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

From: Andy Lutomirski
Date: Tue Oct 14 2014 - 17:19:47 EST

On Tue, Oct 14, 2014 at 2:13 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes:
>> On Tue, Oct 14, 2014 at 01:01:02PM -0700, Eric W. Biederman wrote:
>>> Michael j Theall <mtheall@xxxxxxxxxx> writes:
>>> > Seth Forshee <seth.forshee@xxxxxxxxxxxxx> wrote on 10/14/2014 09:25:55 AM:
>>> >
>>> >> From: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
>>> >> To: Miklos Szeredi <miklos@xxxxxxxxxx>
>>> >> Cc: fuse-devel@xxxxxxxxxxxxxxxxxxxxx, "Serge H. Hallyn"
>>> >> <serge.hallyn@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Seth
>>> >> Forshee <seth.forshee@xxxxxxxxxxxxx>, "Eric W. Biederman"
>>> >> <ebiederm@xxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx
>>> >> 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.
>> So the server could for example return trusted.* xattrs which were not
>> set by a privileged user.
>>> > 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.
>> That I wasn't aware of. In fact I still haven't found where this
>> restriction is implemented.
> My memory may be have been incomplete. What I was thinking of is
> security/commoncap.c the MNT_NOSUID check in get_file_caps.
> Upon inspection that appears limited to file capabilities, and while
> there are a few other MNT_NOSUID checks under security the feel far from
> complete.
> Sigh.
> This deserves a hard look because if MNT_NOSUID is not sufficient than
> it may be possible for me to insert a usb stick with an extN filesystem
> with the right labels having it auto-mounted nosuid and subvert the
> security of something like selinux.

It's this code in selinux/hooks.c:

if ((bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
(bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS))
new_tsec->sid = old_tsec->sid;

One might argue that this should actually generate -EPERM instead of
ignoring the label, but it should be safe against untrusted media.

>> Nonetheless, a userns mount could be done without nosuid (though that
>> mount will also be unaccessible outside of that namespace).
>>> >> 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.
>> Except that unprivileged mounts are normally done by a suid root helper,
>> which is why I've required both global CAP_SYS_ADMIN and a mount option
>> to get the current default behavior.
> If nosuid is sufficient that may break existing setups for no good
> reason.
> Shrug. I won't have much time for a bit but I figured I would highlight
> the potential security hole in existing setups. So someone with time
> this week can look at that.

I think I have a better solution. I'll send it out.

Serge had also mentioned adding some kind of hook to help LSMs handle
user namespaces more intelligently.


> Eric

Andy Lutomirski
AMA Capital Management, LLC
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