Re: [PATCH 8/9] KVM: SVM: Don't apply SEV+SMAP workaround on code fetch or PT access

[Liam Merwick <liam.merwick@oracle.com>]

On 20/01/2022 01:07, Sean Christopherson wrote:
> Resume the guest instead of synthesizing a triple fault shutdown if the
> instruction bytes buffer is empty due to the #NPF being on the code fetch
> itself or on a page table access.  The SMAP errata applies if and only if
> the code fetch was successful and ucode's subsequent data read from the
> code page encountered a SMAP violation.  In practice, the guest is likely
> hosed either way, but crashing the guest on a code fetch to emulated MMIO
> is technically wrong according to the behavior described in the APM.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>


Reviewed-by: Liam Merwick <liam.merwick@oracle.com>

> ---
>   arch/x86/kvm/svm/svm.c | 43 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d324183fc596..a4b02a6217fd 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4262,6 +4262,7 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
>   {
>   	bool smep, smap, is_user;
>   	unsigned long cr4;
> +	u64 error_code;
>   
>   	/* Emulation is always possible when KVM has access to all guest state. */
>   	if (!sev_guest(vcpu->kvm))
> @@ -4325,22 +4326,31 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
>   	 * loap uop with CPL=0 privileges.  If the load hits a SMAP #PF, ucode
>   	 * gives up and does not fill the instruction bytes buffer.
>   	 *
> -	 * Detection:
> -	 * KVM reaches this point if the VM is an SEV guest, the CPU supports
> -	 * DecodeAssist, a #NPF was raised, KVM's page fault handler triggered
> -	 * emulation (e.g. for MMIO), and the CPU returned 0 in GuestIntrBytes
> -	 * field of the VMCB.
> +	 * As above, KVM reaches this point iff the VM is an SEV guest, the CPU
> +	 * supports DecodeAssist, a #NPF was raised, KVM's page fault handler
> +	 * triggered emulation (e.g. for MMIO), and the CPU returned 0 in the
> +	 * GuestIntrBytes field of the VMCB.
>   	 *
>   	 * This does _not_ mean that the erratum has been encountered, as the
>   	 * DecodeAssist will also fail if the load for CS:RIP hits a legitimate
>   	 * #PF, e.g. if the guest attempt to execute from emulated MMIO and
>   	 * encountered a reserved/not-present #PF.
>   	 *
> -	 * To reduce the likelihood of false positives, take action if and only
> -	 * if CR4.SMAP=1 (obviously required to hit the erratum) and CR4.SMEP=0
> -	 * or CPL=3.  If SMEP=1 and CPL!=3, the erratum cannot have been hit as
> -	 * the guest would have encountered a SMEP violation #PF, not a #NPF.
> +	 * To hit the erratum, the following conditions must be true:
> +	 *    1. CR4.SMAP=1 (obviously).
> +	 *    2. CR4.SMEP=0 || CPL=3.  If SMEP=1 and CPL<3, the erratum cannot
> +	 *       have been hit as the guest would have encountered a SMEP
> +	 *       violation #PF, not a #NPF.
> +	 *    3. The #NPF is not due to a code fetch, in which case failure to
> +	 *       retrieve the instruction bytes is legitimate (see abvoe).
> +	 *
> +	 * In addition, don't apply the erratum workaround if the #NPF occurred
> +	 * while translating guest page tables (see below).
>   	 */
> +	error_code = to_svm(vcpu)->vmcb->control.exit_info_1;
> +	if (error_code & (PFERR_GUEST_PAGE_MASK | PFERR_FETCH_MASK))
> +		goto resume_guest;
> +
>   	cr4 = kvm_read_cr4(vcpu);
>   	smep = cr4 & X86_CR4_SMEP;
>   	smap = cr4 & X86_CR4_SMAP;
> @@ -4350,6 +4360,21 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
>   		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>   	}
>   
> +resume_guest:
> +	/*
> +	 * If the erratum was not hit, simply resume the guest and let it fault
> +	 * again.  While awful, e.g. the vCPU may get stuck in an infinite loop
> +	 * if the fault is at CPL=0, it's the lesser of all evils.  Exiting to
> +	 * userspace will kill the guest, and letting the emulator read garbage
> +	 * will yield random behavior and potentially corrupt the guest.
> +	 *
> +	 * Simply resuming the guest is technically not a violation of the SEV
> +	 * architecture.  AMD's APM states that all code fetches and page table
> +	 * accesses for SEV guest are encrypted, regardless of the C-Bit.  The
> +	 * APM also states that encrypted accesses to MMIO are "ignored", but
> +	 * doesn't explicitly define "ignored", i.e. doing nothing and letting
> +	 * the guest spin is technically "ignoring" the access.
> +	 */
>   	return false;
>   }
>