ovl: allow to specify override credentials

Currently overlayfs uses the mounter's credentials for it's
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.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-1-46af55e4ceda@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/overlayfs.rst

@@ -292,13 +292,27 @@ rename or unlink will of course be noticed and handled).
 Permission model
 ----------------
 
+An overlay filesystem stashes credentials that will be used when
+accessing lower or upper filesystems.
+
+In the old mount api the credentials of the task calling mount(2) are
+stashed. In the new mount api the credentials of the task creating the
+superblock through FSCONFIG_CMD_CREATE command of fsconfig(2) are
+stashed.
+
+Starting with kernel v6.15 it is possible to use the "override_creds"
+mount option which will cause the credentials of the calling task to be
+recorded. Note that "override_creds" is only meaningful when used with
+the new mount api as the old mount api combines setting options and
+superblock creation in a single mount(2) syscall.
+
 Permission checking in the overlay filesystem follows these principles:
 
  1) permission check SHOULD return the same result before and after copy up
 
  2) task creating the overlay mount MUST NOT gain additional privileges
 
- 3) non-mounting task MAY gain additional privileges through the overlay,
+ 3) task[*] MAY gain additional privileges through the overlay,
     compared to direct access on underlying lower or upper filesystems
 
 This is achieved by performing two permission checks on each access:
@@ -306,7 +320,7 @@ This is achieved by performing two permission checks on each access:
  a) check if current task is allowed access based on local DAC (owner,
     group, mode and posix acl), as well as MAC checks
 
- b) check if mounting task would be allowed real operation on lower or
+ b) check if stashed credentials would be allowed real operation on lower or
     upper layer based on underlying filesystem permissions, again including
     MAC checks
 
@@ -315,10 +329,10 @@ are copied up.  On the other hand it can result in server enforced
 permissions (used by NFS, for example) being ignored (3).
 
 Check (b) ensures that no task gains permissions to underlying layers that
-the mounting task does not have (2).  This also means that it is possible
+the stashed credentials do not have (2).  This also means that it is possible
 to create setups where the consistency rule (1) does not hold; normally,
-however, the mounting task will have sufficient privileges to perform all
-operations.
+however, the stashed credentials will have sufficient privileges to
+perform all operations.
 
 Another way to demonstrate this model is drawing parallels between::
 

fs/overlayfs/params.c

@@ -59,6 +59,7 @@ enum ovl_opt {
 	Opt_metacopy,
 	Opt_verity,
 	Opt_volatile,
+	Opt_override_creds,
 };
 
 static const struct constant_table ovl_parameter_bool[] = {
@@ -155,6 +156,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = {
 	fsparam_enum("metacopy",            Opt_metacopy, ovl_parameter_bool),
 	fsparam_enum("verity",              Opt_verity, ovl_parameter_verity),
 	fsparam_flag("volatile",            Opt_volatile),
+	fsparam_flag_no("override_creds",   Opt_override_creds),
 	{}
 };
 
@@ -662,6 +664,29 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_userxattr:
 		config->userxattr = true;
 		break;
+	case Opt_override_creds: {
+		const struct cred *cred = NULL;
+
+		if (result.negated) {
+			swap(cred, ofs->creator_cred);
+			put_cred(cred);
+			break;
+		}
+
+		if (!current_in_userns(fc->user_ns)) {
+			err = -EINVAL;
+			break;
+		}
+
+		cred = prepare_creds();
+		if (cred)
+			swap(cred, ofs->creator_cred);
+		else
+			err = -ENOMEM;
+
+		put_cred(cred);
+		break;
+	}
 	default:
 		pr_err("unrecognized mount option \"%s\" or missing value\n",
 		       param->key);

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;