[PATCH v4 4/4] ovl: Optimize override/revert creds

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Use override_creds_light() in ovl_override_creds() and
revert_creds_light() in ovl_revert_creds_light().

The _light() functions do not change the 'usage' of the credentials in
question, as they refer to the credentials associated with the
mounter, which have a longer lifetime.

In ovl_setup_cred_for_create(), do not need to modify the mounter
credentials (returned by override_creds()) 'usage' counter. Add a
warning to verify that we are indeed working with the mounter
credentials (stored in the superblock). Failure in this assumption
means that creds may leak.

Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 fs/overlayfs/dir.c  | 7 ++++++-
 fs/overlayfs/util.c | 4 ++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 09db5eb19242..136a2c7fb9e5 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -571,7 +571,12 @@ static int ovl_setup_cred_for_create(struct dentry *dentry, struct inode *inode,
 		put_cred(override_cred);
 		return err;
 	}
-	put_cred(override_creds(override_cred));
+
+	/*
+	 * We must be called with creator creds already, otherwise we risk
+	 * leaking creds.
+	 */
+	WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
 	put_cred(override_cred);
 
 	return 0;
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 9408046f4f41..3bb107471fb4 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -65,12 +65,12 @@ const struct cred *ovl_override_creds(struct super_block *sb)
 {
 	struct ovl_fs *ofs = OVL_FS(sb);
 
-	return override_creds(ofs->creator_cred);
+	return override_creds_light(ofs->creator_cred);
 }
 
 void ovl_revert_creds(const struct cred *old_cred)
 {
-	revert_creds(old_cred);
+	revert_creds_light(old_cred);
 }
 
 /*
-- 
2.47.0