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Date:   Tue, 23 Jun 2020 13:30:07 +0200
From:   Joerg Roedel <jroedel@suse.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Dave Hansen <dave.hansen@intel.com>,
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Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)
Message-ID: <20200623113007.GH31822@suse.de>
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On Tue, Jun 23, 2020 at 01:07:06PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 28, 2020 at 09:55:12AM +0200, Joerg Roedel wrote:

> So what happens if this #VC triggers on the first access to the #VC
> stack, because the malicious host has craftily mucked with only the #VC
> IST stack page?
> 
> Or on the NMI IST stack, then we get #VC in NMI before the NMI can fix
> you up.
> 
> AFAICT all of that is non-recoverable.

I am not 100% sure, but I think if the #VC stack page is not validated,
the #VC should be promoted to a #DF.

Note that this is an issue only with secure nested paging (SNP), which
is not enabled yet with this patch-set. When it gets enabled a stack
recursion check in the #VC handler is needed which panics the VM. That
also fixes the #VC-in-early-NMI problem.

Regards,

	Joerg