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

Topi Miettinen
2022-04-21T16:48:35Z

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