Re: [PATCH] KVM: x86: only do L1TF workaround on affected processors

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> KVM stores the gfn in MMIO SPTEs as a caching optimization.  These are split
> in two parts, as in "[high 11111 low]", to thwart any attempt to use these bits
> in an L1TF attack.  This works as long as there are 5 free bits between
> MAXPHYADDR and bit 50 (inclusive), leaving bit 51 free so that the MMIO
> access triggers a reserved-bit-set page fault.
>
> The bit positions however were computed wrongly for AMD processors that have
> encryption support.  In this case, x86_phys_bits is reduced (for example
> from 48 to 43, to account for the C bit at position 47 and four bits used
> internally to store the SEV ASID and other stuff) while x86_cache_bits in
> would remain set to 48, and _all_ bits between the reduced MAXPHYADDR
> and bit 51 are set.  Then low_phys_bits would also cover some of the
> bits that are set in the shadow_mmio_value, terribly confusing the gfn
> caching mechanism.
>
> To fix this, avoid splitting gfns as long as the processor does not have
> the L1TF bug (which includes all AMD processors).  When there is no
> splitting, low_phys_bits can be set to the reduced MAXPHYADDR removing
> the overlap.  This fixes "npt=0" operation on EPYC processors.
>
> Thanks to Maxim Levitsky for bisecting this bug.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 52918ed5fcf0 ("KVM: SVM: Override default MMIO mask if memory encryption is enabled")
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 8071952e9cf2..86619631ff6a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -335,6 +335,8 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask, u64 mmio_value, u64 access_mask)
>  {
>  	BUG_ON((u64)(unsigned)access_mask != access_mask);
>  	BUG_ON((mmio_mask & mmio_value) != mmio_value);
> +	WARN_ON(mmio_value & (shadow_nonpresent_or_rsvd_mask << shadow_nonpresent_or_rsvd_mask_len));
> +	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
>  	shadow_mmio_value = mmio_value | SPTE_MMIO_MASK;
>  	shadow_mmio_mask = mmio_mask | SPTE_SPECIAL_MASK;
>  	shadow_mmio_access_mask = access_mask;
> @@ -583,16 +585,15 @@ static void kvm_mmu_reset_all_pte_masks(void)
>  	 * the most significant bits of legal physical address space.
>  	 */
>  	shadow_nonpresent_or_rsvd_mask = 0;
> -	low_phys_bits = boot_cpu_data.x86_cache_bits;
> -	if (boot_cpu_data.x86_cache_bits <
> -	    52 - shadow_nonpresent_or_rsvd_mask_len) {
> +	low_phys_bits = boot_cpu_data.x86_phys_bits;
> +	if (boot_cpu_has_bug(X86_BUG_L1TF) &&
> +	    !WARN_ON_ONCE(boot_cpu_data.x86_cache_bits >=
> +			  52 - shadow_nonpresent_or_rsvd_mask_len)) {
> +		low_phys_bits = boot_cpu_data.x86_cache_bits
> +			- shadow_nonpresent_or_rsvd_mask_len;
>  		shadow_nonpresent_or_rsvd_mask =
> -			rsvd_bits(boot_cpu_data.x86_cache_bits -
> -				  shadow_nonpresent_or_rsvd_mask_len,
> -				  boot_cpu_data.x86_cache_bits - 1);
> -		low_phys_bits -= shadow_nonpresent_or_rsvd_mask_len;
> -	} else
> -		WARN_ON_ONCE(boot_cpu_has_bug(X86_BUG_L1TF));
> +			rsvd_bits(low_phys_bits, boot_cpu_data.x86_cache_bits - 1);
> +	}
>  
>  	shadow_nonpresent_or_rsvd_lower_gfn_mask =
>  		GENMASK_ULL(low_phys_bits - 1, PAGE_SHIFT);

This indeed seems to fix previously-completely-broken 'npt=0' case,
checked with AMD EPYC 7401P.

Tested-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly