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

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

[Pavel Tikhomirov]

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.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Mattias Nissler <mnissler@chromium.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: lkml <linux-kernel@vger.kernel.org>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

---
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);
+	}
+
+	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

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@virtuozzo.com
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Mattias Nissler <mnissler@chromium.org>
> Cc: Aleksa Sarai <cyphar@cyphar.com>
> Cc: Andrei Vagin <avagin@gmail.com>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-api@vger.kernel.org
> Cc: lkml <linux-kernel@vger.kernel.org>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> ---
> 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

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

[Pavel Tikhomirov]

On 14.07.2021 17:43, Christian Brauner wrote:
> 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@virtuozzo.com
>> Cc: Eric W. Biederman <ebiederm@xmission.com>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: Christian Brauner <christian.brauner@ubuntu.com>
>> Cc: Mattias Nissler <mnissler@chromium.org>
>> Cc: Aleksa Sarai <cyphar@cyphar.com>
>> Cc: Andrei Vagin <avagin@gmail.com>
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-api@vger.kernel.org
>> Cc: lkml <linux-kernel@vger.kernel.org>
>> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
>> ---
>> 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();
> }

Thanks for pointing that out! Sent v4 to fix it.

> 
>> +	}
> 
> 
>> +	}
>> +
>> +	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

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.