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From:   Florian Weimer <fweimer@redhat.com>
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     Lennart Poettering <mzxreary@0pointer.de>,
        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
References: <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com>
        <20201022071812.GA324655@gardel-login>
        <87sga6snjn.fsf@oldenburg2.str.redhat.com>
        <511318fd-efde-f2fc-9159-9d16ac8d33a7@gmail.com>
Date:   Thu, 22 Oct 2020 10:25:22 +0200
In-Reply-To: <511318fd-efde-f2fc-9159-9d16ac8d33a7@gmail.com> (Topi
        Miettinen's message of "Thu, 22 Oct 2020 11:17:19 +0300")
Message-ID: <87d01asm4t.fsf@oldenburg2.str.redhat.com>
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* 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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