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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Alexander Graf <graf@amazon.com>,
	Ashish Kalra <Ashish.Kalra@amd.com>,
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	dave.hansen@linux.intel.com, x86@kernel.org
Cc: rafael@kernel.org, peterz@infradead.org, adrian.hunter@intel.com,
	sathyanarayanan.kuppuswamy@linux.intel.com,
	jun.nakajima@intel.com, rick.p.edgecombe@intel.com,
	thomas.lendacky@amd.com, michael.roth@amd.com, seanjc@google.com,
	kai.huang@intel.com, bhe@redhat.com,
	kirill.shutemov@linux.intel.com, bdas@redhat.com,
	dionnaglaze@google.com, anisinha@redhat.com, jroedel@suse.de,
	ardb@kernel.org, kexec@lists.infradead.org,
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 0/4] x86/snp: Add kexec support
Date: Thu, 02 May 2024 14:18:09 +0200	[thread overview]
Message-ID: <87msp8mmpq.fsf@redhat.com> (raw)
In-Reply-To: <26b3b3b5-548d-4ebd-9d21-19580a41e799@amazon.com>

Alexander Graf <graf@amazon.com> writes:

> Hey Ashish,
>
> On 09.04.24 22:42, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@amd.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

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