linux-kernel - Re: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs only with a mount option

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Date:	Tue, 14 Oct 2014 13:01:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Michael j Theall <mtheall@...ibm.com>
Cc:	Seth Forshee <seth.forshee@...onical.com>,
	fuse-devel@...ts.sourceforge.net, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>
Subject: Re: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs only	with a mount option

Michael j Theall <mtheall@...ibm.com> writes:

> Seth Forshee <seth.forshee@...onical.com> wrote on 10/14/2014 09:25:55 AM:
>
>> From: Seth Forshee <seth.forshee@...onical.com>
>> To: Miklos Szeredi <miklos@...redi.hu>
>> Cc: fuse-devel@...ts.sourceforge.net, "Serge H. Hallyn" 
>> <serge.hallyn@...ntu.com>, linux-kernel@...r.kernel.org, Seth 
>> Forshee <seth.forshee@...onical.com>, "Eric W. Biederman" 
>> <ebiederm@...ssion.com>, linux-fsdevel@...r.kernel.org
>> Date: 10/14/2014 09:27 AM
>> Subject: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs
>> only with a mount option
>> 
>> Allowing unprivileged users to provide arbitrary xattrs via fuse
>> mounts bypasses the normal restrictions on setting xattrs. Such
>> mounts should be restricted to reading and writing xattrs in the
>> user.* namespace.
>> 
>
> Can you explain how the normal restrictions on setting xattrs are 
> bypassed?

If the fuse server is not run by root.  Which is a large part of the
point of fuse.

> My filesystem still needs security.* and system.*, and it looks like
> xattr_permission already prevents non-privileged users from accessing
> trusted.*

If the filesystem is mounted with nosuid (typical of a non-privileged
mount of fuse) then the security.* attributes are ignored.

>> It's difficult though to tell whether a mount is being performed
>> on behalf of an unprivileged user since fuse mounts are ususally
>> done via a suid root helper. Thus a new mount option,
>> privileged_xattrs, is added to indicated that xattrs from other
>> namespaces are allowed. This option can only be supplied by
>> system-wide root; supplying the option as an unprivileged user
>> will cause the mount to fail.
>
> I can't say I'm convinced that this is the right direction to head.

With respect to defaults we could keep the current default if you
have the global CAP_SYS_ADMIN privilege when the mount takes place
and then avoid breaking anything.

Eric
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