linux-kernel - Re: [PATCH v2 2/4] seccomp: Refactor notification handler to prepare for new semantics

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Date:   Thu, 27 May 2021 11:41:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rodrigo Campos <rodrigo@...volk.io>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        LKML <linux-kernel@...r.kernel.org>, containers@...ts.linux.dev,
        Tycho Andersen <tycho@...ho.pizza>,
        Andy Lutomirski <luto@...nel.org>,
        Mauricio Vásquez Bernal 
        <mauricio@...volk.io>, Giuseppe Scrivano <gscrivan@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v2 2/4] seccomp: Refactor notification handler to prepare
 for new semantics

On Thu, May 27, 2021 at 01:51:13PM +0200, Rodrigo Campos wrote:
> On Mon, May 17, 2021 at 9:39 PM Sargun Dhillon <sargun@...gun.me> wrote:
> >
> > This refactors the user notification code to have a do / while loop around
> > the completion condition. This has a small change in semantic, in that
> > previously we ignored addfd calls upon wakeup if the notification had been
> > responded to, but instead with the new change we check for an outstanding
> > addfd calls prior to returning to userspace.
> >
> > Rodrigo Campos also identified a bug that can result in addfd causing
> > an early return, when the supervisor didn't actually handle the
> > syscall [1].
> >
> > [1]: https://lore.kernel.org/lkml/20210413160151.3301-1-rodrigo@kinvolk.io/
> >
> > Fixes: 7cf97b125455 ("seccomp: Introduce addfd ioctl to seccomp user notifier")
> > Signed-off-by: Sargun Dhillon <sargun@...gun.me>
> > Acked-by: Tycho Andersen <tycho@...ho.pizza>
> 
> Kees, as I mentioned in the linked thread, this issue is present in
> 5.9+ kernels. Should we add the cc to stable for this patch? Or should
> we cc to stable the one linked, that just fixes the issue without
> semantic changes to userspace?

It sounds like the problem is with Go, using addfd, on 5.9-5.13 kernels,
yes? Would the semantic change be a problem there? (i.e. it sounds like
the semantic change was fine for the 5.14+ kernels, so I'm assuming it's
fine for earlier ones too.)

> Just to be clear, the other patch that fixes the problem without
> userspace visible changes is this:
> https://lore.kernel.org/lkml/20210413160151.3301-1-rodrigo@kinvolk.io/

I'd prefer to use the now-in-next fix if we can. Is it possible to build
a test case that triggers the race so we can have some certainty that
any fix in -stable covers it appropriately?

-Kees

-- 
Kees Cook