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Date:   Tue, 1 Nov 2022 01:10:11 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
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        seanjc@google.com, thomas.lendacky@amd.com,
        elena.reshetova@intel.com, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc_mask()
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On Mon, Oct 31, 2022 at 12:44:15PM -0700, Dave Hansen wrote:
> On 10/31/22 12:27, Andi Kleen wrote:
> >> Moving panic() after earlyprintk working is not good idea as it exposes
> >> kernel more: by the time we already have full #VE handler.
> > 
> > It should be fine to move since there is no user land at this point (the
> > attack requires user land)
> 
> Maybe I'm misunderstanding the exposure.  A normal MMIO #VE goes
> something like this:
> 
> 	1. %rax points to some MMIO
> 	2. Kernel executes: mov (%rax),%rbx, trying to read MMIO
> 	3. #VE handler is triggered
> 	4. Handler emulates the 'mov' with instruction decoding
> 	5. Handler asks the VMM what the value of %rax should be
> 	6. Handler puts VMM value in %rax
> 	7. Return from #VE
> 
> I think the attack scenario subverts a normal MMIO to the following
> (changes from the normal flow are marked with *):
> 
> 	*1. %rax points to some private kernel memory, VMM removes
> 	    Secure-EPT entry for that memory.
> 	 2. Kernel executes: mov (%rax),%rbx as part of normal kernel
> 	    execution, not an MMIO read.
> 	 3. #VE handler is triggered, assuming a MMIO read
> 	 4. Handler emulates the 'mov' with instruction decoding
> 	 5. Handler asks the VMM what the value of %rax should be
> 	*6. Handler puts (malicious) VMM value in %rax
> 	 7. Return from #VE
> 	*8. Now the guest kernel is running with an attacker-controlled
> 	    %rax
> 
> This effectively gives the attacker the ability to override the contents
> of a memory read.
> 
> Am I misunderstanding the attack scenario?  I don't see guest userspace
> needing to be involved at all.

Looks correct to me.

I think Andi refers to attack against syscall gap that also addressed by
the patch.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov