Re: [PATCH 1/2] KVM: SVM: Fix workaround for AMD Errata 1096

["Singh, Brijesh" <brijesh.singh@amd.com>]

On 7/15/19 3:30 PM, Liran Alon wrote:
> According to AMD Errata 1096:
> "On a nested data page fault when CR4.SMAP = 1 and the guest data read generates a SMAP violation, the
> GuestInstrBytes field of the VMCB on a VMEXIT will incorrectly return 0h instead the correct guest instruction
> bytes."
> 
> As stated above, errata is encountered when guest read generates a SMAP violation. i.e. vCPU runs
> with CPL<3 and CR4.SMAP=1. However, code have mistakenly checked if CPL==3 and CR4.SMAP==0.
> 

The SMAP violation will occur from CPL3 so CPL==3 is a valid check.

See [1] for complete discussion

https://patchwork.kernel.org/patch/10808075/#22479271

However, code mistakenly checked CR4.SMAP==0, it should be CR4.SMAP==1

> To avoid future confusion and improve code readbility, comment errata details in code and not
> just in commit message.
> 
> Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>   arch/x86/kvm/svm.c | 17 +++++++++++++----
>   1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 735b8c01895e..79023a41f7a7 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -7123,10 +7123,19 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
>   	bool is_user, smap;
>   
>   	is_user = svm_get_cpl(vcpu) == 3;
> -	smap = !kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
> +	smap = kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
>   

Ah good catch. thank


>   	/*
> -	 * Detect and workaround Errata 1096 Fam_17h_00_0Fh
> +	 * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
> +	 *
> +	 * Errata:
> +	 * On a nested page fault when CR4.SMAP=1 and the guest data read generates
> +	 * a SMAP violation, GuestIntrBytes field of the VMCB on a VMEXIT will
> +	 * incorrectly return 0 instead the correct guest instruction bytes.
> +	 *
> +	 * Workaround:
> +	 * To determine what instruction the guest was executing, the hypervisor
> +	 * will have to decode the instruction at the instruction pointer.
>   	 *
>   	 * In non SEV guest, hypervisor will be able to read the guest
>   	 * memory to decode the instruction pointer when insn_len is zero
> @@ -7137,11 +7146,11 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
>   	 * instruction pointer so we will not able to workaround it. Lets
>   	 * print the error and request to kill the guest.
>   	 */
> -	if (is_user && smap) {
> +	if (!is_user && smap) {
>   		if (!sev_guest(vcpu->kvm))
>   			return true;
>   
> -		pr_err_ratelimited("KVM: Guest triggered AMD Erratum 1096\n");
> +		pr_err_ratelimited("KVM: SEV Guest triggered AMD Erratum 1096\n");
>   		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>   	}
>   
>