Re: [RFC][PATCH v2] mount: In propagate_umount handle overlapping mount propagation trees

[Andrei Vagin]

 On Sat, Oct 22, 2016 at 02:42:03PM -0500, Eric W. Biederman wrote:
> 
> Andrei,
> 
> This fixes the issue you have reported and through a refactoring
> makes the code simpler and easier to verify.  That said I find your
> last test case very interesting.   While looking at it in detail
> I have realized I don't fully understand why we have both lookup_mnt and
> lookup_mnt_last, so I can't say that this change is fully correct.
> 
> Outside of propogate_umount I am don't have concerns but I am not 100%
> convinced that my change to lookup_mnt_last does the right thing
> in the case of propagate_umount.
> 
> I do see why your last test case scales badly.  Long chains of shared
> mounts that we can't skip.  At the same time I don't really understand
> that case.  Part of it has to do with multiple child mounts of the same
> mount on the same mountpoint.
> 
> So I am working through my concerns.  In the mean time I figured it
> would be useful to post this version.  As this version is clearly better
> than the version of this change that have come before it.

Hi Eric,

I have tested this version and it works fine.

As for the the last test case, could you look at the attached patch?
The idea is that we can skip all mounts from a shared group, if one
of them already marked.

> 
> Eric
> 
> From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Date: Thu, 13 Oct 2016 13:27:19 -0500
> 
> Adrei Vagin pointed out that time to executue propagate_umount can go
> non-linear (and take a ludicrious amount of time) when the mount
> propogation trees of the mounts to be unmunted by a lazy unmount
> overlap.
> 
> While investigating the horrible performance I realized that in
> the case overlapping mount trees since the addition of locked
> mount support the code has been failing to unmount all of the
> mounts it should have been unmounting.
> 
> Make the walk of the mount propagation trees nearly linear by using
> MNT_MARK to mark pieces of the mount propagation trees that have
> already been visited, allowing subsequent walks to skip over
> subtrees.
> 
> Make the processing of mounts order independent by adding a list of
> mount entries that need to be unmounted, and simply adding a mount to
> that list when it becomes apparent the mount can safely be unmounted.
> For mounts that are locked on other mounts but otherwise could be
> unmounted move them from their parnets mnt_mounts to mnt_umounts so
> that if and when their parent becomes unmounted these mounts can be
> added to the list of mounts to unmount.
> 
> Add a final pass to clear MNT_MARK and to restore mnt_mounts
> from mnt_umounts for anything that did not get unmounted.
> 
> Add the functions propagation_visit_next and propagation_revisit_next
> to coordinate walking of the mount tree and setting and clearing the
> mount mark.
> 
> The skipping of already unmounted mounts has been moved from
> __lookup_mnt_last to mark_umount_candidates, so that the new
> propagation functions can notice when the propagation tree passes
> through the initial set of unmounted mounts.  Except in umount_tree as
> part of the unmounting process the only place where unmounted mounts
> should be found are in unmounted subtrees.  All of the other callers
> of __lookup_mnt_last are from mounted subtrees so the not checking for
> unmounted mounts should not affect them.
> 
> A script to generate overlapping mount propagation trees:
> $ cat run.sh
> mount -t tmpfs test-mount /mnt
> mount --make-shared /mnt
> for i in `seq $1`; do
>         mkdir /mnt/test.$i
>         mount --bind /mnt /mnt/test.$i
> done
> cat /proc/mounts | grep test-mount | wc -l
> time umount -l /mnt
> $ for i in `seq 10 16`; do echo $i; unshare -Urm bash ./run.sh $i; done
> 
> Here are the performance numbers with and without the patch:
> 
> mhash  |  8192   |  8192  |  8192       | 131072 | 131072      | 104857 | 104857
> mounts | before  | after  | after (sys) | after  | after (sys) |  after | after (sys)
> -------------------------------------------------------------------------------------
>   1024 |  0.071s | 0.020s | 0.000s      | 0.022s | 0.004s      | 0.020s | 0.004s
>   2048 |  0.184s | 0.022s | 0.004s      | 0.023s | 0.004s      | 0.022s | 0.008s
>   4096 |  0.604s | 0.025s | 0.020s      | 0.029s | 0.008s      | 0.026s | 0.004s
>   8912 |  4.471s | 0.053s | 0.020s      | 0.051s | 0.024s      | 0.047s | 0.016s
>  16384 | 34.826s | 0.088s | 0.060s      | 0.081s | 0.048s      | 0.082s | 0.052s
>  32768 |         | 0.216s | 0.172s      | 0.160s | 0.124s      | 0.160s | 0.096s
>  65536 |         | 0.819s | 0.726s      | 0.330s | 0.260s      | 0.338s | 0.256s
> 131072 |         | 4.502s | 4.168s      | 0.707s | 0.580s      | 0.709s | 0.592s
> 
> Andrei Vagin reports fixing the performance problem is part of the
> work to fix CVE-2016-6213.
> 
> A script for a pathlogical set of mounts:
> 
> $ cat pathological.sh
> 
> mount -t tmpfs base /mnt
> mount --make-shared /mnt
> mkdir -p /mnt/b
> 
> mount -t tmpfs test1 /mnt/b
> mount --make-shared /mnt/b
> mkdir -p /mnt/b/10
> 
> mount -t tmpfs test2 /mnt/b/10
> mount --make-shared /mnt/b/10
> mkdir -p /mnt/b/10/20
> 
> mount --rbind /mnt/b /mnt/b/10/20
> 
> unshare -Urm sleep 2
> umount -l /mnt/b
> wait %%
> 
> $ unshare -Urm pathlogical.sh
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: a05964f3917c ("[PATCH] shared mounts handling: umount")
> Fixes: 0c56fe31420c ("mnt: Don't propagate unmounts to locked mounts")
> Reported-by: Andrei Vagin <avagin@xxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
>  fs/mount.h     |   1 +
>  fs/namespace.c |   7 +--
>  fs/pnode.c     | 179 +++++++++++++++++++++++++++++++++++++++++----------------
>  fs/pnode.h     |   2 +-
>  4 files changed, 133 insertions(+), 56 deletions(-)
> 
> diff --git a/fs/mount.h b/fs/mount.h
> index d2e25d7b64b3..00fe0d1d6ba7 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -58,6 +58,7 @@ struct mount {
>  	struct mnt_namespace *mnt_ns;	/* containing namespace */
>  	struct mountpoint *mnt_mp;	/* where is it mounted */
>  	struct hlist_node mnt_mp_list;	/* list mounts with the same mountpoint */
> +	struct list_head mnt_umounts;	/* list of children that are being unmounted */
>  #ifdef CONFIG_FSNOTIFY
>  	struct hlist_head mnt_fsnotify_marks;
>  	__u32 mnt_fsnotify_mask;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index e6c234b1a645..73801391bb00 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -237,6 +237,7 @@ static struct mount *alloc_vfsmnt(const char *name)
>  		INIT_LIST_HEAD(&mnt->mnt_slave_list);
>  		INIT_LIST_HEAD(&mnt->mnt_slave);
>  		INIT_HLIST_NODE(&mnt->mnt_mp_list);
> +		INIT_LIST_HEAD(&mnt->mnt_umounts);
>  #ifdef CONFIG_FSNOTIFY
>  		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
>  #endif
> @@ -650,13 +651,11 @@ struct mount *__lookup_mnt_last(struct vfsmount *mnt, struct dentry *dentry)
>  	p = __lookup_mnt(mnt, dentry);
>  	if (!p)
>  		goto out;
> -	if (!(p->mnt.mnt_flags & MNT_UMOUNT))
> -		res = p;
> +	res = p;
>  	hlist_for_each_entry_continue(p, mnt_hash) {
>  		if (&p->mnt_parent->mnt != mnt || p->mnt_mountpoint != dentry)
>  			break;
> -		if (!(p->mnt.mnt_flags & MNT_UMOUNT))
> -			res = p;
> +		res = p;
>  	}
>  out:
>  	return res;
> diff --git a/fs/pnode.c b/fs/pnode.c
> index 234a9ac49958..8fd1a3fb420c 100644
> --- a/fs/pnode.c
> +++ b/fs/pnode.c
> @@ -134,7 +134,8 @@ void change_mnt_propagation(struct mount *mnt, int type)
>  }
>  
>  /*
> - * get the next mount in the propagation tree.
> + * get the next mount that is not a slave of the current mount in the
> + * propagation tree.
>   * @m: the mount seen last
>   * @origin: the original mount from where the tree walk initiated
>   *
> @@ -143,13 +144,9 @@ void change_mnt_propagation(struct mount *mnt, int type)
>   * vfsmount found while iterating with propagation_next() is
>   * a peer of one we'd found earlier.
>   */
> -static struct mount *propagation_next(struct mount *m,
> -					 struct mount *origin)
> +static struct mount *propagation_next_sib(struct mount *m,
> +						struct mount *origin)
>  {
> -	/* are there any slaves of this mount? */
> -	if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
> -		return first_slave(m);
> -
>  	while (1) {
>  		struct mount *master = m->mnt_master;
>  
> @@ -164,6 +161,26 @@ static struct mount *propagation_next(struct mount *m,
>  	}
>  }
>  
> +/*
> + * get the next mount in the propagation tree.
> + * @m: the mount seen last
> + * @origin: the original mount from where the tree walk initiated
> + *
> + * Note that peer groups form contiguous segments of slave lists.
> + * We rely on that in get_source() to be able to find out if
> + * vfsmount found while iterating with propagation_next() is
> + * a peer of one we'd found earlier.
> + */
> +static struct mount *propagation_next(struct mount *m,
> +					 struct mount *origin)
> +{
> +	/* are there any slaves of this mount? */
> +	if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
> +		return first_slave(m);
> +
> +	return propagation_next_sib(m, origin);
> +}
> +
>  static struct mount *next_group(struct mount *m, struct mount *origin)
>  {
>  	while (1) {
> @@ -389,57 +406,92 @@ void propagate_mount_unlock(struct mount *mnt)
>  	}
>  }
>  
> -/*
> - * Mark all mounts that the MNT_LOCKED logic will allow to be unmounted.
> - */
> -static void mark_umount_candidates(struct mount *mnt)
> +static struct mount *propagation_visit_child(struct mount *last_child,
> +					    struct mount *origin_child)
>  {
> -	struct mount *parent = mnt->mnt_parent;
> -	struct mount *m;
> +	struct mount *m = last_child->mnt_parent;
> +	struct mount *origin = origin_child->mnt_parent;
> +	struct dentry *mountpoint = origin_child->mnt_mountpoint;
> +	struct mount *child;
>  
> -	BUG_ON(parent == mnt);
> +	/* Has this part of the propgation tree already been visited? */
> +	if (IS_MNT_MARKED(last_child))
> +		return NULL;
>  
> -	for (m = propagation_next(parent, parent); m;
> -			m = propagation_next(m, parent)) {
> -		struct mount *child = __lookup_mnt_last(&m->mnt,
> -						mnt->mnt_mountpoint);
> -		if (child && (!IS_MNT_LOCKED(child) || IS_MNT_MARKED(m))) {
> -			SET_MNT_MARK(child);
> -		}
> +	SET_MNT_MARK(last_child);
> +
> +	m = propagation_next(m, origin);
> +	while (m) {
> +		child = __lookup_mnt_last(&m->mnt, mountpoint);
> +		if (child && !IS_MNT_MARKED(child))
> +			return child;
> +
> +		if (!child)
> +			m = propagation_next(m, origin);
> +		else
> +			m = propagation_next_sib(m, origin);
>  	}
> +	return NULL;
>  }
>  
> -/*
> - * NOTE: unmounting 'mnt' naturally propagates to all other mounts its
> - * parent propagates to.
> - */
> -static void __propagate_umount(struct mount *mnt)
> +static struct mount *propagation_revisit_child(struct mount *last_child,
> +					       struct mount *origin_child)
>  {
> -	struct mount *parent = mnt->mnt_parent;
> -	struct mount *m;
> +	struct mount *m = last_child->mnt_parent;
> +	struct mount *origin = origin_child->mnt_parent;
> +	struct dentry *mountpoint = origin_child->mnt_mountpoint;
> +	struct mount *child;
>  
> -	BUG_ON(parent == mnt);
> +	/* Has this part of the propgation tree already been revisited? */
> +	if (!IS_MNT_MARKED(last_child))
> +		return NULL;
>  
> -	for (m = propagation_next(parent, parent); m;
> -			m = propagation_next(m, parent)) {
> +	CLEAR_MNT_MARK(last_child);
>  
> -		struct mount *child = __lookup_mnt_last(&m->mnt,
> -						mnt->mnt_mountpoint);
> -		/*
> -		 * umount the child only if the child has no children
> -		 * and the child is marked safe to unmount.
> -		 */
> -		if (!child || !IS_MNT_MARKED(child))
> -			continue;
> -		CLEAR_MNT_MARK(child);
> -		if (list_empty(&child->mnt_mounts)) {
> -			list_del_init(&child->mnt_child);
> -			child->mnt.mnt_flags |= MNT_UMOUNT;
> -			list_move_tail(&child->mnt_list, &mnt->mnt_list);
> -		}
> +	m = propagation_next(m, origin);
> +	while (m) {
> +		child = __lookup_mnt_last(&m->mnt, mountpoint);
> +		if (child && IS_MNT_MARKED(child))
> +			return child;
> +
> +		if (!child)
> +			m = propagation_next(m, origin);
> +		else
> +			m = propagation_next_sib(m, origin);
>  	}
> +	return NULL;
>  }
>  
> +static void start_umount_propagation(struct mount *child,
> +				     struct list_head *to_umount)
> +{
> +	do {
> +		struct mount *parent = child->mnt_parent;
> +
> +		if ((child->mnt.mnt_flags & MNT_UMOUNT) ||
> +		    !list_empty(&child->mnt_mounts))
> +			return;
> +
> +		if (!IS_MNT_LOCKED(child))
> +			list_move_tail(&child->mnt_child, to_umount);
> +		else
> +			list_move_tail(&child->mnt_child, &parent->mnt_umounts);
> +
> +		child = NULL;
> +		if (IS_MNT_MARKED(parent))
> +			child = parent;
> +	} while (child);
> +}
> +
> +static void end_umount_propagation(struct mount *child)
> +{
> +	struct mount *parent = child->mnt_parent;
> +
> +	if (!list_empty(&parent->mnt_umounts))
> +		list_splice_tail_init(&parent->mnt_umounts, &parent->mnt_mounts);
> +}
> +
> +
>  /*
>   * collect all mounts that receive propagation from the mount in @list,
>   * and return these additional mounts in the same list.
> @@ -447,14 +499,39 @@ static void __propagate_umount(struct mount *mnt)
>   *
>   * vfsmount lock must be held for write
>   */
> -int propagate_umount(struct list_head *list)
> +void propagate_umount(struct list_head *list)
>  {
>  	struct mount *mnt;
> +	LIST_HEAD(to_umount);
> +	LIST_HEAD(tmp_list);
> +
> +	/* Find candidates for unmounting */
> +	list_for_each_entry(mnt, list, mnt_list) {
> +		struct mount *child;
> +		for (child = propagation_visit_child(mnt, mnt); child;
> +		     child = propagation_visit_child(child, mnt))
> +			start_umount_propagation(child, &to_umount);
> +	}
>  
> -	list_for_each_entry_reverse(mnt, list, mnt_list)
> -		mark_umount_candidates(mnt);
> +	/* Begin unmounting */
> +	while (!list_empty(&to_umount)) {
> +		mnt = list_first_entry(&to_umount, struct mount, mnt_child);
>  
> -	list_for_each_entry(mnt, list, mnt_list)
> -		__propagate_umount(mnt);
> -	return 0;
> +		list_del_init(&mnt->mnt_child);
> +		mnt->mnt.mnt_flags |= MNT_UMOUNT;
> +		list_move_tail(&mnt->mnt_list, &tmp_list);
> +
> +		if (!list_empty(&mnt->mnt_umounts))
> +			list_splice_tail_init(&mnt->mnt_umounts, &to_umount);
> +	}
> +
> +	/* Cleanup the mount propagation tree */
> +	list_for_each_entry(mnt, list, mnt_list) {
> +		struct mount *child;
> +		for (child = propagation_revisit_child(mnt, mnt); child;
> +		     child = propagation_revisit_child(child, mnt))
> +			end_umount_propagation(child);
> +	}
> +
> +	list_splice_tail(&tmp_list, list);
>  }
> diff --git a/fs/pnode.h b/fs/pnode.h
> index 550f5a8b4fcf..38c6cdb96b34 100644
> --- a/fs/pnode.h
> +++ b/fs/pnode.h
> @@ -41,7 +41,7 @@ static inline void set_mnt_shared(struct mount *mnt)
>  void change_mnt_propagation(struct mount *, int);
>  int propagate_mnt(struct mount *, struct mountpoint *, struct mount *,
>  		struct hlist_head *);
> -int propagate_umount(struct list_head *);
> +void propagate_umount(struct list_head *);
>  int propagate_mount_busy(struct mount *, int);
>  void propagate_mount_unlock(struct mount *);
>  void mnt_release_group_id(struct mount *);
> -- 
> 2.10.1
>