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

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Date:   Sun, 14 May 2017 06:56:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Eric Biggers <ebiggers3@...il.com>,
        "Darrick J. Wong" <darrick.wong@...cle.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 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.

>>>
>>>>> 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.