linux-kernel - Re: [PATCH v4 4/6] mm: Make PR_MDWE_REFUSE_EXEC

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Message-ID: <CABRcYmJzWLQ2jPH6WmugUrePX+=JMo5iqP0S=U4n191GCm9ChA@mail.gmail.com>
Date:   Fri, 22 Sep 2023 15:10:47 +0200
From:   Florent Revest <revest@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        catalin.marinas@....com, anshuman.khandual@....com,
        joey.gouly@....com, mhocko@...e.com, keescook@...omium.org,
        david@...hat.com, peterx@...hat.com, izbyshev@...ras.ru,
        broonie@...nel.org, szabolcs.nagy@....com, kpsingh@...nel.org,
        gthelen@...gle.com, toiwoton@...il.com, ayush.jain3@....com,
        stable@...r.kernel.org
Subject: Re: [PATCH v4 4/6] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long

On Fri, Sep 22, 2023 at 3:29 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 28 Aug 2023 17:08:56 +0200 Florent Revest <revest@...omium.org> wrote:
>
> > Defining a prctl flag as an int is a footgun because on a 64 bit machine
> > and with a variadic implementation of prctl (like in musl and glibc),
> > when used directly as a prctl argument, it can get casted to long with
> > garbage upper bits which would result in unexpected behaviors.
> >
> > This patch changes the constant to an unsigned long to eliminate that
> > possibilities. This does not break UAPI.
> >
> > Fixes: b507808ebce2 ("mm: implement memory-deny-write-execute as a prctl")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Florent Revest <revest@...omium.org>
> > Suggested-by: Alexey Izbyshev <izbyshev@...ras.ru>
> > Reviewed-by: David Hildenbrand <david@...hat.com>
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Acked-by: Catalin Marinas <catalin.marinas@....com>
>
> Why is this being offered to -stable?  Does it fix any known problem?

The background for this was discussed in these threads:
v1: https://lore.kernel.org/all/66900d0ad42797a55259061f757beece@ispras.ru/
v2: https://lore.kernel.org/all/d7e3749c-a718-df94-92af-1cb0fecab772@redhat.com/

Cc-ing stable was suggested by David and Alexey:

> On Mon, May 22, 2023 at 8:58 PM Alexey Izbyshev <izbyshev@...ras.ru> wrote:
> > On 2023-05-22 19:22, David Hildenbrand wrote:
> > > Which raises the question if we want to tag this here with a "Fixes"
> > > and eventually cc stable (hmm ...)?
> >
> > Yes, IMO the faster we propagate this change, the better.
>
> Okay, will do

I think that a stable backport would be "nice to have": to reduce the
chances that users build binaries that could end up with garbage bits
in their MDWE prctl arguments. We are not aware of anyone having yet
encountered this corner case with MDWE prctls but a backport would
reduce the likelihood it happens, since this sort of issues has
happened with other prctls. But If this is perceived as a backporting
burden, I suppose we could also live without a stable backport.