samples/seccomp: Zero out members based on seccomp_notif_sizes

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>
Link: https://lore.kernel.org/r/20191230203503.4925-1-sargun@sargun.me
Fixes: fec7b6690541 ("samples: add an example of seccomp user trap")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
index 6d0125ca8af714e712babe7a425eab4695efed04..20291ec6489f31e4d3380c6d102711fc8e2c9692 100644 (file)
--- a/samples/seccomp/user-trap.c
+++ b/samples/seccomp/user-trap.c
@@ -298,14 +298,14 @@ int main(void)
                req = malloc(sizes.seccomp_notif);
                if (!req)
                        goto out_close;
-               memset(req, 0, sizeof(*req));
 
                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, sizes.seccomp_notif);
                        if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
                                perror("ioctl recv");
                                goto out_resp;