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Date:   Thu, 22 Oct 2020 13:02:18 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     Szabolcs Nagy <szabolcs.nagy@arm.com>,
        Jeremy Linton <jeremy.linton@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>, libc-alpha@sourceware.org,
        systemd-devel@lists.freedesktop.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Dave Martin <dave.martin@arm.com>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Salvatore Mesoraca <s.mesoraca16@gmail.com>,
        kernel-hardening@lists.openwall.com,
        linux-hardening@vger.kernel.org
Subject: Re: BTI interaction between seccomp filters in systemd and glibc
 mprotect calls, causing service failures
Message-ID: <202010221256.A4F95FD11@keescook>
References: <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com>
 <20201022075447.GO3819@arm.com>
 <78464155-f459-773f-d0ee-c5bdbeb39e5d@gmail.com>
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On Thu, Oct 22, 2020 at 01:39:07PM +0300, Topi Miettinen wrote:
> But I think SELinux has a more complete solution (execmem) which can track
> the pages better than is possible with seccomp solution which has a very
> narrow field of view. Maybe this facility could be made available to
> non-SELinux systems, for example with prctl()? Then the in-kernel MDWX could
> allow mprotect(PROT_EXEC | PROT_BTI) in case the backing file hasn't been
> modified, the source filesystem isn't writable for the calling process and
> the file descriptor isn't created with memfd_create().

Right. The problem here is that systemd is attempting to mediate a
state change using only syscall details (i.e. with seccomp) instead of
a stateful analysis. Using a MAC is likely the only sane way to do that.
SELinux is a bit difficult to adjust "on the fly" the way systemd would
like to do things, and the more dynamic approach seen with SARA[1] isn't
yet in the kernel. Trying to enforce memory W^X protection correctly
via seccomp isn't really going to work well, as far as I can see.

Regardless, it makes sense to me to have the kernel load the executable
itself with BTI enabled by default. I prefer gaining Catalin's suggested
patch[2]. :)

[1] https://lore.kernel.org/kernel-hardening/1562410493-8661-1-git-send-email-s.mesoraca16@gmail.com/
[2] https://lore.kernel.org/linux-arm-kernel/20201022093104.GB1229@gaia/

-- 
Kees Cook