SELinux: Handle opening of a unioned file

Handle the opening of a unioned file by trying to derive the label that would
be attached to the union-layer inode if it doesn't exist.

If the union-layer inode does exist (as it necessarily does in overlayfs, but
not in unionmount), we assume that it has the right label and use that.
Otherwise we try to get it from the superblock.

If the superblock has a globally-applied label, we use that, otherwise we try
to transition to an appropriate label.  This union label is then stored in the
file_security_struct.

We then perform an additional check to make sure that the calling task is
granted permission by the union-layer inode label to open the file in addition
to a check to make sure that the task is granted permission to open the lower
file with the lower inode label.

Signed-off-by: David Howells <dhowells@redhat.com>

security/selinux/hooks.c

@@ -3474,10 +3474,72 @@ static int selinux_file_receive(struct file *file)
 	return file_has_perm(cred, file, file_to_av(file));
 }
 
+/*
+ * We have a file opened on a unioned file system that falls through to a file
+ * on a lower layer.  If there is a union inode, we try to get the label from
+ * that, otherwise we need to get it from the superblock.
+ *
+ * file->f_path points to the union layer and file->f_inode points to the lower
+ * layer.
+ */
+static int selinux_file_open_union(struct file *file,
+				   struct file_security_struct *fsec,
+				   const struct cred *cred)
+{
+	const struct superblock_security_struct *sbsec;
+	const struct inode_security_struct *isec, *dsec, *fisec;
+	const struct task_security_struct *tsec = current_security();
+	struct common_audit_data ad;
+	struct dentry *union_dentry = file->f_path.dentry;
+	const struct inode *union_inode = d_inode(union_dentry);
+	const struct inode *lower_inode = file_inode(file);
+	struct dentry *dir;
+	int rc;
+
+	sbsec = union_dentry->d_sb->s_security;
+
+	if (union_inode) {
+		isec = union_inode->i_security;
+		fsec->union_isid = isec->sid;
+	} else if ((sbsec->flags & SE_SBINITIALIZED) &&
+		   (sbsec->behavior == SECURITY_FS_USE_MNTPOINT)) {
+		fsec->union_isid = sbsec->mntpoint_sid;
+	} else {
+		dir = dget_parent(union_dentry);
+		dsec = d_inode(dir)->i_security;
+
+		rc = security_transition_sid(
+			tsec->sid, dsec->sid,
+			inode_mode_to_security_class(lower_inode->i_mode),
+			&union_dentry->d_name,
+			&fsec->union_isid);
+		dput(dir);
+		if (rc) {
+			pr_warn("%s:  security_transition_sid failed, rc=%d (name=%pD)\n",
+				__func__, -rc, file);
+			return rc;
+		}
+	}
+
+	/* We need to check that the union file is allowed to be opened as well
+	 * as checking that the lower file is allowed to be opened.
+	 */
+	if (unlikely(IS_PRIVATE(lower_inode)))
+		return 0;
+
+	ad.type = LSM_AUDIT_DATA_PATH;
+	ad.u.path = file->f_path;
+
+	fisec = lower_inode->i_security;
+	return avc_has_perm(cred_sid(cred), fsec->union_isid, fisec->sclass,
+			    open_file_to_av(file), &ad);
+}
+
 static int selinux_file_open(struct file *file, const struct cred *cred)
 {
 	struct file_security_struct *fsec;
 	struct inode_security_struct *isec;
+	int rc;
 
 	fsec = file->f_security;
 	isec = file_inode(file)->i_security;
@@ -3498,6 +3560,13 @@ static int selinux_file_open(struct file *file, const struct cred *cred)
 	 * new inode label or new policy.
 	 * This check is not redundant - do not remove.
 	 */
+
+	if (d_inode(file->f_path.dentry) != file->f_inode) {
+		rc = selinux_file_open_union(file, fsec, cred);
+		if (rc < 0)
+			return rc;
+	}
+
 	return file_path_has_perm(cred, file, open_file_to_av(file));
 }
 

security/selinux/include/objsec.h

@@ -54,6 +54,7 @@ struct file_security_struct {
 	u32 sid;		/* SID of open file description */
 	u32 fown_sid;		/* SID of file owner (for SIGIO) */
 	u32 isid;		/* SID of inode at the time of file open */
+	u32 union_isid;		/* SID of would-be inodes in union top (or 0) */
 	u32 pseqno;		/* Policy seqno at the time of file open */
 };