Re: [PATCH v3 1/2] move_mount: allow to add a mount into an existing group

[Christian Brauner]

On Wed, Jul 14, 2021 at 04:27:53PM +0300, Pavel Tikhomirov wrote:
> Previously a sharing group (shared and master ids pair) can be only
> inherited when mount is created via bindmount. This patch adds an
> ability to add an existing private mount into an existing sharing group.
> 
> With this functionality one can first create the desired mount tree from
> only private mounts (without the need to care about undesired mount
> propagation or mount creation order implied by sharing group
> dependencies), and next then setup any desired mount sharing between
> those mounts in tree as needed.
> 
> This allows CRIU to restore any set of mount namespaces, mount trees and
> sharing group trees for a container.
> 
> We have many issues with restoring mounts in CRIU related to sharing
> groups and propagation:
> - reverse sharing groups vs mount tree order requires complex mounts
>   reordering which mostly implies also using some temporary mounts
> (please see https://lkml.org/lkml/2021/3/23/569 for more info)
> 
> - mount() syscall creates tons of mounts due to propagation
> - mount re-parenting due to propagation
> - "Mount Trap" due to propagation
> - "Non Uniform" propagation, meaning that with different tricks with
>   mount order and temporary children-"lock" mounts one can create mount
>   trees which can't be restored without those tricks
> (see https://www.linuxplumbersconf.org/event/7/contributions/640/)
> 
> With this new functionality we can resolve all the problems with
> propagation at once.
> 
> Link: https://lore.kernel.org/r/20210714132754.94633-1-ptikhomirov@xxxxxxxxxxxxx
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> Cc: Mattias Nissler <mnissler@xxxxxxxxxxxx>
> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Cc: Andrei Vagin <avagin@xxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-api@xxxxxxxxxxxxxxx
> Cc: lkml <linux-kernel@xxxxxxxxxxxxxxx>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
> ---
> This is a rework of "mnt: allow to add a mount into an existing group"
> patch from Andrei. https://lkml.org/lkml/2017/4/28/20
> 
> New do_set_group is similar to do_move_mount, but with some restrictions
> of do_move_mount removed and that's why:
> 
> 1) Allow "cross-namespace" sharing group set. If we allow operation only
> with mounts from current+anon mount namespace one would still be able to
> setns(from_mntns) + open_tree(from, OPEN_TREE_CLONE) + setns(to_mntns) +
> move_mount(anon, to, MOVE_MOUNT_SET_GROUP) to set sharing group to mount
> in different mount namespace with source mount. But with this approach
> we would need to create anon mount namespace and mount copy each time,
> which is just a waste of resources. So instead lets just check if we are
> allowed to modify both mount namespaces (which looks equivalent to what
> setns-es and open_tree check).
> 
> 2) Skip checks wich only apply to actually moving mount which we have in
> do_move_mount and open_tree. We don't need to check for MNT_LOCKED,
> d_is_dir matching, unbindable, nsfs loops and ancestor relation as we
> don't move mounts.
> 
> Also let's add some new checks:
> 
> 1) Don't allow to copy sharing from mount with narrow root to a wider
> root, so that user does not have power to receive more propagations when
> user already has. (proposed by Andrei)
> 
> 2) Don't allow to copy sharing from mount with locked children for the
> same reason, as user shouldn't see propagations to areas overmounted by
> locked mounts (if the user could not already do it before sharing
> adjustment).
> 
> 3) If From is private for MOVE_MOUNT_SET_GROUP let's report an error
> instead of just doing nothing, so that the user knows that there is
> probably some logical usage error. (proposed by Christian)
> 
> Security note: there would be no (new) loops in sharing groups tree,
> because this new move_mount(MOVE_MOUNT_SET_GROUP) operation only adds
> one _private_ mount to one group (without moving between groups), the
> sharing groups tree itself stays unchanged after it.
> 
> In Virtuozzo we have "mount-v2" implementation, based with the original
> kernel patch from Andrei, tested for almost a year and it actually
> decreased number of bugs with mounts a lot. One can take a look on the
> implementation of sharing group restore in CRIU in "mount-v2" here:
> 
> https://src.openvz.org/projects/OVZ/repos/criu/browse/criu/mount-v2.c#898
> 
> This works almost the same with current version of patch if we replace
> mount(MS_SET_GROUP) to move_mount(MOVE_MOUNT_SET_GROUP), please see
> super-draft port for mainstream criu, this at least passes
> non-user-namespaced mount tests (zdtm.py --mounts-v2 -f ns).
> 
> https://github.com/Snorch/criu/commits/mount-v2-poc
> 
> v2: Solve the problem mentioned by Andrei:
> - check mnt_root of "to" is in the sub-tree of mnt_root of "from"
> - also check "from" has no locked mounts in subroot of "to"
> v3: Add checks:
> - check paths to be mount root dentries
> - return EINVAL if From is private (no sharing to copy)
> 
> ---
>  fs/namespace.c             | 75 +++++++++++++++++++++++++++++++++++++-
>  include/uapi/linux/mount.h |  3 +-
>  2 files changed, 76 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ab4174a3c802..a7828e695e03 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2684,6 +2684,76 @@ static bool check_for_nsfs_mounts(struct mount *subtree)
>  	return ret;
>  }
>  
> +static int do_set_group(struct path *from_path, struct path *to_path)
> +{
> +	struct mount *from, *to;
> +	int err;
> +
> +	from = real_mount(from_path->mnt);
> +	to = real_mount(to_path->mnt);
> +
> +	namespace_lock();
> +
> +	err = -EINVAL;
> +	/* To and From must be mounted */
> +	if (!is_mounted(&from->mnt))
> +		goto out;
> +	if (!is_mounted(&to->mnt))
> +		goto out;
> +
> +	err = -EPERM;
> +	/* We should be allowed to modify mount namespaces of both mounts */
> +	if (!ns_capable(from->mnt_ns->user_ns, CAP_SYS_ADMIN))
> +		goto out;
> +	if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN))
> +		goto out;
> +
> +	err = -EINVAL;
> +	/* To and From paths should be mount roots */
> +	if (from_path->dentry != from_path->mnt->mnt_root)
> +		goto out;
> +	if (to_path->dentry != to_path->mnt->mnt_root)
> +		goto out;
> +
> +	/* Setting sharing groups is only allowed across same superblock */
> +	if (from->mnt.mnt_sb != to->mnt.mnt_sb)
> +		goto out;
> +
> +	/* From mount root should be wider than To mount root */
> +	if (!is_subdir(to->mnt.mnt_root, from->mnt.mnt_root))
> +		goto out;
> +
> +	/* From mount should not have locked children in place of To's root */
> +	if (has_locked_children(from, to->mnt.mnt_root))
> +		goto out;
> +
> +	/* Setting sharing groups is only allowed on private mounts */
> +	if (IS_MNT_SHARED(to) || IS_MNT_SLAVE(to))
> +		goto out;
> +
> +	/* From should not be private */
> +	if (!IS_MNT_SHARED(from) && !IS_MNT_SLAVE(from))
> +		goto out;
> +
> +	if (IS_MNT_SLAVE(from)) {
> +		struct mount *m = from->mnt_master;
> +
> +		list_add(&to->mnt_slave, &m->mnt_slave_list);
> +		to->mnt_master = m;
> +	}
> +
> +	if (IS_MNT_SHARED(from)) {
> +		to->mnt_group_id = from->mnt_group_id;
> +		list_add(&to->mnt_share, &from->mnt_share);
> +		set_mnt_shared(to);

Thanks for the updated patch.

Note, set_mnt_shared() writes to the vfsmount so this should be:

if (IS_MNT_SHARED(from)) {
	to->mnt_group_id = from->mnt_group_id;
	list_add(&to->mnt_share, &from->mnt_share);
	lock_mount_hash();
	set_mnt_shared(to);
	unlock_mount_hash();
}

> +	}


> +	}
> +
> +	err = 0;
> +out:
> +	namespace_unlock();
> +	return err;
> +}
> +
>  static int do_move_mount(struct path *old_path, struct path *new_path)
>  {
>  	struct mnt_namespace *ns;
> @@ -3669,7 +3739,10 @@ SYSCALL_DEFINE5(move_mount,
>  	if (ret < 0)
>  		goto out_to;
>  
> -	ret = do_move_mount(&from_path, &to_path);
> +	if (flags & MOVE_MOUNT_SET_GROUP)
> +		ret = do_set_group(&from_path, &to_path);
> +	else
> +		ret = do_move_mount(&from_path, &to_path);
>  
>  out_to:
>  	path_put(&to_path);
> diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> index dd7a166fdf9c..4d93967f8aea 100644
> --- a/include/uapi/linux/mount.h
> +++ b/include/uapi/linux/mount.h
> @@ -73,7 +73,8 @@
>  #define MOVE_MOUNT_T_SYMLINKS		0x00000010 /* Follow symlinks on to path */
>  #define MOVE_MOUNT_T_AUTOMOUNTS		0x00000020 /* Follow automounts on to path */
>  #define MOVE_MOUNT_T_EMPTY_PATH		0x00000040 /* Empty to path permitted */
> -#define MOVE_MOUNT__MASK		0x00000077
> +#define MOVE_MOUNT_SET_GROUP		0x00000100 /* Set sharing group instead */
> +#define MOVE_MOUNT__MASK		0x00000177
>  
>  /*
>   * fsopen() flags.
> -- 
> 2.31.1