seccomp: Check that seccomp_notif is zeroed out by the user

This patch is a small change in enforcement of the uapi for
SECCOMP_IOCTL_NOTIF_RECV ioctl. Specifically, the datastructure which
is passed (seccomp_notif) must be zeroed out. Previously any of its
members could be set to nonsense values, and we would ignore it.

This ensures all fields are set to their zero value.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>
Acked-by: Tycho Andersen <tycho@tycho.ws>
Link: https://lore.kernel.org/r/20191229062451.9467-2-sargun@sargun.me
Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 12d2227e5786794260a7018a9141f083b575f732..b6ea3dcb57bfefa47233ada64d74f822b6666ab6 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1026,6 +1026,13 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
        struct seccomp_notif unotif;
        ssize_t ret;
 
+       /* Verify that we're not given garbage to keep struct extensible. */
+       ret = check_zeroed_user(buf, sizeof(unotif));
+       if (ret < 0)
+               return ret;
+       if (!ret)
+               return -EINVAL;
+
        memset(&unotif, 0, sizeof(unotif));
 
        ret = down_interruptible(&filter->notif->request);