procd: mount /dev with noexec

/dev is writable. Allowing execution inside it makes it a possible attack
vector. Kees Cook recently sent a kernel patch [1] in order to mount /dev as
noexec and nosuid for systems which rely on CONFIG_DEVTMPFS_MOUNT=y to
create/populate /dev, which isn't our case (it's procd's responsibility).

Add noexec to the /dev mount flags, since we already use nosuid (and keep the
coldplug flags symmetric, while at it). This carries the risk of breaking very
old, pre-KMS graphics drivers [2], but it shouldn't be a problem for systems
built in the last ~15 years. The vast majority of our targets doesn't have a
GPU, anyway. :)

[1] https://lore.kernel.org/all/YcMfDOyrg647RCmd@debian-BULLSEYE-live-builder-AMD64/
[2] https://lore.kernel.org/all/CAPXgP12e5LpN6XVxaXOHhH=u8XXN==2reTaJDCoCk4tP4QduDQ@mail.gmail.com/

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

initd/early.c

@@ -61,7 +61,7 @@ early_mounts(void)
 		mount("proc", "/proc", "proc", MS_NOATIME | MS_NODEV | MS_NOEXEC | MS_NOSUID, 0);
 		mount("sysfs", "/sys", "sysfs", MS_NOATIME | MS_NODEV | MS_NOEXEC | MS_NOSUID, 0);
 		mount("cgroup2", "/sys/fs/cgroup", "cgroup2",  MS_NODEV | MS_NOEXEC | MS_NOSUID | MS_RELATIME, "nsdelegate");
-		mount("tmpfs", "/dev", "tmpfs", MS_NOATIME | MS_NOSUID, "mode=0755,size=512K");
+		mount("tmpfs", "/dev", "tmpfs", MS_NOATIME | MS_NOEXEC | MS_NOSUID, "mode=0755,size=512K");
 		ignore(symlink("/tmp/shm", "/dev/shm"));
 		mkdir("/dev/pts", 0755);
 		mount("devpts", "/dev/pts", "devpts", MS_NOATIME | MS_NOEXEC | MS_NOSUID, "mode=600");

plug/coldplug.c

@@ -48,7 +48,7 @@ void procd_coldplug(void)
 	if (!is_container()) {
 		umount2("/dev/pts", MNT_DETACH);
 		umount2("/dev/", MNT_DETACH);
-		mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755,size=512K");
+		mount("tmpfs", "/dev", "tmpfs", MS_NOATIME | MS_NOEXEC | MS_NOSUID, "mode=0755,size=512K");
 		mkdir("/dev/pts", 0755);
 		mount("devpts", "/dev/pts", "devpts", MS_NOEXEC | MS_NOSUID, 0);
 	}