Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

From: Salvatore Mesoraca <s.mesoraca16@gmail.com>
To: Topi Miettinen <toiwoton@gmail.com>
Cc: Kees Cook <keescook@chromium.org>,
	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>,
	 Kernel Hardening <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
Date: Sat, 24 Oct 2020 15:12:36 +0100	[thread overview]
Message-ID: <CAJHCu1JskXZTvSsspQD-wk4L59FxesvVJdjMSX=jiHg-R2zCuQ@mail.gmail.com> (raw)
In-Reply-To: <3cb894d4-049f-aa25-4450-d1df36a1b92e@gmail.com>

On Sat, 24 Oct 2020 at 12:34, Topi Miettinen <toiwoton@gmail.com> wrote:
>
> On 23.10.2020 20.52, Salvatore Mesoraca wrote:
> > Hi,
> >
> > On Thu, 22 Oct 2020 at 23:24, Topi Miettinen <toiwoton@gmail.com> wrote:
> >> SARA looks interesting. What is missing is a prctl() to enable all W^X
> >> protections irrevocably for the current process, then systemd could
> >> enable it for services with MemoryDenyWriteExecute=yes.
> >
> > SARA actually has a procattr[0] interface to do just that.
> > There is also a library[1] to help using it.
>
> That means that /proc has to be available and writable at that point, so
> setting up procattrs has to be done before mount namespaces are set up.
> In general, it would be nice for sandboxing facilities in kernel if
> there would be a way to start enforcing restrictions only at next
> execve(), like setexeccon() for SELinux and aa_change_onexec() for
> AppArmor. Otherwise the exact order of setting up various sandboxing
> options can be very tricky to arrange correctly, since each option may
> have a subtle effect to the sandboxing features enabled later. In case
> of SARA, the operations done between shuffling the mount namespace and
> before execve() shouldn't be affected so it isn't important. Even if it
> did (a new sandboxing feature in the future would need trampolines or
> JIT code generation), maybe the procattr file could be opened early but
> it could be written closer to execve().

A new "apply on exec" procattr file seems reasonable and relatively easy to add.
As Kees pointed out, the main obstacle here is the fact that SARA is
not upstream :(

Salvatore