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
