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Date:   Thu, 21 Apr 2022 09:42:23 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Topi Miettinen <toiwoton@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lennart Poettering <lennart@...ttering.net>,
        Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Mark Brown <broonie@...nel.org>,
        Jeremy Linton <jeremy.linton@....com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-abi-devel@...ts.sourceforge.net,
        linux-hardening@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Igor Zhbanov <izh1979@...il.com>
Subject: Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for
 memory-deny-write-execute (MDWE)

On Thu, Apr 21, 2022 at 04:35:15PM +0100, Catalin Marinas wrote:
> 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.

I think "was PROT_WRITE" is an important part of the defense that
couldn't be done with a simple seccomp filter (which is why the filter
ended up being a problem in the first place).

-- 
Kees Cook

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