Date:   Fri, 14 Jun 2019 07:08:50 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Brauner <christian@...uner.io>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Regression for MS_MOVE on kernel v5.1

Christian Brauner <christian@...uner.io> writes:

> On Thu, Jun 13, 2019 at 04:59:24PM -0500, Eric W. Biederman wrote:
>> Miklos Szeredi <miklos@...redi.hu> writes:
>> 
>> > On Thu, Jun 13, 2019 at 8:35 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> >>
>> >> Christian Brauner <christian@...uner.io> writes:
>> >>
>> >> > On Wed, Jun 12, 2019 at 06:00:39PM -1000, Linus Torvalds wrote:
>> >> >> On Wed, Jun 12, 2019 at 12:54 PM Christian Brauner <christian@...uner.io> wrote:
>> >> >> >
>> >> >> > The commit changes the internal logic to lock mounts when propagating
>> >> >> > mounts (user+)mount namespaces and - I believe - causes do_mount_move()
>> >> >> > to fail at:
>> >> >>
>> >> >> You mean 'do_move_mount()'.
>> >> >>
>> >> >> > if (old->mnt.mnt_flags & MNT_LOCKED)
>> >> >> >         goto out;
>> >> >> >
>> >> >> > If that's indeed the case we should either revert this commit (reverts
>> >> >> > cleanly, just tested it) or find a fix.
>> >> >>
>> >> >> Hmm.. I'm not entirely sure of the logic here, and just looking at
>> >> >> that commit 3bd045cc9c4b ("separate copying and locking mount tree on
>> >> >> cross-userns copies") doesn't make me go "Ahh" either.
>> >> >>
>> >> >> Al? My gut feel is that we need to just revert, since this was in 5.1
>> >> >> and it's getting reasonably late in 5.2 too. But maybe you go "guys,
>> >> >> don't be silly, this is easily fixed with this one-liner".
>> >> >
>> >> > David and I have been staring at that code today for a while together.
>> >> > I think I made some sense of it.
>> >> > One thing we weren't absolutely sure is if the old MS_MOVE behavior was
>> >> > intentional or a bug. If it is a bug we have a problem since we quite
>> >> > heavily rely on this...
>> >>
>> >> It was intentional.
>> >>
>> >> The only mounts that are locked in propagation are the mounts that
>> >> propagate together.  If you see the mounts come in as individuals you
>> >> can always see/manipulate/work with the underlying mount.
>> >>
>> >> I can think of only a few ways for MNT_LOCKED to become set:
>> >> a) unshare(CLONE_NEWNS)
>> >> b) mount --rclone /path/to/mnt/tree /path/to/propagation/point
>> >> c) mount --move /path/to/mnt/tree /path/to/propgation/point
>> >>
>> >> Nothing in the target namespace should be locked on the propgation point
>> >> but all of the new mounts that came across as a unit should be locked
>> >> together.
>> >
>> > Locked together means the root of the new mount tree doesn't have
>> > MNT_LOCKED set, but all mounts below do have MNT_LOCKED, right?
>> >
>> > Isn't the bug here that the root mount gets MNT_LOCKED as well?
>
> Yes, we suspected this as well. We just couldn't pinpoint where the
> surgery would need to start.
>
>> 
>> Yes, and the code to remove MNT_LOCKED is still sitting there in
>> propogate_one right after it calls copy_tree.  It should be a trivial
>> matter of moving that change to after the lock_mnt_tree call.
>> 
>> Now that I have been elightened about anonymous mount namespaces
>> I am suspecting that we want to take the user_namespace of the anonymous
>> namespace into account when deciding to lock the mounts.
>> 
>> >> Then it breaking is definitely a regression that needs to be fixed.
>> >>
>> >> I believe the problematic change as made because the new mount
>> >> api allows attaching floating mounts.  Or that was the plan last I
>> >> looked.   Those floating mounts don't have a mnt_ns so will result
>> >> in a NULL pointer dereference when they are attached.
>> >
>> > Well, it's called anonymous namespace.  So there *is* an mnt_ns, and
>> > its lifetime is bound to the file returned by fsmount().
>> 
>> Interesting.  That has changed since I last saw the patches.
>> 
>> Below is what will probably be a straight forward fix for the regression.
>
> Tested the patch just now applied on top of v5.1. It fixes the
> regression.
> Can you please send a proper patch, Eric?
>
> Tested-by: Christian Brauner <christian@...uner.io>
> Acked-by: Christian Brauner <christian@...uner.io>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

I will let Al or whoever take this over the finish line.
I am too sleep deprived at the moment to say anything about the quality
of my patch.

Eric

>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index ffb13f0562b0..a39edeecbc46 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -2105,6 +2105,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
>>                 /* Notice when we are propagating across user namespaces */
>>                 if (child->mnt_parent->mnt_ns->user_ns != user_ns)
>>                         lock_mnt_tree(child);
>> +               child->mnt.mnt_flags &= ~MNT_LOCKED;
>>                 commit_tree(child);
>>         }
>>         put_mountpoint(smp);
>> diff --git a/fs/pnode.c b/fs/pnode.c
>> index 7ea6cfb65077..012be405fec0 100644
>> --- a/fs/pnode.c
>> +++ b/fs/pnode.c
>> @@ -262,7 +262,6 @@ static int propagate_one(struct mount *m)
>>         child = copy_tree(last_source, last_source->mnt.mnt_root, type);
>>         if (IS_ERR(child))
>>                 return PTR_ERR(child);
>> -       child->mnt.mnt_flags &= ~MNT_LOCKED;
>>         mnt_set_mountpoint(m, mp, child);
>>         last_dest = m;
>>         last_source = child;
>> 
>> 

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