Patch "mnt: In umount propagation reparent in a separate pass" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mnt: In umount propagation reparent in a separate pass

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mnt-in-umount-propagation-reparent-in-a-separate-pass.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Mon, 15 May 2017 14:42:07 -0500
Subject: mnt: In umount propagation reparent in a separate pass

From: Eric W. Biederman <[email protected]>

commit 570487d3faf2a1d8a220e6ee10f472163123d7da upstream.

It was observed that in some pathlogical cases that the current code does not unmount everything it should. After investigation it was determined that the issue is that mnt_change_mntpoint can can change which mounts are available to be unmounted during mount propagation which is wrong.

The trivial reproducer is:

$ cat ./pathological.sh

mount -t tmpfs test-base /mnt
cd /mnt
mkdir 1 2 1/1
mount --bind 1 1
mount --make-shared 1
mount --bind 1 2
mount --bind 1/1 1/1
mount --bind 1/1 1/1
echo
grep test-base /proc/self/mountinfo
umount 1/1
echo
grep test-base /proc/self/mountinfo

$ unshare -Urm ./pathological.sh

The expected output looks like:
46 31 0:25 / /mnt rw,relatime - tmpfs test-base rw,uid=1000,gid=1000 47 46 0:25 /1 /mnt/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 48 46 0:25 /1 /mnt/2 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 49 54 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 50 53 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 51 49 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 54 47 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 53 48 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 52 50 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000

46 31 0:25 / /mnt rw,relatime - tmpfs test-base rw,uid=1000,gid=1000 47 46 0:25 /1 /mnt/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 48 46 0:25 /1 /mnt/2 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000

The output without the fix looks like:
46 31 0:25 / /mnt rw,relatime - tmpfs test-base rw,uid=1000,gid=1000 47 46 0:25 /1 /mnt/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 48 46 0:25 /1 /mnt/2 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 49 54 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 50 53 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 51 49 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 54 47 0:25 /1/1 /mnt/1/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 53 48 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 52 50 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000

46 31 0:25 / /mnt rw,relatime - tmpfs test-base rw,uid=1000,gid=1000 47 46 0:25 /1 /mnt/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 48 46 0:25 /1 /mnt/2 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000 52 48 0:25 /1/1 /mnt/2/1 rw,relatime shared:1 - tmpfs test-base rw,uid=1000,gid=1000

That last mount in the output was in the propgation tree to be unmounted but was missed because the mnt_change_mountpoint changed it's parent before the walk through the mount propagation tree observed it.

---
 fs/mount.h     |    1 +
 fs/namespace.c |    1 +
 fs/pnode.c     |   35 ++++++++++++++++++++++++++++++-----
 3 files changed, 32 insertions(+), 5 deletions(-)

@@ -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_reparent;	/* reparent list entry */
 #ifdef CONFIG_FSNOTIFY
 	struct hlist_head mnt_fsnotify_marks;
 	__u32 mnt_fsnotify_mask;

@@ -237,6 +237,7 @@ static struct mount *alloc_vfsmnt(const
 		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_reparent);
 #ifdef CONFIG_FSNOTIFY
 		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
 #endif

@@ -441,7 +441,7 @@ static void mark_umount_candidates(struc
  * NOTE: unmounting 'mnt' naturally propagates to all other mounts its
  * parent propagates to.
  */
-static void __propagate_umount(struct mount *mnt)
+static void __propagate_umount(struct mount *mnt, struct list_head *to_reparent)
 {
 	struct mount *parent = mnt->mnt_parent;
 	struct mount *m;

@@ -466,17 +466,38 @@ static void __propagate_umount(struct mo
 		 */
 		topper = find_topper(child);
 		if (topper)
-			mnt_change_mountpoint(child->mnt_parent, child->mnt_mp,
-					      topper);
+			list_add_tail(&topper->mnt_reparent, to_reparent);
 
-		if (list_empty(&child->mnt_mounts)) {
+		if (topper || list_empty(&child->mnt_mounts)) {
 			list_del_init(&child->mnt_child);
+			list_del_init(&child->mnt_reparent);
 			child->mnt.mnt_flags |= MNT_UMOUNT;
 			list_move_tail(&child->mnt_list, &mnt->mnt_list);
 		}
 	}
 }
 
+static void reparent_mounts(struct list_head *to_reparent)
+{
+	while (!list_empty(to_reparent)) {
+		struct mount *mnt, *parent;
+		struct mountpoint *mp;
+
+		mnt = list_first_entry(to_reparent, struct mount, mnt_reparent);
+		list_del_init(&mnt->mnt_reparent);
+
+		/* Where should this mount be reparented to? */
+		mp = mnt->mnt_mp;
+		parent = mnt->mnt_parent;
+		while (parent->mnt.mnt_flags & MNT_UMOUNT) {
+			mp = parent->mnt_mp;
+			parent = parent->mnt_parent;
+		}
+
+		mnt_change_mountpoint(parent, mp, mnt);
+	}
+}
+
 /*
  * collect all mounts that receive propagation from the mount in @list,
  * and return these additional mounts in the same list.

@@ -487,11 +508,15 @@ static void __propagate_umount(struct mo
 int propagate_umount(struct list_head *list)
 {
 	struct mount *mnt;
+	LIST_HEAD(to_reparent);
 
 	list_for_each_entry_reverse(mnt, list, mnt_list)
 		mark_umount_candidates(mnt);
 
 	list_for_each_entry(mnt, list, mnt_list)
-		__propagate_umount(mnt);
+		__propagate_umount(mnt, &to_reparent);
+
+	reparent_mounts(&to_reparent);
+
 	return 0;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mnt-in-propgate_umount-handle-visiting-mounts-in-any-order.patch queue-4.9/selftests-capabilities-fix-the-test_execve-test.patch queue-4.9/mnt-in-umount-propagation-reparent-in-a-separate-pass.patch queue-4.9/mnt-make-propagate_umount-less-slow-for-overlapping-mount-propagation-trees.patch