linux-kernel - Re: [PATCH v4 2/5] fuse: Support fuse filesystems outside of init_user

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Message-ID: <CALCETrXtHGzBbxFjZ_S3Zb0KkN1y6Uqm6CNkYSY37BLLcocFGw@mail.gmail.com>
Date:	Wed, 15 Oct 2014 10:05:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...capital.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel@...ts.sourceforge.net,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>
Subject: Re: [PATCH v4 2/5] fuse: Support fuse filesystems outside of init_user_ns

On Wed, Oct 15, 2014 at 8:05 AM, Seth Forshee
<seth.forshee@...onical.com> wrote:
> On Wed, Oct 15, 2014 at 07:49:39AM -0700, Andy Lutomirski wrote:
>> On 10/14/2014 07:25 AM, Seth Forshee wrote:
>> > Update fuse to translate uids and gids to/from the user namspace
>> > of the process servicing requests on /dev/fuse. Any ids which do
>> > not map into the namespace will result in errors. inodes will
>> > also be marked bad when unmappable ids are received from
>> > userspace.
>> >
>> > Due to security concerns the namespace used should be fixed,
>> > otherwise a user might be able to gain elevated privileges or
>> > influence processes that the user would otherwise be unable to
>> > manipulate. Thus the namespace of the mounting process is used
>> > for all translations, and this namespace is required to be the
>> > same as the one in use when /dev/fuse was opened.
>> >
>>
>> I'm not sure that this is necessary if my nosuid patch goes in, but I
>> also don't think it makes any sense to hold this up while we find a
>> perfect solution.
>>
>> Is there a decent way to extend this to different translation schemes in
>> the future (e.g. a flag at fs setup that could be used)?
>
> I think it would be possible to relax the translation scheme
> restrictions in the future, certainly that's easier than tightening down
> a looser restriction. I still favor picking one namespace to use for
> translation (surely that's how it would work with other filesystems
> anyway) rather than using the current namespace during /dev/fuse I/O. I
> did an implementation using the latter technique, and it's far more
> complex with no benefits that I can see.

Long term, I think we'll want more flexible translations for
filesystems on removable media, even when both the mounter and the
accessing process are in the init user namespace.  But this can wait.

--Andy

>
> Thanks,
> Seth



-- 
Andy Lutomirski
AMA Capital Management, LLC
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