security, overlayfs: Provide hook to correctly label newly created files

During a new file creation we need to make sure new file is created with the
right label. New file is created in upper/ so effectively file should get
label as if task had created file in upper/.

We switched to mounter's creds for actual file creation. Also if there is a
whiteout present, then file will be created in work/ dir first and then
renamed in upper. In none of the cases file will be labeled as we want it to
be.

This patch introduces a new hook dentry_create_files_as(), which determines
the label/context dentry will get if it had been created by task in upper
and modify passed set of creds appropriately. Caller makes use of these new
creds for file creation.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: fix whitespace issues found with checkpatch.pl]
[PM: changes to use stat->mode in ovl_create_or_link()]
Signed-off-by: Paul Moore <paul@paul-moore.com>

fs/overlayfs/dir.c

@@ -435,6 +435,15 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 	if (override_cred) {
 		override_cred->fsuid = inode->i_uid;
 		override_cred->fsgid = inode->i_gid;
+		if (!hardlink) {
+			err = security_dentry_create_files_as(dentry,
+					stat->mode, &dentry->d_name, old_cred,
+					override_cred);
+			if (err) {
+				put_cred(override_cred);
+				goto out_revert_creds;
+			}
+		}
 		put_cred(override_creds(override_cred));
 		put_cred(override_cred);
 
@@ -445,6 +454,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 			err = ovl_create_over_whiteout(dentry, inode, stat,
 							link, hardlink);
 	}
+out_revert_creds:
 	revert_creds(old_cred);
 	if (!err) {
 		struct inode *realinode = d_inode(ovl_dentry_upper(dentry));

include/linux/lsm_hooks.h

@@ -151,6 +151,16 @@
  *	@name name of the last path component used to create file
  *	@ctx pointer to place the pointer to the resulting context in.
  *	@ctxlen point to place the length of the resulting context.
+ * @dentry_create_files_as:
+ *	Compute a context for a dentry as the inode is not yet available
+ *	and set that context in passed in creds so that new files are
+ *	created using that context. Context is calculated using the
+ *	passed in creds and not the creds of the caller.
+ *	@dentry dentry to use in calculating the context.
+ *	@mode mode used to determine resource type.
+ *	@name name of the last path component used to create file
+ *	@old creds which should be used for context calculation
+ *	@new creds to modify
  *
  *
  * Security hooks for inode operations.
@@ -1375,6 +1385,10 @@ union security_list_options {
 	int (*dentry_init_security)(struct dentry *dentry, int mode,
 					const struct qstr *name, void **ctx,
 					u32 *ctxlen);
+	int (*dentry_create_files_as)(struct dentry *dentry, int mode,
+					struct qstr *name,
+					const struct cred *old,
+					struct cred *new);
 
 
 #ifdef CONFIG_SECURITY_PATH
@@ -1675,6 +1689,7 @@ struct security_hook_heads {
 	struct list_head sb_clone_mnt_opts;
 	struct list_head sb_parse_opts_str;
 	struct list_head dentry_init_security;
+	struct list_head dentry_create_files_as;
 #ifdef CONFIG_SECURITY_PATH
 	struct list_head path_unlink;
 	struct list_head path_mkdir;

include/linux/security.h

@@ -242,6 +242,10 @@ int security_sb_parse_opts_str(char *options, struct security_mnt_opts *opts);
 int security_dentry_init_security(struct dentry *dentry, int mode,
 					const struct qstr *name, void **ctx,
 					u32 *ctxlen);
+int security_dentry_create_files_as(struct dentry *dentry, int mode,
+					struct qstr *name,
+					const struct cred *old,
+					struct cred *new);
 
 int security_inode_alloc(struct inode *inode);
 void security_inode_free(struct inode *inode);
@@ -600,6 +604,14 @@ static inline int security_dentry_init_security(struct dentry *dentry,
 	return -EOPNOTSUPP;
 }
 
+static inline int security_dentry_create_files_as(struct dentry *dentry,
+						  int mode, struct qstr *name,
+						  const struct cred *old,
+						  struct cred *new)
+{
+	return 0;
+}
+
 
 static inline int security_inode_init_security(struct inode *inode,
 						struct inode *dir,

security/security.c

@@ -364,6 +364,15 @@ int security_dentry_init_security(struct dentry *dentry, int mode,
 }
 EXPORT_SYMBOL(security_dentry_init_security);
 
+int security_dentry_create_files_as(struct dentry *dentry, int mode,
+				    struct qstr *name,
+				    const struct cred *old, struct cred *new)
+{
+	return call_int_hook(dentry_create_files_as, 0, dentry, mode,
+				name, old, new);
+}
+EXPORT_SYMBOL(security_dentry_create_files_as);
+
 int security_inode_init_security(struct inode *inode, struct inode *dir,
 				 const struct qstr *qstr,
 				 const initxattrs initxattrs, void *fs_data)
@@ -1635,6 +1644,8 @@ struct security_hook_heads security_hook_heads = {
 		LIST_HEAD_INIT(security_hook_heads.sb_parse_opts_str),
 	.dentry_init_security =
 		LIST_HEAD_INIT(security_hook_heads.dentry_init_security),
+	.dentry_create_files_as =
+		LIST_HEAD_INIT(security_hook_heads.dentry_create_files_as),
 #ifdef CONFIG_SECURITY_PATH
 	.path_unlink =	LIST_HEAD_INIT(security_hook_heads.path_unlink),
 	.path_mkdir =	LIST_HEAD_INIT(security_hook_heads.path_mkdir),