linux-kernel - Re: [PATCH v4 0/4] x86/snp: Add kexec support

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Date: Fri, 3 May 2024 10:32:13 +0200
From: Alexander Graf <graf@...zon.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>, Ashish Kalra
	<Ashish.Kalra@....com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>, <x86@...nel.org>
CC: <rafael@...nel.org>, <peterz@...radead.org>, <adrian.hunter@...el.com>,
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, <jun.nakajima@...el.com>,
	<rick.p.edgecombe@...el.com>, <thomas.lendacky@....com>,
	<michael.roth@....com>, <seanjc@...gle.com>, <kai.huang@...el.com>,
	<bhe@...hat.com>, <kirill.shutemov@...ux.intel.com>, <bdas@...hat.com>,
	<dionnaglaze@...gle.com>, <anisinha@...hat.com>, <jroedel@...e.de>,
	<ardb@...nel.org>, <kexec@...ts.infradead.org>, <linux-coco@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/4] x86/snp: Add kexec support


On 02.05.24 14:18, Vitaly Kuznetsov wrote:
> Alexander Graf <graf@...zon.com> writes:
>
>> Hey Ashish,
>>
>> On 09.04.24 22:42, Ashish Kalra wrote:
>>> From: Ashish Kalra <ashish.kalra@....com>
>>>
>>> The patchset adds bits and pieces to get kexec (and crashkernel) work on
>>> SNP guest.
>>
>> With this patch set (and similar for the TDX one), you enable the
>> typical kdump case, which is great!
>>
>> However, if a user is running with direct kernel boot - which is very
>> typical in SEV-SNP setup, especially for Kata Containers and similar -
>> the initial launch measurement is a natural indicator of the target
>> environment. Kexec basically allows them to completely bypass that: You
>> would be able to run a completely different environment than the one you
>> measure through the launch digest. I'm not sure it's a good idea to even
>> allow that by default in CoCo environments - at least not if the kernel
>> is locked down.
> Isn't it the same when we just allow loading kernel modules? I'm sure
> you can also achieve a 'completely different environment' with that :-)
> With SecureBoot / lockdown we normally require modules to pass signature
> check, I guess we can employ the same mechanism for kexec. I.e. in
> lockdown, we require signature check on the kexec-ed kernel. Also, it
> may make sense to check initramfs too (with direct kernel boot it's also
> part of launch measurements, right?) and there's UKI for that already).


Correct. With IMA, you even do exactly that: Enforce a signature check 
of the next binary with kexec.

The problem is that you typically want to update the system because 
something is broken; most likely your original environment had a 
security issue somewhere. From a pure SEV-SNP attestation point of view, 
you can not distinguish between the patched and unpatched environment: 
Both look the same.

So while kexec isn't the problem, it's the fact that you can't tell 
anyone that you're now running a fixed version of the code :).


> Personally, I believe that if we simply forbid kexec for CoCo in
> lockdown mode, the feature will become mostly useless in 'full stack'
> (which boot through firmware) production envrironments.


I'm happy for CoCo to stay smoke and mirrors :). But I believe that if 
you want to genuinely draw a trust chain back to an AMD/Intel 
certificate, we need to come up with a good way of making updates work 
with a working trust chain so that whoever checks whether you're running 
sanctioned code is able to validate the claim.


Alex