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Date:   Thu, 21 Apr 2022 19:48:27 +0300
From:   Topi Miettinen <toiwoton@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Kees Cook <keescook@...omium.org>
Cc:     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 21.4.2022 18.35, 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 we'd want existing installations with MemoryDenyWriteExecute=yes 
not start failing when the implementation is changed to in-kernel 
version. The implementation could be very simple and not even check 
existing PROT_ flags (except for BTI case) to be maximally compatible to 
BPF version. So I'd leave "was PROT_WRITE" and other checks to more 
advanced versions, enabled with a different PR_MDWX_FLAG_.

-Topi

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