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Date:   Thu, 21 Apr 2022 19:48:27 +0300
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Subject: Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for
 memory-deny-write-execute (MDWE)
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To:     Catalin Marinas <catalin.marinas@arm.com>,
        Kees Cook <keescook@chromium.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        Lennart Poettering <lennart@poettering.net>,
        =?UTF-8?Q?Zbigniew_J=c4=99drzejewski-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>
References: <20220413134946.2732468-1-catalin.marinas@arm.com>
 <202204141028.0482B08@keescook> <YmAEDsGtxhim46UI@arm.com>
 <c62170c6-5993-2417-4143-5a37a98b227c@gmail.com>
 <202204201610.093C9D5FE8@keescook> <YmF5s4KqT5WL4O0G@arm.com>
From:   Topi Miettinen <toiwoton@gmail.com>
In-Reply-To: <YmF5s4KqT5WL4O0G@arm.com>
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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