Message-ID: <scmyycf2trich22v25s6gpe3ib6ejawflwf76znxg7sedqablp@ejfycd34xvpa>
Date: Wed, 17 Sep 2025 11:25:55 +0200
From: Jan Kara <jack@...e.cz>
To: Amir Goldstein <amir73il@...il.com>
Cc: Jan Kara <jack@...e.cz>, Jakub Acs <acsjakub@...zon.de>, 
	linux-unionfs@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, Christian Brauner <brauner@...nel.org>, 
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ovl: check before dereferencing s_root field

On Tue 16-09-25 15:29:35, Amir Goldstein wrote:
> On Tue, Sep 16, 2025 at 1:30 PM Jan Kara <jack@...e.cz> wrote:
> >
> > On Mon 15-09-25 17:29:40, Amir Goldstein wrote:
> > > On Mon, Sep 15, 2025 at 4:07 PM Jan Kara <jack@...e.cz> wrote:
> > > > > > diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
> > > > > > index 83f80fdb1567..424c73188e06 100644
> > > > > > --- a/fs/overlayfs/export.c
> > > > > > +++ b/fs/overlayfs/export.c
> > > > > > @@ -195,6 +195,8 @@ static int ovl_check_encode_origin(struct inode *inode)
> > > > > >         if (!ovl_inode_lower(inode))
> > > > > >                 return 0;
> > > > > >
> > > > > > +       if (!inode->i_sb->s_root)
> > > > > > +               return -ENOENT;
> > > > >
> > > > > For a filesystem method to have to check that its own root is still alive sounds
> > > > > like the wrong way to me.
> > > > > That's one of the things that should be taken for granted by fs code.
> > > > >
> > > > > I don't think this is an overlayfs specific issue, because other fs would be
> > > > > happy if encode_fh() would be called with NULL sb->s_root.
> > > >
> > > > Actually, I don't see where that would blow up? Generally references to
> > > > sb->s_root in filesystems outside of mount / remount code are pretty rare.
> > > > Also most of the code should be unreachable by the time we set sb->s_root
> > > > to NULL because there are no open files at that moment, no exports etc. But
> > > > as this report shows, there are occasional surprises (I remember similar
> > > > issue with ext4 sysfs files handlers using s_root without checking couple
> > > > years back).
> > > >
> > >
> > > I am not sure that I understand what you are arguing for.
> > > I did a very naive grep s_root fs/*/export.c and quickly found:
> >
> > You're better with grep than me ;). I was grepping for '->s_root' as well
> > but all the hits I had looked into were related to mounting and similar and
> > eventually I got bored. Restricting the grep to export ops indeed shows
> > ceph, gfs2 and overlayfs are vulnerable to this kind of problem.
> >
> > > static int gfs2_encode_fh(struct inode *inode, __u32 *p, int *len,
> > >                           struct inode *parent)
> > > {
> > > ...
> > >         if (!parent || inode == d_inode(sb->s_root))
> > >                 return *len;
> > >
> > > So it's not an overlayfs specific issue, just so happens that zysbot
> > > likes to test overlayfs.
> > >
> > > Are you suggesting that we fix all of those one by one?
> >
> > No. I agree we need to figure out a way to make sure export ops are not
> > called on a filesystem being unmounted. Standard open_by_handle() or NFS
> > export cannot race with generic_shutdown_super() (they hold the fs mounted)
> > so fsnotify is a special case here.
> >
> > I actually wonder if fanotify event (e.g. from inode deletion postponed to
> > some workqueue or whatever) cannot race with umount as well and cause the
> > same problem...
> >
> 
> Oy. I was thinking that all event happen when holding some mnt ref
> but yeh fsnotify_inoderemove() does look like it could be a problem
> from sb shutdown context.

Well, but there's also fun like fs/kernfs/file.c: kernfs_notify() which
queues work which calls fsnotify for some inodes and, frankly, proper
exclusion with umount seems non-existent there (but I can be missing
something).

Also we have fsnotify_sb_error() which can happen practically anytime
before the fs gets fully shutdown in ->kill_sb() and may try to encode fh
of an inode.

So there are not many cases where this can happen but enough that I'd say
that handling some events specially to avoid encoding fh on fs while it is
unmounted is fragile and prone to breaking again sooner or later.

> How about skipping fsnotify_inoderemove() in case sb is in shutdown?

Also how would you like to handle that in a race-free manner? We'd need to
hold s_umount for that which we cannot really afford in that context. But
maybe you have some better idea...

> > > > > Can we change the order of generic_shutdown_super() so that
> > > > > fsnotify_sb_delete(sb) is called before setting s_root to NULL?
> > > > >
> > > > > Or is there a better solution for this race?
> > > >
> > > > Regarding calling fsnotify_sb_delete() before setting s_root to NULL:
> > > > In 2019 (commit 1edc8eb2e9313 ("fs: call fsnotify_sb_delete after
> > > > evict_inodes")) we've moved the call after evict_inodes() because otherwise
> > > > we were just wasting cycles scanning many inodes without watches. So moving
> > > > it earlier wouldn't be great...
> > >
> > > Yes, I noticed that and I figured there were subtleties.
> >
> > Right. After thinking more about it I think calling fsnotify_sb_delete()
> > earlier is the only practical choice we have (not clearing sb->s_root isn't
> > much of an option - we need to prune all dentries to quiesce the filesystem
> > and leaving s_root alive would create odd corner cases). But you don't want
> > to be iterating millions of inodes just to clear couple of marks so we'll
> > have to figure out something more clever there.
> 
> I think we only need to suppress the fsnotify_inoderemove() call.
> It sounds doable and very local to fs/super.c.
> 
> Regarding show_mark_fhandle() WDYT about my suggestion to
> guard it with super_trylock_shared()?

Yes, super_trylock_shared() for that callsite looks like a fine solution
for that call site. Occasional random failures in encoding fh because the
trylock fails are unlikely to have any bad consequences there. But I think
we need to figure out other possibly racing call-sites as well first.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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