linux-kernel - Re: MAP_FIXED_NOREPLACE appears to break older i386 binaries

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Message-ID: <201910061817.9ECFDFE182@keescook>
Date:   Sun, 6 Oct 2019 18:22:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Michal Hocko <mhocko@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: MAP_FIXED_NOREPLACE appears to break older i386 binaries

On Sun, Oct 06, 2019 at 02:14:59PM -0700, Linus Torvalds wrote:
> On Sun, Oct 6, 2019 at 11:07 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Yes, we should get this fixed. But I continue to ask you to point to
> > the actual binaries for testing..
> 
> Just to bring the resolution back publicly to lkml after rmk sent me
> test binaries in private email, the end result is commit b212921b13bd
> ("elf: don't use MAP_FIXED_NOREPLACE for elf executable mappings").

Unsurprisingly, I'm not a fan of reverting that patch, but obviously we
must since it breaks old userspace. :) I'm travelling tomorrow, so if
Michal doesn't fix it before I'm back, I'll take a stab at it. I'd like
to retain the general best-effort-defense of "don't let mappings collide"
but I think it'll require us retaining more details about what the ELF
told us to collide with. (i.e. the LOADs can collide, but not into
stack, brk, etc.)

And better yet, we need self-tests here. execve has SO many corner
cases... I'd like to figure out some way to capture all these.

-- 
Kees Cook