linux-kernel - Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS

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Date:   Thu, 1 Jul 2021 09:58:34 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Vivek Goyal <vgoyal@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc:     Theodore Ts'o <tytso@....edu>, Daniel Walsh <dwalsh@...hat.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "virtio-fs@...hat.com" <virtio-fs@...hat.com>,
        "berrange@...hat.com" <berrange@...hat.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special
 files if caller has CAP_SYS_RESOURCE

On 7/1/2021 6:10 AM, Vivek Goyal wrote:
> On Thu, Jul 01, 2021 at 09:48:33AM +0100, Dr. David Alan Gilbert wrote:
>> * Theodore Ts'o (tytso@....edu) wrote:
>>> On Wed, Jun 30, 2021 at 04:01:42PM +0100, Dr. David Alan Gilbert wrote:
>>>> Even if you fix symlinks, I don't think it fixes device nodes or
>>>> anything else where the permissions bitmap isn't purely used as the
>>>> permissions on the inode.
>>> I think we're making a mountain out of a molehill.  Again, very few
>>> people are using quota these days.  And if you give someone write
>>> access to a 8TB disk, do you really care if they can "steal" 32k worth
>>> of space (which is the maximum size of an xattr, enforced by the VFS).
>>>
>>> OK, but what about character mode devices?  First of all, most users
>>> don't have access to huge number of devices, but let's assume
>>> something absurd.  Let's say that a user has write access to *1024*
>>> devices.  (My /dev has 233 character mode devices, and I have write
>>> access to well under a dozen.)
>>>
>>> An 8TB disk costs about $200.  So how much of the "stolen" quota space
>>> are we talking about, assuming the user has access to 1024 devices,
>>> and the file system actually supports a 32k xattr.
>>>
>>>     32k * 1024 * $200 / 8TB / (1024*1024*1024) = $0.000763 = 0.0763 cents
>>>
>>> A 2TB SSD is less around $180, so even if we calculate the prices
>>> based on SSD space, we're still talking about a quarter of a penny.
>>>
>>> Why are we worrying about this?
>> I'm not worrying about storage cost, but we would need to define what
>> the rules are on who can write and change a user.* xattr on a device
>> node.  It doesn't feel sane to make it anyone who can write to the
>> device; then everyone can start leaving droppings on /dev/null.
>>
>> The other evilness I can imagine, is if there's a 32k limit on xattrs on
>> a node, an evil user could write almost 32k of junk to the node
>> and then break the next login that tries to add an acl or breaks the
>> next relabel.
> I guess 64k is per xattr VFS size limit.
>
> #define XATTR_SIZE_MAX 65536
>
> I just wrote a simple program to write "user.<N>" xattrs of size 1K
> each and could easily write 1M xattrs. So that 1G worth data right
> there. I did not try to push it further.
>
> So a user can write lot of data in the form of user.* xattrs on
> symlinks and device nodes if were to open it unconditionally. Hence
> permission semantics will probably will have to defined properly.
>
> I am wondering will it be alright if owner of the file (or CAP_FOWNER),
> is allowed to write user.* xattrs on symlinks and special files.

That would be sensible.
That's independent of your xattr mapping scheme.

>
> Vivek
>