linux-kernel - Re: [PATCH v3 0/4] Add seccomp notifier ioctl that enables adding fds

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Date:   Wed, 3 Jun 2020 19:44:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Matt Denton <mpdenton@...gle.com>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Robert Sesek <rsesek@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 0/4] Add seccomp notifier ioctl that enables adding fds

On Wed, Jun 03, 2020 at 04:56:59PM -0700, Sargun Dhillon wrote:
> On Wed, Jun 3, 2020 at 4:42 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Tue, Jun 02, 2020 at 06:10:40PM -0700, Sargun Dhillon wrote:
> > > Sargun Dhillon (4):
> > >   fs, net: Standardize on file_receive helper to move fds across
> > >     processes
> > >   pid: Use file_receive helper to copy FDs
> >
> > The fixes (that should add open-coded cgroups stuff) should be separate
> > patches so they can be backported.
> Patch 1/4, and 2/4 are separated so they can be backported. Patch 1 should
> go into long term, and patch 2 should land in stable.
> 
> Do you see anything in 1/4, and 2/4 that shouldn't be there?

So, my thinking was to open code the fixes to minimize the code churn
in the -stable trees and isolate logical changes. However, in looking
at the commits (3.6, 3.8) and the age of the rest of the nearby code in
SCM_RIGHTS (3.7), and the actual oldest supported kernel release (3.16),
I guess it would be better to split it like you've done.

> > The helper doesn't take the __user pointer I thought we'd agreed it
> > should to avoid changing any SCM_RIGHTS behaviors?
> >
> It doesn't change the SCM_RIGHTS behaviour because it continues
> to have the logic which allocates the file descriptor outside of the
> helper.
> 1. Allocate FD (this happens in scm.c)
> 2. Copy FD # to userspace (this happens in scm.c)
> 3. Receive FD (this happens in the new helper)

Sorry, I was not writing very clearly: I was meaning to have said:

I was expecting the helper to take the __user pointer (like I thought
we agreed[1]) so we could both avoid changing SCM_RIGHTS behavior and
avoid copy/pasting of the get_unused/put_unused code in 3 places. (I
get into this more in the other thread[2]).

So, let's finalize this decision in the thread at [2]. Again, sorry I
wasted your time due to my confusion!

-Kees

[1] Apologies, I misread your "1" in [3] to be "suggestion 1" from the
    quoted text from me in that email, rather than the "[1]" it was,
    which was a link to your counter-proposal. And then I wasted your
    time by saying "agreed".
[2] https://lore.kernel.org/lkml/202006031845.F587F85A@keescook/
[3] https://lore.kernel.org/lkml/20200530011054.GA14852@ircssh-2.c.rugged-nimbus-611.internal/

-- 
Kees Cook