linux-kernel - Re: [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

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Date:   Thu, 22 Oct 2020 12:27:00 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Mark Rutland <mark.rutland@....com>,
        systemd-devel@...ts.freedesktop.org,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, libc-alpha@...rceware.org,
        Dave Martin <dave.martin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in
 systemd and glibc mprotect calls, causing service failures

* Topi Miettinen:

> Allowing mprotect(PROT_EXEC|PROT_BTI) would mean that all you need to
> circumvent MDWX is to add PROT_BTI flag. I'd suggest getting the flags 
> right at mmap() time or failing that, reverting the PROT_BTI for
> legacy programs later.
>
> Could the kernel tell the loader of the BTI situation with auxiliary
> vectors? Then it would be easy for the loader to always use the best 
> mmap() flags without ever needing to mprotect().

I think what we want is a mprotect2 call with a flags argument (separate
from protection flags) that tells the kernel that the request *removes*
protection flags and should fail otherwise.  seccomp could easily filter
that then.

But like the other proposals, the migration story isn't great.  You
would need kernel and seccomp/systemd etc. updates before glibc starts
working, even if glibc has a fallback from mprotect2 to mprotect
(because the latter would be blocked).

Thanks,
Florian
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