linux-kernel - Re: [PATCH v3 4/7] KVM: VMX: Handle vectoring error in check_emulate

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Message-ID: <Z2MWzyoq8c2FfJnM@google.com>
Date: Wed, 18 Dec 2024 10:39:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ivan Orlov <iorlov@...zon.com>
Cc: bp@...en8.de, dave.hansen@...ux.intel.com, mingo@...hat.com, 
	pbonzini@...hat.com, shuah@...nel.org, tglx@...utronix.de, hpa@...or.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, x86@...nel.org, dwmw@...zon.co.uk, 
	pdurrant@...zon.co.uk, jalliste@...zon.co.uk
Subject: Re: [PATCH v3 4/7] KVM: VMX: Handle vectoring error in check_emulate_instruction

On Tue, Dec 17, 2024, Ivan Orlov wrote:
> Move unhandleable vmexit during vectoring error detection
> into check_emulate_instruction. Implement the function which prohibits
> the emulation if EMULTYPE_PF is set when vectoring, otherwise such a
> situation may occur:

I definitely think it's worth explaining that moving the detection covers new
emulation cases, and also calling out that handle_ept_misconfig() consults
vmx_check_emulate_instruction(), i.e. that moving the detection shouldn't
affect KVM's overall handlng of EPT Misconfig.

--

Move handling of emulation during event vectoring, which KVM doesn't
support, into VMX's check_emulate_instruction(), so that KVM detects
all unsupported emulation, not just cached emulated MMIO (EPT misconfig).
E.g. on emulated MMIO that isn't cached (EPT Violation) or occurs with
legacy shadow paging (#PF).

Rejecting emulation on other sources of emulation also fixes a largely
theoretical flaw (thanks to the "unprotect and retry" logic), where KVM
could incorrectly inject a #DF:

  1. CPU executes an instruction and hits a #GP
  2. While vectoring the #GP, a shadow #PF occurs
  3. On the #PF VM-Exit, KVM re-injects #GP
  4. KVM emulates because of the write-protected page
  5. KVM "successfully" emulates and also detects the #GP
  6. KVM synthesizes a #GP, and since #GP has already been injected,
     incorrectly escalates to a #DF.

Fix the comment about EMULTYPE_PF as this flag doesn't necessarily
mean MMIO anymore: it can also be set due to the write protection
violation.

Note, handle_ept_misconfig() checks vmx_check_emulate_instruction() before
attempting emulation of any kind.