blob: 3c8fe8a3cfed3c239c8dfadf56447b64a1f3ede3 [file] [log] [blame]
From 7fb46f35930c9017078d5ff3369f1d0b81dbc5b9 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 19 May 2020 05:34:41 -0400
Subject: [PATCH] KVM: x86: only do L1TF workaround on affected processors
commit d43e2675e96fc6ae1a633b6a69d296394448cc32 upstream.
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@vger.kernel.org
Fixes: 52918ed5fcf0 ("KVM: SVM: Override default MMIO mask if memory encryption is enabled")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 5a5b831360f5..aac4f6f6b64b 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -323,6 +323,8 @@ 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_SPECIAL_MASK;
shadow_mmio_mask = mmio_mask | SPTE_SPECIAL_MASK;
shadow_mmio_access_mask = access_mask;
@@ -543,16 +545,15 @@ 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);
--
2.7.4