Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE)

From: Catalin Marinas <catalin.marinas@arm.com>
To: Kees Cook <keescook@chromium.org>
Cc: "Topi Miettinen" <toiwoton@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Lennart Poettering" <lennart@poettering.net>,
	"Zbigniew Jędrzejewski-Szmek" <zbyszek@in.waw.pl>,
	"Will Deacon" <will@kernel.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Szabolcs Nagy" <szabolcs.nagy@arm.com>,
	"Mark Brown" <broonie@kernel.org>,
	"Jeremy Linton" <jeremy.linton@arm.com>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	linux-abi-devel@lists.sourceforge.net,
	linux-hardening@vger.kernel.org, "Jann Horn" <jannh@google.com>,
	"Salvatore Mesoraca" <s.mesoraca16@gmail.com>,
	"Igor Zhbanov" <izh1979@gmail.com>
Subject: Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE)
Date: Thu, 21 Apr 2022 16:35:15 +0100	[thread overview]
Message-ID: <YmF5s4KqT5WL4O0G@arm.com> (raw)
In-Reply-To: <202204201610.093C9D5FE8@keescook>

On Wed, Apr 20, 2022 at 04:21:45PM -0700, Kees Cook wrote:
> On Wed, Apr 20, 2022 at 10:34:33PM +0300, Topi Miettinen wrote:
> > For systemd, feature compatibility with the BPF version is important so that
> > we could automatically switch to the kernel version once available without
> > regressions. So I think PR_MDWX_MMAP (or maybe PR_MDWX_COMPAT) should match
> > exactly what MemoryDenyWriteExecute=yes as implemented with BPF has: only
> > forbid mmap(PROT_EXEC|PROT_WRITE) and mprotect(PROT_EXEC). Like BPF, once
> > installed there should be no way to escape and ELF flags should be also
> > ignored. ARM BTI should be allowed though (allow PROT_EXEC|PROT_BTI if the
> > old flags had PROT_EXEC).

I agree.

> > Then we could have improved versions (other PR_MDWX_ prctls) with lots more
> > checks. This could be enabled with MemoryDenyWriteExecute=strict or so.
> > 
> > Perhaps also more relaxed versions (like SARA) could be interesting (system
> > service running Python with FFI, or perhaps JVM etc), enabled with for
> > example MemoryDenyWriteExecute=trampolines. That way even those programs
> > would get some protection (though there would be a gap in the defences).
> 
> Yup, I think we're all on the same page. Catalin, can you respin with a
> prctl for enabling MDWE? I propose just:
> 
> 	prctl(PR_MDWX_SET, flags);
> 	prctl(PR_MDWX_GET);
> 
> 	PR_MDWX_FLAG_MMAP
> 		disallows PROT_EXEC on any VMA that is or was PROT_WRITE,
> 		covering at least: mmap, mprotect, pkey_mprotect, and shmat.

Do we want the "was PROT_WRITE" or we just reject mprotect(PROT_EXEC) if
the vma is not already PROT_EXEC? The latter is closer to the current
systemd approach. The former allows an mprotect(PROT_EXEC) if the
mapping was PROT_READ only for example.

I'd drop the "was PROT_WRITE" for now if the aim is a drop-in
replacement for BPF MDWE.

-- 
Catalin