From: Kai Huang kai.huang@linux.intel.com
[ Upstream commit 61455bf26236e7f3d72705382a6437fdfd1bd0af ]
Currently KVM sets 5 most significant bits of physical address bits reported by CPUID (boot_cpu_data.x86_phys_bits) for nonpresent or reserved bits SPTE to mitigate L1TF attack from guest when using shadow MMU. However for some particular Intel CPUs the physical address bits of internal cache is greater than physical address bits reported by CPUID.
Use the kernel's existing boot_cpu_data.x86_cache_bits to determine the five most significant bits. Doing so improves KVM's L1TF mitigation in the unlikely scenario that system RAM overlaps the high order bits of the "real" physical address space as reported by CPUID. This aligns with the kernel's warnings regarding L1TF mitigation, e.g. in the above scenario the kernel won't warn the user about lack of L1TF mitigation if x86_cache_bits is greater than x86_phys_bits.
Also initialize shadow_nonpresent_or_rsvd_mask explicitly to make it consistent with other 'shadow_{xxx}_mask', and opportunistically add a WARN once if KVM's L1TF mitigation cannot be applied on a system that is marked as being susceptible to L1TF.
Reviewed-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Kai Huang kai.huang@linux.intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/mmu.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index e5af08b58132..2e558c814883 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -460,16 +460,24 @@ static void kvm_mmu_reset_all_pte_masks(void) * If the CPU has 46 or less physical address bits, then set an * appropriate mask to guard against L1TF attacks. Otherwise, it is * assumed that the CPU is not vulnerable to L1TF. + * + * Some Intel CPUs address the L1 cache using more PA bits than are + * reported by CPUID. Use the PA width of the L1 cache when possible + * to achieve more effective mitigation, e.g. if system RAM overlaps + * the most significant bits of legal physical address space. */ - low_phys_bits = boot_cpu_data.x86_phys_bits; - if (boot_cpu_data.x86_phys_bits < + 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) { shadow_nonpresent_or_rsvd_mask = - rsvd_bits(boot_cpu_data.x86_phys_bits - + rsvd_bits(boot_cpu_data.x86_cache_bits - shadow_nonpresent_or_rsvd_mask_len, - boot_cpu_data.x86_phys_bits - 1); + 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)); + shadow_nonpresent_or_rsvd_lower_gfn_mask = GENMASK_ULL(low_phys_bits - 1, PAGE_SHIFT); }