linux-kernel - Re: [PATCH] fix writing to the filesystem after unmount

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 6 Sep 2023 17:58:18 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Zdenek Kabelac <zkabelac@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <djwong@...nel.org>
Subject: Re: [PATCH] fix writing to the filesystem after unmount

On Wed, Sep 06, 2023 at 05:33:32PM +0200, Christian Brauner wrote:
> > Currently, if we freeze a filesystem with "fsfreeze" and unmount it, the 
> > mount point is removed, but the filesystem stays active and it is leaked. 
> > You can't unfreeze it with "fsfreeze --unfreeze" because the mount point 
> > is gone. (the only way how to recover it is "echo j>/proc/sysrq-trigger").
> 
> You can of course always remount and unfreeze it.
> 
> > > IOW, you'd also hang on any umount of a bind-mount. IOW, every
> > > single container making use of this filesystems via bind-mounts would
> > > hang on umount and shutdown.
> > 
> > bind-mount doesn't modify "s->s_writers.frozen", so the patch does nothing 
> > in this case. I tried unmounting bind-mounts and there was no deadlock.
> 
> With your patch what happens if you do the following?
> 
> #!/bin/sh -ex
> modprobe brd rd_size=4194304
> vgcreate vg /dev/ram0
> lvcreate -L 16M -n lv vg
> mkfs.ext4 /dev/vg/lv
> 
> mount -t ext4 /dev/vg/lv /mnt/test
> mount --bind /mnt/test /opt
> mount --make-private /opt
> 
> dmsetup suspend /dev/vg/lv
> (sleep 1; dmsetup resume /dev/vg/lv) &
> 
> umount /opt # I'd expect this to hang
> 
> md5sum /dev/vg/lv
> md5sum /dev/vg/lv
> dmsetup remove_all
> rmmod brd
> 
> > BTW. what do you think that unmount of a frozen filesystem should properly 
> > do? Fail with -EBUSY? Or, unfreeze the filesystem and unmount it? Or 
> > something else?
> 
> In my opinion we should refuse to unmount frozen filesystems and log an
> error that the filesystem is frozen. Waiting forever isn't a good idea
> in my opinion.
> 
> But this is a significant uapi change afaict so this would need to be
> hidden behind a config option, a sysctl, or it would have to be a new
> flag to umount2() MNT_UNFROZEN which would allow an administrator to use
> this flag to not unmount a frozen filesystems.

That's probably too careful. I think we could risk starting to return an
error when trying to unmount a frozen filesystem. And if that causes
regressions we could go and look at another option like MNT_UNFROZEN or
whatever.