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Date:   Wed, 25 Jan 2017 13:48:19 -0800
From:   Andy Lutomirski <>
To:     Ben Hutchings <>
Cc:     Andy Lutomirski <>,
        "" <>,
        Konstantin Khlebnikov <>,
        Alexander Viro <>,
        Kees Cook <>, Willy Tarreau <>,
        "" <>,
        Andrew Morton <>,
        yalin wang <>,
        Linux Kernel Mailing List <>,
        Jan Kara <>,
        Linux FS Devel <>,
        stable <>
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 <> 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.


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