp/user-trap.c
+++ b/samples/seccomp/user-trap.c
@@ -302,10 +302,10 @@ int main(void)
 		resp = malloc(sizes.seccomp_notif_resp);
 		if (!resp)
 			goto out_req;
-		memset(resp, 0, sizeof(*resp));
+		memset(resp, 0, sizes.seccomp_notif_resp);
 
 		while (1) {
-			memset(req, 0, sizeof(*req));
+			memset(req, 0, sizes.seccomp_notif);
 			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
 				perror("ioctl recv");
 				goto out_resp;
-- 
2.20.1

