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

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Date: Fri, 23 Oct 2020 18:52:50 +0100
From: Salvatore Mesoraca <s.mesoraca16@...il.com>
To: Topi Miettinen <toiwoton@...il.com>
Cc: Kees Cook <keescook@...omium.org>, Szabolcs Nagy <szabolcs.nagy@....com>, 
	Jeremy Linton <jeremy.linton@....com>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, libc-alpha@...rceware.org, 
	systemd-devel@...ts.freedesktop.org, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>, 
	Mark Brown <broonie@...nel.org>, Dave Martin <dave.martin@....com>, 
	Catalin Marinas <Catalin.Marinas@....com>, Will Deacon <will.deacon@....com>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-hardening@...r.kernel.org
Subject: Re: BTI interaction between seccomp filters in systemd and glibc
 mprotect calls, causing service failures

Hi,

On Thu, 22 Oct 2020 at 23:24, Topi Miettinen <toiwoton@...il.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.

> I didn't also see specific measures against memfd_create() or file
> system W&X, but perhaps those can be added later.

You are right, there are no measures against those vectors.
It would be interesting to add them, though.

> Maybe pkey_mprotect()
> is not handled either unless it uses the same LSM hook as mprotect().

IIRC mprotect is implemented more or less as a pkey_mprotect with -1 as pkey.
The same LSM hook should cover both.

Salvatore

[0] https://lore.kernel.org/lkml/1562410493-8661-10-git-send-email-s.mesoraca16@gmail.com/
[1] https://github.com/smeso/libsara

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