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

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Message-ID: <87msp8mmpq.fsf@redhat.com>
Date: Thu, 02 May 2024 14:18:09 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Alexander Graf <graf@...zon.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

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).

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.

-- 
Vitaly