selinux: allow context mounts on tmpfs, ramfs, devpts within user namespaces

commit aad82892 ("selinux: Add support for
unprivileged mounts from user namespaces") prohibited any use of context
mount options within non-init user namespaces.  However, this breaks
use of context mount options for tmpfs mounts within user namespaces,
which are being used by Docker/runc.  There is no reason to block such
usage for tmpfs, ramfs or devpts.  Exempt these filesystem types
from this restriction.

Before:
sh$ userns_child_exec  -p -m -U -M '0 1000 1' -G '0 1000 1' bash
sh# mount -t tmpfs -o context=system_u:object_r:user_tmp_t:s0:c13 none /tmp
mount: tmpfs is write-protected, mounting read-only
mount: cannot mount tmpfs read-only

After:
sh$ userns_child_exec  -p -m -U -M '0 1000 1' -G '0 1000 1' bash
sh# mount -t tmpfs -o context=system_u:object_r:user_tmp_t:s0:c13 none /tmp
sh# ls -Zd /tmp
unconfined_u:object_r:user_tmp_t:s0:c13 /tmp
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -834,10 +834,14 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 	}
 
 	/*
-	 * If this is a user namespace mount, no contexts are allowed
-	 * on the command line and security labels must be ignored.
+	 * If this is a user namespace mount and the filesystem type is not
+	 * explicitly whitelisted, then no contexts are allowed on the command
+	 * line and security labels must be ignored.
 	 */
-	if (sb->s_user_ns != &init_user_ns) {
+	if (sb->s_user_ns != &init_user_ns &&
+	    strcmp(sb->s_type->name, "tmpfs") &&
+	    strcmp(sb->s_type->name, "ramfs") &&
+	    strcmp(sb->s_type->name, "devpts")) {
 		if (context_sid || fscontext_sid || rootcontext_sid ||
 		    defcontext_sid) {
 			rc = -EACCES;