Avoid use of untrusted security labels when s_user_ns !=
init_user_ns:
 - smk_fetch: refuse to read labels from disk
 - smack_inode_init_security: return -ENOTSUPP
 - smack_d_instantiate: don't use security xattrs from disk

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 security/smack/smack_lsm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index a143328f75eb..6a849da94f47 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -255,6 +255,9 @@ static struct smack_known *smk_fetch(const char *name, struct inode *ip,
 	char *buffer;
 	struct smack_known *skp = NULL;
 
+	if (ip->i_sb->s_user_ns != &init_user_ns)
+		return NULL;
+
 	if (ip->i_op->getxattr == NULL)
 		return ERR_PTR(-EOPNOTSUPP);
 
@@ -833,6 +836,9 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 	struct smack_known *dsp = smk_of_inode(dir);
 	int may;
 
+	if (inode->i_sb->s_user_ns != &init_user_ns)
+		return -ENOTSUPP;
+
 	if (name)
 		*name = XATTR_SMACK_SUFFIX;
 
@@ -3176,11 +3182,13 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
 		}
 		/*
 		 * No xattr support means, alas, no SMACK label.
-		 * Use the aforeapplied default.
+		 * Use the aforeapplied default. Also don't use
+		 * xattrs from userns mounts.
 		 * It would be curious if the label of the task
 		 * does not match that assigned.
 		 */
-		if (inode->i_op->getxattr == NULL)
+		if (inode->i_sb->s_user_ns != &init_user_ns ||
+		    inode->i_op->getxattr == NULL)
 			break;
 		/*
 		 * Get the dentry for xattr.
-- 
1.9.1