linux-kernel - Re: [PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

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Date:	Wed, 30 Mar 2016 15:18:28 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...hat.com, linux-raid@...r.kernel.org,
	linux-mtd@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov
Subject: Re: [PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

Seth Forshee <seth.forshee@...onical.com> writes:

> On Tue, Mar 29, 2016 at 08:36:09PM -0500, Eric W. Biederman wrote:
>> Seth Forshee <seth.forshee@...onical.com> writes:
>> 
>> > On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:
>> >> In general this is only an issue if uids and gids on the filesystem
>> >> do not map into the user namespace.
>> >> 
>> >> Therefore the general fix is to limit the logic of checking for
>> >> capabilities in s_user_ns if we are dealing with INVALID_UID and
>> >> INVALID_GID.  For proc and kernfs that should never be the case
>> >> so the problem becomes a non-issue.
>> >> 
>> >> Further I would look at limiting that relaxation to just
>> >> inode_change_ok. 
>> >
>> > Finally got around to implementing this today; is the patch below what
>> > you had in mind?
>> 
>> Pretty much.
>> 
>> For the same reason that capble_wrt_inode_uidgid(inode) had to look
>> at both inode->i_uid and inode->i_gid I think we need to look at
>> both inode->i_uid and inode->i_gid in those case.
>> 
>> I am worried about chgrp_ok in cases such as inode->i_uid is valid
>> but unmapped.  I have a similiar worry about chown_ok where
>> inode->i_gid is valid but unmapped (although that worry is less
>> serious).
>
> That makes sense.
>
> So then what is wanted is to check that the other id is either invalid,
> or else it maps into s_user_ns. So for chown_ok() something like this:
>
>     if (!uid_valid(inode->i_uid) &&
>         (!gid_valid(inode->i_gid) || kgid_has_mapping(inode->i_sb->s_user_ns, inode->i_gid)) &&
>         ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
>             return true;
>
> and likewise for chgrp_ok(). Does that satisfy your concerns?

Yes it does.

Eric