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

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Message-ID: <87d01asm4t.fsf@oldenburg2.str.redhat.com>
Date:   Thu, 22 Oct 2020 10:25:22 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        Mark Rutland <mark.rutland@....com>,
        systemd-devel@...ts.freedesktop.org,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        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:

>> 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?

Is that architecturally supported?  How costly is the mprotect change if
the pages have not been faulted in yet?

> 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?

Eventually, I expect that mainstream distributions build everything for
BTI, so yes, the PROT_BTI removal would only be needed for legacy
programs.

Thanks,
Florian
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