linux-hardening - Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT

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Message-ID: <87h7q54ghy.fsf@oldenburg2.str.redhat.com>
Date:   Wed, 04 Nov 2020 10:29:29 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha@...rceware.org, Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ
 #26831]

* Will Deacon:

> Is there real value in this seccomp filter if it only looks at mprotect(),
> or was it just implemented because it's easy to do and sounds like a good
> idea?

It seems bogus to me.  Everyone will just create alias mappings instead,
just like they did for the similar SELinux feature.  See “Example code
to avoid execmem violations” in:

  <https://www.akkadia.org/drepper/selinux-mem.html>

As you can see, this reference implementation creates a PROT_WRITE
mapping aliased to a PROT_EXEC mapping, so it actually reduces security
compared to something that generates the code in an anonymous mapping
and calls mprotect to make it executable.

Furthermore, it requires unusual cache flushing code on some AArch64
implementations (a requirement that is not shared by any Linux other
architecture to which libffi has been ported), resulting in
hard-to-track-down real-world bugs.

Thanks,
Florian
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