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Date:   Thu, 22 Oct 2020 10:38:23 +0200
From:   Lennart Poettering <mzxreary@0pointer.de>
To:     Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc:     Topi Miettinen <toiwoton@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        systemd-devel@lists.freedesktop.org,
        Kees Cook <keescook@chromium.org>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Brown <broonie@kernel.org>, libc-alpha@sourceware.org,
        Dave Martin <dave.martin@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in
 systemd and glibc mprotect calls, causing service failures
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On Do, 22.10.20 09:29, Szabolcs Nagy (szabolcs.nagy@arm.com) wrote:

> > > The dynamic loader has to process the LOAD segments to get to the ELF
> > > note that says to enable BTI.  Maybe we could do a first pass and load
> > > only the segments that cover notes.  But that requires lots of changes
> > > to generic code in the loader.
> >
> > What if the loader always enabled BTI for PROT_EXEC pages, but then when
> > discovering that this was a mistake, mprotect() the pages without BTI? Then
> > both BTI and MDWX would work and the penalty of not getting MDWX would fall
> > to non-BTI programs. What's the expected proportion of BTI enabled code vs.
> > disabled in the future, is it perhaps expected that a distro would enable
> > the flag globally so eventually only a few legacy programs might be
> > unprotected?
>
> i thought mprotect(PROT_EXEC) would get filtered
> with or without bti, is that not the case?

We can adjust the filter in systemd to match any combination of
flags to allow and to deny.

Lennart

--
Lennart Poettering, Berlin