linux-kernel - Re: [PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove

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Date:   Wed, 25 Jan 2017 13:48:19 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Ben Hutchings <ben@...adent.org.uk>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "security@...nel.org" <security@...nel.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>, Willy Tarreau <w@....eu>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        yalin wang <yalin.wang2010@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove_suid()

On Wed, Jan 25, 2017 at 1:43 PM, Ben Hutchings <ben@...adent.org.uk> wrote:
> On Wed, 2017-01-25 at 13:06 -0800, Andy Lutomirski wrote:
>> If an unprivileged program opens a setgid file for write and passes
>> the fd to a privileged program and the privileged program writes to
>> it, we currently fail to clear the setgid bit.  Fix it by checking
>> f_cred instead of current's creds whenever a struct file is
>> involved.
> [...]
>
> What if, instead, a privileged program passes the fd to an un
> unprivileged program?  It sounds like a bad idea to start with, but at
> least currently the unprivileged program is going to clear the setgid
> bit when it writes.  This change would make that behaviour more
> dangerous.

Hmm.  Although, if a privileged program does something like:

(sudo -u nobody echo blah) >setuid_program

presumably it wanted to make the change.

>
> Perhaps there should be a capability check on both the current
> credentials and file credentials?  (I realise that we've considered
> file credential checks to be sufficient elsewhere, but those cases
> involved virtual files with special semantics, where it's clearer that
> a privileged process should not pass them to an unprivileged process.)
>

I could go either way.

What I really want to do is to write a third patch that isn't for
-stable that just removes the capable() check entirely.  I'm
reasonably confident it won't break things for a silly reason: because
it's capable() and not ns_capable(), anything it would break would
also be broken in an unprivileged container, and I haven't seen any
reports of package managers or similar breaking for this reason.

--Andy