linux-ext4 - Re: [PATCH] ioctl_getfsmap.2: document the GETFSMAP ioctl

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Message-ID: <20170518020416.GF4514@birch.djwong.org>
Date:   Wed, 17 May 2017 19:04:16 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Eric Biggers <ebiggers3@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Theodore Ts'o" <tytso@....edu>, Jann Horn <jannh@...gle.com>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        linux-xfs@...r.kernel.org,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH] ioctl_getfsmap.2: document the GETFSMAP ioctl

On Sun, May 14, 2017 at 06:56:10AM -0700, Andy Lutomirski wrote:
> On Sat, May 13, 2017 at 6:41 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> > On May 10, 2017, at 11:10 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> >>
> >> On Wed, May 10, 2017 at 01:14:37PM -0700, Darrick J. Wong wrote:
> >>> [cc btrfs, since afaict that's where most of the dedupe tool authors hang out]
> 
> >> Yes, PIDs have traditionally been global, but today we have PID namespaces, and
> >> many other isolation features such as mount namespaces.  Nothing is perfect, of
> >> course, and containers are a lot worse than VMs, but it seems weird to use that
> >> as an excuse to knowingly make things worse...
> >>
> 
> Indeed.  Not only PID namespaces -- we have hidepid and we can simply
> unmount /proc.  "There are other info leaks" is a poor excuse.

Eh.  From the sounds of it I'm not all that impressed at the isolation
and leakproofness of any of these schemes.  Regardless, I will rephrase
the manpage to emphasize more strongly that filesystems are under no
obligation to share inode numbers, privileged callers or otherwise.

> >>>
> >>>>> Fortunately, the days of timesharing seem to well behind us.  For
> >>>>> those people who think that containers are as secure as VM's (hah,
> >>>>> hah, hah), it might be that best way to handle this is to have a mount
> >>>>> option that requires root access to this functionality.  For those
> >>>>> people who really care about this, they can disable access.
> >>>
> >>> Or use separate filesystems for each container so that exploitable bugs
> >>> that shut down the filesystem can't be used to kill the other
> >>> containers.  You could use a torrent of metadata-heavy operations
> >>> (fallocate a huge file, punch every block, truncate file, repeat) to DoS
> >>> the other containers.
> >>>
> >>>> What would be the reason for not putting this behind
> >>>> capable(CAP_SYS_ADMIN)?
> >>>>
> >>>> What possible legitimate function could this functionality serve to
> >>>> users who don't own your filesystem?
> >>>
> >>> As I've said before, it's to enable dedupe tools to decide, given a set
> >>> of files with shareable blocks, roughly how many other times each of
> >>> those shareable blocks are shared so that they can make better decisions
> >>> about which file keeps its shareable blocks, and which file gets
> >>> remapped.  Dedupe is not a privileged operation, nor are any of the
> >>> tools.
> >>>
> >>
> >> So why does the ioctl need to return all extent mappings for the entire
> >> filesystem, instead of just the share count of each block in the file that the
> >> ioctl is called on?
> >
> > One possibility is that the ioctl() can return the mapping for all inodes
> > owned by the calling PID (or others if CAP_SYS_ADMIN, CAP_DAC_OVERRIDE,
> > or CAP_FOWNER is set), and return an "filesystem aggregate inode" (or more
> > than one if there is a reason to do so) with all the other allocated blocks
> > for inodes the user doesn't have permission to access?
> 
> Sounds like it could be reasonable.  But you don't want "owned by the
> calling PID" precisely -- you also need to check
> kgid_has_mapping(current_user_ns(), inode->i_gid), I think.

Not to mention that I don't want to go xfs_igetting every inode across
the entire filesystem... :)

--D

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