[PATCH] fs/ceph/super: add mount options "snapdir{mode,uid,gid}"

[Max Kellermann <max.kellermann@ionos.com>]

By default, the ".snap" directory inherits the parent's permissions
and ownership, which allows all users to create new cephfs snapshots
in arbitrary directories they have write access on.

In some environments, giving everybody this capability is not
desirable, but there is currently no way to disallow only some users
to create snapshots.  It is only possible to revoke the permission to
the whole client (i.e. all users on the computer which mounts the
cephfs).

This patch allows overriding the permissions and ownership of all
virtual ".snap" directories in a cephfs mount, which allows
restricting (read and write) access to snapshots.

For example, the mount options:

 snapdirmode=0751,snapdiruid=0,snapdirgid=4

... allows only user "root" to create or delete snapshots, and group
"adm" (gid=4) is allowed to get a list of snapshots.  All others are
allowed to read the contents of existing snapshots (if they know the
name).

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/ceph/inode.c |  7 ++++---
 fs/ceph/super.c | 33 +++++++++++++++++++++++++++++++++
 fs/ceph/super.h |  4 ++++
 3 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 56c53ab3618e..0e9388af2821 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -80,6 +80,7 @@ struct inode *ceph_get_snapdir(struct inode *parent)
 	};
 	struct inode *inode = ceph_get_inode(parent->i_sb, vino);
 	struct ceph_inode_info *ci = ceph_inode(inode);
+	struct ceph_mount_options *const fsopt = ceph_inode_to_client(parent)->mount_options;
 
 	if (IS_ERR(inode))
 		return inode;
@@ -96,9 +97,9 @@ struct inode *ceph_get_snapdir(struct inode *parent)
 		goto err;
 	}
 
-	inode->i_mode = parent->i_mode;
-	inode->i_uid = parent->i_uid;
-	inode->i_gid = parent->i_gid;
+	inode->i_mode = fsopt->snapdir_mode == (umode_t)-1 ? parent->i_mode : fsopt->snapdir_mode;
+	inode->i_uid = uid_eq(fsopt->snapdir_uid, INVALID_UID) ? parent->i_uid : fsopt->snapdir_uid;
+	inode->i_gid = gid_eq(fsopt->snapdir_gid, INVALID_GID) ? parent->i_gid : fsopt->snapdir_gid;
 	inode->i_mtime = parent->i_mtime;
 	inode->i_ctime = parent->i_ctime;
 	inode->i_atime = parent->i_atime;
diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 40140805bdcf..5e5713946f7b 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -143,6 +143,9 @@ enum {
 	Opt_readdir_max_entries,
 	Opt_readdir_max_bytes,
 	Opt_congestion_kb,
+	Opt_snapdirmode,
+	Opt_snapdiruid,
+	Opt_snapdirgid,
 	/* int args above */
 	Opt_snapdirname,
 	Opt_mds_namespace,
@@ -200,6 +203,9 @@ static const struct fs_parameter_spec ceph_mount_parameters[] = {
 	fsparam_flag_no ("require_active_mds",		Opt_require_active_mds),
 	fsparam_u32	("rsize",			Opt_rsize),
 	fsparam_string	("snapdirname",			Opt_snapdirname),
+	fsparam_u32oct	("snapdirmode",			Opt_snapdirmode),
+	fsparam_u32	("snapdiruid",			Opt_snapdiruid),
+	fsparam_u32	("snapdirgid",			Opt_snapdirgid),
 	fsparam_string	("source",			Opt_source),
 	fsparam_string	("mon_addr",			Opt_mon_addr),
 	fsparam_u32	("wsize",			Opt_wsize),
@@ -414,6 +420,22 @@ static int ceph_parse_mount_param(struct fs_context *fc,
 		fsopt->snapdir_name = param->string;
 		param->string = NULL;
 		break;
+	case Opt_snapdirmode:
+		fsopt->snapdir_mode = result.uint_32;
+		if (fsopt->snapdir_mode & ~0777)
+			return invalfc(fc, "Invalid snapdirmode");
+		fsopt->snapdir_mode |= S_IFDIR;
+		break;
+	case Opt_snapdiruid:
+		fsopt->snapdir_uid = make_kuid(current_user_ns(), result.uint_32);
+		if (!uid_valid(fsopt->snapdir_uid))
+			return invalfc(fc, "Invalid snapdiruid");
+		break;
+	case Opt_snapdirgid:
+		fsopt->snapdir_gid = make_kgid(current_user_ns(), result.uint_32);
+		if (!gid_valid(fsopt->snapdir_gid))
+			return invalfc(fc, "Invalid snapdirgid");
+		break;
 	case Opt_mds_namespace:
 		if (!namespace_equals(fsopt, param->string, strlen(param->string)))
 			return invalfc(fc, "Mismatching mds_namespace");
@@ -734,6 +756,14 @@ static int ceph_show_options(struct seq_file *m, struct dentry *root)
 		seq_printf(m, ",readdir_max_bytes=%u", fsopt->max_readdir_bytes);
 	if (strcmp(fsopt->snapdir_name, CEPH_SNAPDIRNAME_DEFAULT))
 		seq_show_option(m, "snapdirname", fsopt->snapdir_name);
+	if (fsopt->snapdir_mode != (umode_t)-1)
+		seq_printf(m, ",snapdirmode=%o", fsopt->snapdir_mode);
+	if (!uid_eq(fsopt->snapdir_uid, INVALID_UID))
+		seq_printf(m, ",snapdiruid=%o",
+			   from_kuid_munged(&init_user_ns, fsopt->snapdir_uid));
+	if (!gid_eq(fsopt->snapdir_gid, INVALID_GID))
+		seq_printf(m, ",snapdirgid=%o",
+			   from_kgid_munged(&init_user_ns, fsopt->snapdir_gid));
 
 	return 0;
 }
@@ -1335,6 +1365,9 @@ static int ceph_init_fs_context(struct fs_context *fc)
 	fsopt->wsize = CEPH_MAX_WRITE_SIZE;
 	fsopt->rsize = CEPH_MAX_READ_SIZE;
 	fsopt->rasize = CEPH_RASIZE_DEFAULT;
+	fsopt->snapdir_mode = (umode_t)-1;
+	fsopt->snapdir_uid = INVALID_UID;
+	fsopt->snapdir_gid = INVALID_GID;
 	fsopt->snapdir_name = kstrdup(CEPH_SNAPDIRNAME_DEFAULT, GFP_KERNEL);
 	if (!fsopt->snapdir_name)
 		goto nomem;
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index d44a366b2f1b..3c930816078d 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -85,6 +85,10 @@ struct ceph_mount_options {
 	unsigned int max_readdir;       /* max readdir result (entries) */
 	unsigned int max_readdir_bytes; /* max readdir result (bytes) */
 
+	umode_t snapdir_mode;
+	kuid_t snapdir_uid;
+	kgid_t snapdir_gid;
+
 	bool new_dev_syntax;
 
 	/*
-- 
2.35.1