[PATCH 6/7] selinux: Ignore security labels on user namespace mounts

[Seth Forshee <seth.forshee@canonical.com>]

Unprivileged users should not be able to supply security labels
in filesystems, nor should they be able to supply security
contexts in unprivileged mounts. For any mount where s_user_ns is
not init_user_ns, force the use of SECURITY_FS_USE_NONE behavior
and return EPERM if any contexts are supplied in the mount
options.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 security/selinux/hooks.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 459e71ddbc9d..eeb71e45ab82 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -732,6 +732,19 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 	    !strcmp(sb->s_type->name, "pstore"))
 		sbsec->flags |= SE_SBGENFS;
 
+	/*
+	 * If this is a user namespace mount, no contexts are allowed
+	 * on the command line and security labels mus be ignored.
+	 */
+	if (sb->s_user_ns != &init_user_ns) {
+		if (context_sid || fscontext_sid || rootcontext_sid ||
+		    defcontext_sid)
+			return -EPERM;
+		sbsec->behavior = SECURITY_FS_USE_NONE;
+		goto out_set_opts;
+	}
+
+
 	if (!sbsec->behavior) {
 		/*
 		 * Determine the labeling behavior to use for this
@@ -813,6 +826,7 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 		sbsec->def_sid = defcontext_sid;
 	}
 
+out_set_opts:
 	rc = sb_finish_set_opts(sb);
 out:
 	mutex_unlock(&sbsec->lock);
-- 
1.9.1