| Message ID | 20211123114227.3124056-1-brauner@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | Extend and tweak mapping support | expand |
On Tue, Nov 23, 2021 at 12:42:17PM +0100, Christian Brauner wrote: > From: Christian Brauner <christian.brauner@ubuntu.com> > > Hey, > > This series extend the mapping infrastructure in order to support mapped > mounts of mapped filesystems in the future. Friendly ping. Would be good to get a review for this. Christian
On Tue, Nov 23, 2021 at 2:16 PM Christian Brauner <brauner@kernel.org> wrote: > > From: Christian Brauner <christian.brauner@ubuntu.com> > > Hey, > > This series extend the mapping infrastructure in order to support mapped > mounts of mapped filesystems in the future. > > Currently we only support mapped mounts of filesystems mounted without an > idmapping. This was a consicous decision mentioned in multiple places. For > example, see [1]. > > In our mapping documentation in [3] we explained in detail that it is > perfectly fine to extend support for mapped mounts to filesystem's mounted > with an idmapping should the need arise. The need has been there for some > time now (cf. [2]). > > Before we can port any such filesystem we need to first extend the mapping > helpers to account for the filesystem's idmapping in the remapping helpers. > This again, is explained at length in our documentation at [3]. > > Currently, the low-level mapping helpers implement the remapping algorithms > described in [3] in a simplified manner. Because we could rely on the fact > that all filesystems supporting mapped mounts are mounted without an > idmapping the translation step from or into the filesystem idmapping could > be skipped. > > In order to support mapped mounts of filesystem's mountable with an > idmapping the translation step we were able to skip before cannot be > skipped anymore. A filesystem mounted with an idmapping is very likely to > not use an identity mapping and will instead use a non-identity mapping. So > the translation step from or into the filesystem's idmapping in the > remapping algorithm cannot be skipped for such filesystems. More details > with examples can be found in [3]. > > This series adds a few new as well as prepares and tweaks some already > existing low-level mapping helpers to perform the full translation > algorithm explained in [3]. The low-level helpers can be written in a way > that they only perform the additional translation step when the filesystem > is indeed mounted with an idmapping. > > Since we don't yet support such a filesystem yet a kernel was compiled > carrying a trivial patch making ext4 mountable with an idmapping: > > # We're located on the host with the initial idmapping. > ubuntu@f2-vm:~$ cat /proc/self/uid_map > 0 0 4294967295 > > # Mount an ext4 filesystem with the initial idmapping. > ubuntu@f2-vm:~$ sudo mount -t ext4 /dev/loop0 /mnt > > # The filesystem contains two files. One owned by id 0 and another one owned by > # id 1000 in the initial idmapping. > ubuntu@f2-vm:~$ ls -al /mnt/ > total 8 > drwxrwxrwx 2 root root 4096 Nov 22 17:04 . > drwxr-xr-x 24 root root 4096 Nov 20 11:24 .. > -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_0 > -rw-r--r-- 1 ubuntu ubuntu 0 Nov 22 17:04 file_init_mapping_1000 > > # Umount it again so we we can mount it in another namespace later. > ubuntu@f2-vm:~$ sudo umount /mnt > > # Use the lxc-usernsexec binary to run a shell in a user and mount namespace > # with an idmapping of 0:10000:100000000. > # > # This idmapping will have the effect that files which are owned by i_{g,u}id > # 10000 and files that are owned by i_{g,u}id 11000 will be owned by {g,u}id > # 0 and {g,u}id 1000 with that namespace respectively. > ubuntu@f2-vm:~$ sudo lxc-usernsexec -m b:0:10000:100000000 -- bash > > # Verify that we're really running with the expected idmapping. > root@f2-vm:/home/ubuntu# cat /proc/self/uid_map > 0 10000 100000000 > > # Mount the ext4 filesystem in the user and mountns with the idmapping > # 0:10000:100000000. > # > # Note, that this requires a test kernel that makes ext4 mountable in a > # non-initial userns. The patch is simply: > # > # diff --git a/fs/ext4/super.c b/fs/ext4/super.c > # index 4e33b5eca694..0221e8211e5b 100644 > # --- a/fs/ext4/super.c > # +++ b/fs/ext4/super.c > # @@ -6584,7 +6584,7 @@ static struct file_system_type ext4_fs_type = { > # .name = "ext4", > # .mount = ext4_mount, > # .kill_sb = kill_block_super, > # - .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP, > # + .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_USERNS_MOUNT, > # }; > # MODULE_ALIAS_FS("ext4"); > root@f2-vm:/home/ubuntu# mount -t ext4 /dev/loop0 /mnt > Hi Christian, I have a question not directly related to the patches, but to the test hack above. I may be wrong, but it looks like an idmapped sb would be desired for some use cases(?). My question is - could we use fsconfig() to allow CAP_SYS_ADMIN to attach a newly mounted sb (e.g. ext4) to user ns without allowing mount of ext4 from within userns? Wouldn't that add usability to some users without adding any new risks over the risks already subjected with idmapped mounts? Thanks, Amir.
From: Christian Brauner <christian.brauner@ubuntu.com> Hey, This series extend the mapping infrastructure in order to support mapped mounts of mapped filesystems in the future. Currently we only support mapped mounts of filesystems mounted without an idmapping. This was a consicous decision mentioned in multiple places. For example, see [1]. In our mapping documentation in [3] we explained in detail that it is perfectly fine to extend support for mapped mounts to filesystem's mounted with an idmapping should the need arise. The need has been there for some time now (cf. [2]). Before we can port any such filesystem we need to first extend the mapping helpers to account for the filesystem's idmapping in the remapping helpers. This again, is explained at length in our documentation at [3]. Currently, the low-level mapping helpers implement the remapping algorithms described in [3] in a simplified manner. Because we could rely on the fact that all filesystems supporting mapped mounts are mounted without an idmapping the translation step from or into the filesystem idmapping could be skipped. In order to support mapped mounts of filesystem's mountable with an idmapping the translation step we were able to skip before cannot be skipped anymore. A filesystem mounted with an idmapping is very likely to not use an identity mapping and will instead use a non-identity mapping. So the translation step from or into the filesystem's idmapping in the remapping algorithm cannot be skipped for such filesystems. More details with examples can be found in [3]. This series adds a few new as well as prepares and tweaks some already existing low-level mapping helpers to perform the full translation algorithm explained in [3]. The low-level helpers can be written in a way that they only perform the additional translation step when the filesystem is indeed mounted with an idmapping. Since we don't yet support such a filesystem yet a kernel was compiled carrying a trivial patch making ext4 mountable with an idmapping: # We're located on the host with the initial idmapping. ubuntu@f2-vm:~$ cat /proc/self/uid_map 0 0 4294967295 # Mount an ext4 filesystem with the initial idmapping. ubuntu@f2-vm:~$ sudo mount -t ext4 /dev/loop0 /mnt # The filesystem contains two files. One owned by id 0 and another one owned by # id 1000 in the initial idmapping. ubuntu@f2-vm:~$ ls -al /mnt/ total 8 drwxrwxrwx 2 root root 4096 Nov 22 17:04 . drwxr-xr-x 24 root root 4096 Nov 20 11:24 .. -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 ubuntu ubuntu 0 Nov 22 17:04 file_init_mapping_1000 # Umount it again so we we can mount it in another namespace later. ubuntu@f2-vm:~$ sudo umount /mnt # Use the lxc-usernsexec binary to run a shell in a user and mount namespace # with an idmapping of 0:10000:100000000. # # This idmapping will have the effect that files which are owned by i_{g,u}id # 10000 and files that are owned by i_{g,u}id 11000 will be owned by {g,u}id # 0 and {g,u}id 1000 with that namespace respectively. ubuntu@f2-vm:~$ sudo lxc-usernsexec -m b:0:10000:100000000 -- bash # Verify that we're really running with the expected idmapping. root@f2-vm:/home/ubuntu# cat /proc/self/uid_map 0 10000 100000000 # Mount the ext4 filesystem in the user and mountns with the idmapping # 0:10000:100000000. # # Note, that this requires a test kernel that makes ext4 mountable in a # non-initial userns. The patch is simply: # # diff --git a/fs/ext4/super.c b/fs/ext4/super.c # index 4e33b5eca694..0221e8211e5b 100644 # --- a/fs/ext4/super.c # +++ b/fs/ext4/super.c # @@ -6584,7 +6584,7 @@ static struct file_system_type ext4_fs_type = { # .name = "ext4", # .mount = ext4_mount, # .kill_sb = kill_block_super, # - .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP, # + .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_USERNS_MOUNT, # }; # MODULE_ALIAS_FS("ext4"); root@f2-vm:/home/ubuntu# mount -t ext4 /dev/loop0 /mnt # We verify that we can still interact with the files and that they are # correctly owned from witin the namespace. # # As mentioned before, a file owned by {g,u}id 0 within the userns is mapped to # {g,u}id 10000 in the initial userns. So inode->i_{g,u}id will contain # 10000. Similarly a file owned by {g,u}id 1000 within the userns means that # inode->i_{g,u}id will contain 11000. the inode->i_{g,u}id root@f2-vm:/home/ubuntu# ls -al /mnt/ total 8 drwxrwxrwx 2 root root 4096 Nov 22 17:04 . drwxr-xr-x 24 nobody nogroup 4096 Nov 20 11:24 .. -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 ubuntu ubuntu 0 Nov 22 17:04 file_init_mapping_1000 # Since we own the superblock we can also create idmapped mounts of the # filesystem. # First, we create an idmapped mount that maps {g,u}id 1000 to {g,u}id 0. # Note that the idmapping 1000:0:1 as written from userspace is different from # the idmapping the kernel actually stores. The idmapping is always translated # into the initial userns. So the kernel actually stores the idmapping # 1000:10000:1. root@f2-vm:/home/ubuntu# /mount-idmapped --map-mount b:1000:0:1 /mnt /opt # Verify that files owned by {g,u}id 1000 are now owned by {g,u}id 0 and all # others are owned by the overflow{g,u}id. root@f2-vm:/home/ubuntu# ls -al /opt total 8 drwxrwxrwx 2 nobody nogroup 4096 Nov 22 17:04 . drwxr-xr-x 24 nobody nogroup 4096 Nov 20 11:24 .. -rw-r--r-- 1 nobody nogroup 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_1000 # Umount the idmapped mount. root@f2-vm:/mnt# umount /opt # Now create another idmapped mount with a mapping of 0:20000:1000000. # # Similar to what we said above the kernel doesn't store 0:20000:1000000 but # 0:30000:1000000. root@f2-vm:/mnt# /mount-idmapped --map-mount b:0:20000:1000000 /mnt /opt # Verify that files owned by {g,u}id 0 in the filesystem's idmapping are owned # by {g,u}id 20000 in the mount's idmapping and that files owned by {g,u}id # 10000 in the filesystem's idmapping are owned by {g,u}id 21000 in the # mount's idmapping. root@f2-vm:/mnt# ls -al /opt total 8 drwxrwxrwx 2 20000 20000 4096 Nov 22 17:04 . drwxr-xr-x 24 nobody nogroup 4096 Nov 20 11:24 .. -rw-r--r-- 1 20000 20000 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 21000 21000 0 Nov 22 17:04 file_init_mapping_1000 # Use the lxc-usernsexec binary to run a shell in a user and mount namespace # with an idmapping of 0:20000:1000000. # # Similar to what we said above the kernel doesn't store 0:20000:1000000 but # 0:30000:1000000. root@f2-vm:/mnt# lxc-usernsexec -m b:0:20000:1000000 -- bash # Verify that we managed to use the correct nested mapping. root@f2-vm:/mnt# cat /proc/self/uid_map 0 20000 1000000 # Verify that files owned by {g,u}id 20000 in the idmapped mount when accessed # from the ancestor userns are now owned by {g,u}id 0 in the idmapped mount # when accessed from the new namespace. Similarly, verify that files owned by # {g,u}id 21000 in the idmapped mount when accessed from the ancestor userns # are now owned by {g,u}id 1000 in the idmapped mount when accesed from the new # namespace. root@f2-vm:/mnt# ls -al /opt/ total 8 drwxrwxrwx 2 root root 4096 Nov 22 17:04 . drwxr-xr-x 24 nobody nogroup 4096 Nov 20 11:24 .. -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 ubuntu ubuntu 0 Nov 22 17:04 file_init_mapping_1000 # Create a file as id 0 in the current namespace through the idmapped mount. root@f2-vm:/mnt# touch /opt/file_from_nested_ns # Verify that the newly created file is owned by {g,u}id 0. root@f2-vm:/mnt# ls -al /opt total 8 drwxrwxrwx 2 root root 4096 Nov 22 17:32 . drwxr-xr-x 24 nobody nogroup 4096 Nov 20 11:24 .. -rw-r--r-- 1 root root 0 Nov 22 17:32 file_from_nested_ns -rw-r--r-- 1 root root 0 Nov 22 17:04 file_init_mapping_0 -rw-r--r-- 1 ubuntu ubuntu 0 Nov 22 17:04 file_init_mapping_1000 [1]: commit 2ca4dcc4909d ("fs/mount_setattr: tighten permission checks") [2]: https://github.com/containers/podman/issues/10374 [3]: Documentations/filesystems/idmappings.rst Thanks! Christian Christian Brauner (10): fs: add is_mapped_mnt() helper fs: move mapping helpers fs: tweak fsuidgid_has_mapping() fs: account for filesystem mappings docs: update mapping documentation fs: use low-level mapping helpers fs: remove unused low-level mapping helpers fs: port higher-level mapping helpers fs: add i_user_ns() helper fs: support mapped mounts of mapped filesystems Documentation/filesystems/idmappings.rst | 72 ------- fs/cachefiles/bind.c | 2 +- fs/ecryptfs/main.c | 2 +- fs/ksmbd/smbacl.c | 18 +- fs/ksmbd/smbacl.h | 4 +- fs/namespace.c | 38 ++-- fs/nfsd/export.c | 2 +- fs/open.c | 7 +- fs/overlayfs/super.c | 2 +- fs/posix_acl.c | 16 +- fs/proc_namespace.c | 2 +- fs/xfs/xfs_inode.c | 10 +- fs/xfs/xfs_symlink.c | 5 +- include/linux/fs.h | 141 ++++---------- include/linux/mnt_mapping.h | 234 +++++++++++++++++++++++ security/commoncap.c | 14 +- 16 files changed, 339 insertions(+), 230 deletions(-) create mode 100644 include/linux/mnt_mapping.h base-commit: 136057256686de39cc3a07c2e39ef6bc43003ff6