Merge tag 'vfs-6.15-rc1.overlayfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs overlayfs updates from Christian Brauner:
 "Currently overlayfs uses the mounter's credentials for its
  override_creds() calls. That provides a consistent permission model.

  This patches allows a caller to instruct overlayfs to use its
  credentials instead. The caller must be located in the same user
  namespace hierarchy as the user namespace the overlayfs instance will
  be mounted in. This provides a consistent and simple security model.

  With this it is possible to e.g., mount an overlayfs instance where
  the mounter must have CAP_SYS_ADMIN but the credentials used for
  override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage
  of custom fs{g,u}id different from the callers and other tweaks"

* tag 'vfs-6.15-rc1.overlayfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  selftests/ovl: add third selftest for "override_creds"
  selftests/ovl: add second selftest for "override_creds"
  selftests/filesystems: add utils.{c,h}
  selftests/ovl: add first selftest for "override_creds"
  ovl: allow to specify override credentials

1 files changed, 15 insertions, 1 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 86ae6f6da36b..61e21c3129e8 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1305,6 +1305,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct ovl_fs_context *ctx = fc->fs_private;
+	const struct cred *old_cred = NULL;
 	struct dentry *root_dentry;
 	struct ovl_entry *oe;
 	struct ovl_layer *layers;
@@ -1318,10 +1319,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_d_op = &ovl_dentry_operations;
 
 	err = -ENOMEM;
-	ofs->creator_cred = cred = prepare_creds();
+	if (!ofs->creator_cred)
+		ofs->creator_cred = cred = prepare_creds();
+	else
+		cred = (struct cred *)ofs->creator_cred;
 	if (!cred)
 		goto out_err;
 
+	old_cred = ovl_override_creds(sb);
+
 	err = ovl_fs_params_verify(ctx, &ofs->config);
 	if (err)
 		goto out_err;
@@ -1481,11 +1487,19 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sb->s_root = root_dentry;
 
+	ovl_revert_creds(old_cred);
 	return 0;
 
 out_free_oe:
 	ovl_free_entry(oe);
 out_err:
+	/*
+	 * Revert creds before calling ovl_free_fs() which will call
+	 * put_cred() and put_cred() requires that the cred's that are
+	 * put are not the caller's creds, i.e., current->cred.
+	 */
+	if (old_cred)
+		ovl_revert_creds(old_cred);
 	ovl_free_fs(ofs);
 	sb->s_fs_info = NULL;
 	return err;