security,overlayfs: Provide security hook for copy up of xattrs for overlay file

Provide a security hook which is called when xattrs of a file are being
copied up. This hook is called once for each xattr and LSM can return
0 if the security module wants the xattr to be copied up, 1 if the
security module wants the xattr to be discarded on the copy, -EOPNOTSUPP
if the security module does not handle/manage the xattr, or a -errno
upon an error.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: whitespace cleanup for checkpatch.pl]
Signed-off-by: Paul Moore <paul@paul-moore.com>

fs/overlayfs/copy_up.c

@@ -103,6 +103,13 @@ retry:
 			goto retry;
 		}
 
+		error = security_inode_copy_up_xattr(name);
+		if (error < 0 && error != -EOPNOTSUPP)
+			break;
+		if (error == 1) {
+			error = 0;
+			continue; /* Discard */
+		}
 		error = vfs_setxattr(new, name, value, size, 0);
 		if (error)
 			break;

include/linux/lsm_hooks.h

@@ -410,6 +410,14 @@
  *	@src indicates the union dentry of file that is being copied up.
  *	@new pointer to pointer to return newly allocated creds.
  *	Returns 0 on success or a negative error code on error.
+ * @inode_copy_up_xattr:
+ *	Filter the xattrs being copied up when a unioned file is copied
+ *	up from a lower layer to the union/overlay layer.
+ *	@name indicates the name of the xattr.
+ *	Returns 0 to accept the xattr, 1 to discard the xattr, -EOPNOTSUPP if
+ *	security module does not know about attribute or a negative error code
+ *	to abort the copy up. Note that the caller is responsible for reading
+ *	and writing the xattrs as this hook is merely a filter.
  *
  * Security hooks for file operations
  *
@@ -1435,6 +1443,7 @@ union security_list_options {
 					size_t buffer_size);
 	void (*inode_getsecid)(struct inode *inode, u32 *secid);
 	int (*inode_copy_up)(struct dentry *src, struct cred **new);
+	int (*inode_copy_up_xattr)(const char *name);
 
 	int (*file_permission)(struct file *file, int mask);
 	int (*file_alloc_security)(struct file *file);
@@ -1707,6 +1716,7 @@ struct security_hook_heads {
 	struct list_head inode_listsecurity;
 	struct list_head inode_getsecid;
 	struct list_head inode_copy_up;
+	struct list_head inode_copy_up_xattr;
 	struct list_head file_permission;
 	struct list_head file_alloc_security;
 	struct list_head file_free_security;

include/linux/security.h

@@ -283,6 +283,7 @@ int security_inode_setsecurity(struct inode *inode, const char *name, const void
 int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer_size);
 void security_inode_getsecid(struct inode *inode, u32 *secid);
 int security_inode_copy_up(struct dentry *src, struct cred **new);
+int security_inode_copy_up_xattr(const char *name);
 int security_file_permission(struct file *file, int mask);
 int security_file_alloc(struct file *file);
 void security_file_free(struct file *file);
@@ -764,6 +765,11 @@ static inline int security_inode_copy_up(struct dentry *src, struct cred **new)
 	return 0;
 }
 
+static inline int security_inode_copy_up_xattr(const char *name)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int security_file_permission(struct file *file, int mask)
 {
 	return 0;

security/security.c

@@ -754,6 +754,12 @@ int security_inode_copy_up(struct dentry *src, struct cred **new)
 }
 EXPORT_SYMBOL(security_inode_copy_up);
 
+int security_inode_copy_up_xattr(const char *name)
+{
+	return call_int_hook(inode_copy_up_xattr, -EOPNOTSUPP, name);
+}
+EXPORT_SYMBOL(security_inode_copy_up_xattr);
+
 int security_file_permission(struct file *file, int mask)
 {
 	int ret;
@@ -1692,6 +1698,8 @@ struct security_hook_heads security_hook_heads = {
 		LIST_HEAD_INIT(security_hook_heads.inode_getsecid),
 	.inode_copy_up =
 		LIST_HEAD_INIT(security_hook_heads.inode_copy_up),
+	.inode_copy_up_xattr =
+		LIST_HEAD_INIT(security_hook_heads.inode_copy_up_xattr),
 	.file_permission =
 		LIST_HEAD_INIT(security_hook_heads.file_permission),
 	.file_alloc_security =