linux-kernel - Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user

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Message-ID: <CALCETrU_Ap-cENF4nczn1FrtRxKfWBT0TdHN0cU4rJt=Nvxpgw@mail.gmail.com>
Date:	Fri, 21 Nov 2014 10:38:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Seth Forshee <seth.forshee@...onical.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns

On Fri, Nov 21, 2014 at 10:14 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> - Tweak the file capability code to look at s_user_ns and treat it
>   properly.
>

> - Tweak the security checks to allow setting file capabilities and
>   other security xattrs if we have the appropriate capabilities in
>   s_user_ns.
>

Thinking about this some more, what do you mean by tweaking the file
capability code to look at s_user_ns and treat it properly?

I think that the semantics should be that cap_inode_setxattr should
check ns_capable wrt s_user_ns, but that the fscap *consumer* should
check the mount as in my may_suid patch (and maybe also check
s_user_ns).  There is legacy code that starts a FUSE server as global
root, mounts the thing in a mount namespace belonging to an
unprivileged user ns, and (I think) hands the /dev/fuse fd to that
unprivileged code.

Without the mount ns check, that FUSE server can take over the system.
With the mount ns check, it's safe.

--Andy

>
> When those bits are done we can tweak the fuse patches to also set
> s_user_ns.
>
> As for MNT_NO_SUID if fuse wants to enforce that in some way.  I don't
> particularly care, but I don't think that makes sense as a vfs property.
>
> Eric



-- 
Andy Lutomirski
AMA Capital Management, LLC
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