[PATCH RFC v2 1/2] ovl: allow to specify override credentials

[Christian Brauner <brauner@xxxxxxxxxx>]

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 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.

Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 fs/overlayfs/params.c | 22 ++++++++++++++++++++++
 fs/overlayfs/super.c  | 11 ++++++++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index 1115c22deca0..f2bc8acf6bf1 100644
--- a/fs/overlayfs/params.c
+++ b/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 +664,26 @@ 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 = ofs->creator_cred;
+
+		if (!result.negated) {
+			if (fc->user_ns != current_user_ns()) {
+				err = -EINVAL;
+				break;
+			}
+
+			ofs->creator_cred = prepare_creds();
+			if (!ofs->creator_cred) {
+				err = -EINVAL;
+				break;
+			}
+		} else {
+			ofs->creator_cred = NULL;
+		}
+		put_cred(cred);
+		break;
+	}
 	default:
 		pr_err("unrecognized mount option \"%s\" or missing value\n",
 		       param->key);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 86ae6f6da36b..a85071fe18fd 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;
@@ -1481,6 +1487,7 @@ 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:
@@ -1488,6 +1495,8 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 out_err:
 	ovl_free_fs(ofs);
 	sb->s_fs_info = NULL;
+	if (old_cred)
+		ovl_revert_creds(old_cred);
 	return err;
 }
 

-- 
2.47.2