security/selinux: allow security_sb_clone_mnt_opts to enable/disable native labeling behavior

When an NFSv4 client performs a mount operation, it first mounts the
NFSv4 root and then does path walk to the exported path and performs a
submount on that, cloning the security mount options from the root's
superblock to the submount's superblock in the process.

Unless the NFS server has an explicit fsid=0 export with the
"security_label" option, the NFSv4 root superblock will not have
SBLABEL_MNT set, and neither will the submount superblock after cloning
the security mount options.  As a result, setxattr's of security labels
over NFSv4.2 will fail.  In a similar fashion, NFSv4.2 mounts mounted
with the context= mount option will not show the correct labels because
the nfs_server->caps flags of the cloned superblock will still have
NFS_CAP_SECURITY_LABEL set.

Allowing the NFSv4 client to enable or disable SECURITY_LSM_NATIVE_LABELS
behavior will ensure that the SBLABEL_MNT flag has the correct value
when the client traverses from an exported path without the
"security_label" option to one with the "security_label" option and
vice versa.  Similarly, checking to see if SECURITY_LSM_NATIVE_LABELS is
set upon return from security_sb_clone_mnt_opts() and clearing
NFS_CAP_SECURITY_LABEL if necessary will allow the correct labels to
be displayed for NFSv4.2 mounts mounted with the context= mount option.

Resolves: https://github.com/SELinuxProject/selinux-kernel/issues/35

Signed-off-by: Scott Mayhew <[email protected]>
Reviewed-by: Stephen Smalley <[email protected]>
Tested-by: Stephen Smalley <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

security/selinux/hooks.c

@@ -525,8 +525,16 @@ static int sb_finish_set_opts(struct super_block *sb)
 	}
 
 	sbsec->flags |= SE_SBINITIALIZED;
+
+	/*
+	 * Explicitly set or clear SBLABEL_MNT.  It's not sufficient to simply
+	 * leave the flag untouched because sb_clone_mnt_opts might be handing
+	 * us a superblock that needs the flag to be cleared.
+	 */
 	if (selinux_is_sblabel_mnt(sb))
 		sbsec->flags |= SBLABEL_MNT;
+	else
+		sbsec->flags &= ~SBLABEL_MNT;
 
 	/* Initialize the root inode. */
 	rc = inode_doinit_with_dentry(root_inode, root);
@@ -959,8 +967,11 @@ mismatch:
 }
 
 static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
-					struct super_block *newsb)
+					struct super_block *newsb,
+					unsigned long kern_flags,
+					unsigned long *set_kern_flags)
 {
+	int rc = 0;
 	const struct superblock_security_struct *oldsbsec = oldsb->s_security;
 	struct superblock_security_struct *newsbsec = newsb->s_security;
 
@@ -975,6 +986,13 @@ static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
 	if (!ss_initialized)
 		return 0;
 
+	/*
+	 * Specifying internal flags without providing a place to
+	 * place the results is not allowed.
+	 */
+	if (kern_flags && !set_kern_flags)
+		return -EINVAL;
+
 	/* how can we clone if the old one wasn't set up?? */
 	BUG_ON(!(oldsbsec->flags & SE_SBINITIALIZED));
 
@@ -990,6 +1008,18 @@ static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
 	newsbsec->def_sid = oldsbsec->def_sid;
 	newsbsec->behavior = oldsbsec->behavior;
 
+	if (newsbsec->behavior == SECURITY_FS_USE_NATIVE &&
+		!(kern_flags & SECURITY_LSM_NATIVE_LABELS) && !set_context) {
+		rc = security_fs_use(newsb);
+		if (rc)
+			goto out;
+	}
+
+	if (kern_flags & SECURITY_LSM_NATIVE_LABELS && !set_context) {
+		newsbsec->behavior = SECURITY_FS_USE_NATIVE;
+		*set_kern_flags |= SECURITY_LSM_NATIVE_LABELS;
+	}
+
 	if (set_context) {
 		u32 sid = oldsbsec->mntpoint_sid;
 
@@ -1009,8 +1039,9 @@ static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
 	}
 
 	sb_finish_set_opts(newsb);
+out:
 	mutex_unlock(&newsbsec->lock);
-	return 0;
+	return rc;
 }
 
 static int selinux_parse_opts_str(char *options,