linux-kernel - [PATCH v3 10/11] fs/fuse: allow idmapped mounts

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Message-Id: <20240815092429.103356-11-aleksandr.mikhalitsyn@canonical.com>
Date: Thu, 15 Aug 2024 11:24:27 +0200
From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: mszeredi@...hat.com
Cc: brauner@...nel.org,
	stgraber@...raber.org,
	linux-fsdevel@...r.kernel.org,
	Seth Forshee <sforshee@...nel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>,
	Bernd Schubert <bschubert@....com>,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 10/11] fs/fuse: allow idmapped mounts

Now we have everything in place and we can allow idmapped mounts
by setting the FS_ALLOW_IDMAP flag. Notice that real availability
of idmapped mounts will depend on the fuse daemon. Fuse daemon
have to set FUSE_ALLOW_IDMAP flag in the FUSE_INIT reply.

To discuss:
- we enable idmapped mounts support only if "default_permissions" mode is enabled,
because otherwise we would need to deal with UID/GID mappings in the userspace side OR
provide the userspace with idmapped req->in.h.uid/req->in.h.gid values which is not
something that we probably want to. Idmapped mounts phylosophy is not about faking
caller uid/gid.

Some extra links and examples:

- libfuse support
https://github.com/mihalicyn/libfuse/commits/idmap_support

- fuse-overlayfs support:
https://github.com/mihalicyn/fuse-overlayfs/commits/idmap_support

- cephfs-fuse conversion example
https://github.com/mihalicyn/ceph/commits/fuse_idmap

- glusterfs conversion example
https://github.com/mihalicyn/glusterfs/commits/fuse_idmap

Cc: Christian Brauner <brauner@...nel.org>
Cc: Seth Forshee <sforshee@...nel.org>
Cc: Miklos Szeredi <miklos@...redi.hu>
Cc: Amir Goldstein <amir73il@...il.com>
Cc: Bernd Schubert <bschubert@....com>
Cc: <linux-fsdevel@...r.kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
---
v2:
	- simplified and get rid of ->allow_idmap global VFS callback
v3:
	- now use a new SB_I_NOIDMAP flag
---
 fs/fuse/inode.c           | 14 +++++++++++---
 include/uapi/linux/fuse.h |  5 ++++-
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 6c205731c844..b840189ac8be 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1345,6 +1345,12 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 				fm->sb->s_export_op = &fuse_export_fid_operations;
 			if (flags & FUSE_OWNER_UID_GID_EXT)
 				fc->owner_uid_gid_ext = 1;
+			if (flags & FUSE_ALLOW_IDMAP) {
+				if (fc->owner_uid_gid_ext && fc->default_permissions)
+					fm->sb->s_iflags &= ~SB_I_NOIDMAP;
+				else
+					ok = false;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1392,7 +1398,8 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
 		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
-		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_OWNER_UID_GID_EXT;
+		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_OWNER_UID_GID_EXT |
+		FUSE_ALLOW_IDMAP;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
@@ -1569,6 +1576,7 @@ static void fuse_sb_defaults(struct super_block *sb)
 	sb->s_time_gran = 1;
 	sb->s_export_op = &fuse_export_operations;
 	sb->s_iflags |= SB_I_IMA_UNVERIFIABLE_SIGNATURE;
+	sb->s_iflags |= SB_I_NOIDMAP;
 	if (sb->s_user_ns != &init_user_ns)
 		sb->s_iflags |= SB_I_UNTRUSTED_MOUNTER;
 	sb->s_flags &= ~(SB_NOSEC | SB_I_VERSION);
@@ -1981,7 +1989,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
 static struct file_system_type fuse_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "fuse",
-	.fs_flags	= FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
+	.fs_flags	= FS_HAS_SUBTYPE | FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
 	.init_fs_context = fuse_init_fs_context,
 	.parameters	= fuse_fs_parameters,
 	.kill_sb	= fuse_kill_sb_anon,
@@ -2002,7 +2010,7 @@ static struct file_system_type fuseblk_fs_type = {
 	.init_fs_context = fuse_init_fs_context,
 	.parameters	= fuse_fs_parameters,
 	.kill_sb	= fuse_kill_sb_blk,
-	.fs_flags	= FS_REQUIRES_DEV | FS_HAS_SUBTYPE,
+	.fs_flags	= FS_REQUIRES_DEV | FS_HAS_SUBTYPE | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("fuseblk");
 
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index d9ecc17fd13b..b23e8247ce43 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -221,6 +221,7 @@
  *  7.41
  *  - add FUSE_EXT_OWNER_UID_GID
  *  - add FUSE_OWNER_UID_GID_EXT
+ *  - add FUSE_ALLOW_IDMAP
  */
 
 #ifndef _LINUX_FUSE_H
@@ -256,7 +257,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 40
+#define FUSE_KERNEL_MINOR_VERSION 41
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -427,6 +428,7 @@ struct fuse_file_lock {
  *		    of the request ID indicates resend requests
  * FUSE_OWNER_UID_GID_EXT: add inode owner UID/GID info to create, mkdir,
  *			   symlink and mknod
+ * FUSE_ALLOW_IDMAP: allow creation of idmapped mounts
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -473,6 +475,7 @@ struct fuse_file_lock {
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP
 #define FUSE_OWNER_UID_GID_EXT	(1ULL << 40)
+#define FUSE_ALLOW_IDMAP	(1ULL << 41)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.34.1