[PATCH v2 1/6] ovl: add ovl_override_creator_creds cred guard

[Christian Brauner <brauner@kernel.org>]

The current code to override credentials for creation operations is
pretty difficult to understand. We effectively override the credentials
twice:

(1) override with the mounter's credentials
(2) copy the mounts credentials and override the fs{g,u}id with the inode {u,g}id

And then we elide the revert because it would be an idempotent revert.
That elision doesn't buy us anything anymore though because I've made it
all work without any reference counting anyway. All it does is mix the
two credential overrides together.

We can use a cleanup guard to clarify the creation codepaths and make
them easier to understand.

This just introduces the cleanup guard keeping the patch reviewable.
We'll convert the caller in follow-up patches and then drop the
duplicated code.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/overlayfs/dir.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 4fd6ddd6f4ef..3eb0bb0b8f3b 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -575,6 +575,42 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	goto out_dput;
 }
 
+static const struct cred *ovl_override_creator_creds(struct dentry *dentry, struct inode *inode, umode_t mode)
+{
+	int err;
+
+	if (WARN_ON_ONCE(current->cred != ovl_creds(dentry->d_sb)))
+		return ERR_PTR(-EINVAL);
+
+	CLASS(prepare_creds, override_cred)();
+	if (!override_cred)
+		return ERR_PTR(-ENOMEM);
+
+	override_cred->fsuid = inode->i_uid;
+	override_cred->fsgid = inode->i_gid;
+
+	err = security_dentry_create_files_as(dentry, mode, &dentry->d_name,
+					      current->cred, override_cred);
+	if (err)
+		return ERR_PTR(err);
+
+	return override_creds(no_free_ptr(override_cred));
+}
+
+static void ovl_revert_creator_creds(const struct cred *old_cred)
+{
+	const struct cred *override_cred;
+
+	override_cred = revert_creds(old_cred);
+	put_cred(override_cred);
+}
+
+DEFINE_CLASS(ovl_override_creator_creds,
+	     const struct cred *,
+	     if (!IS_ERR_OR_NULL(_T)) ovl_revert_creator_creds(_T),
+	     ovl_override_creator_creds(dentry, inode, mode),
+	     struct dentry *dentry, struct inode *inode, umode_t mode)
+
 static const struct cred *ovl_setup_cred_for_create(struct dentry *dentry,
 						    struct inode *inode,
 						    umode_t mode,

-- 
2.47.3