linux-kernel - Re: [PATCH RESEND v2 16/18] fuse: Support fuse filesystems outside of init_user

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Date:	Mon, 14 Mar 2016 21:58:43 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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>,
	Kernel Mailing List <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 <linux-fsdevel@...r.kernel.org>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	LSM <linux-security-module@...r.kernel.org>,
	selinux@...ho.nsa.gov
Subject: Re: [PATCH RESEND v2 16/18] fuse: Support fuse filesystems outside of init_user_ns

On Wed, Mar 9, 2016 at 6:07 PM, Seth Forshee <seth.forshee@...onical.com> wrote:
> On Wed, Mar 09, 2016 at 04:51:42PM +0100, Miklos Szeredi wrote:
>> On Wed, Mar 9, 2016 at 4:25 PM, Seth Forshee <seth.forshee@...onical.com> wrote:
>> > On Wed, Mar 09, 2016 at 03:48:22PM +0100, Miklos Szeredi wrote:
>>
>> >> Can't we use current_cred()->uid/gid? Or fsuid/fsgid maybe?
>> >
>> > That would be a departure from the current behavior in the !allow_other
>> > case for unprivileged users. Since those mounts are done by an suid
>> > helper all of those ids would be root in the userns, wouldn't they?
>>
>> Well, actually this is what the helper does:
>>
>>     sprintf(d, "fd=%i,rootmode=%o,user_id=%u,group_id=%u",
>>         fd, rootmode, getuid(), getgid());
>
> Sorry, I was thinking of euid. So this may not be a problem.
>
>> So it just uses the current uid/gid.  Apparently no reason to do this
>> in userland, we could just as well set these in the kernel.  Except
>> for possible backward compatibility problems for things not using the
>> helper.
>>
>> BUT if the mount is unprivileged or it's a userns mount, or anything
>> previously not possible, then we are not constrained by the backward
>> compatibility issues, and can go with the saner solution.
>>
>> Does that not make sense?
>
> But we generally do want backwards compatibility, and we want userspace
> software to be able to expect the same behavior whether or not it's
> running in a user namespaced container. Obviously we can't always have
> things 100% identical, but we shouldn't break things unless we really
> need to.
>
> However it may be that this isn't actually going to break assumptions of
> existing software like I had feared. My preference is still to not
> change any userspace-visible behaviors since we never know what software
> might have made assumptions based on those behaviors. But if you're
> confident that it won't break anything I'm willing to give it a try.

I'm quite confident it won't make a difference.

Thanks,
Miklos