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SubjectRe: [PATCH 4.14 057/126] KVM: SVM: Override default MMIO mask if memory encryption is enabled
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On 3/10/20 1:19 PM, Sean Christopherson wrote:
> Has this been tested on the stable kernels? There's a recent bug report[*]
> that suggests the 4.19 backport of this patch may be causing issues.

I missed this went the stable patches went by... when backported to the
older version of kvm_mmu_set_mmio_spte_mask() in the stable kernels (4.14
and 4.19), the call should have been:

kvm_mmu_set_mmio_spte_mask(mask, mask) and not:

kvm_mmu_set_mmio_spte_mask(mask, PT_WRITABLE_MASK | PT_USER_MASK);

The call in the original upstream patch was:

kvm_mmu_set_mmio_spte_mask(mask, mask, PT_WRITABLE_MASK | PT_USER_MASK);

Tom

>
> [*] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D206795&data=02%7C01%7Cthomas.lendacky%40amd.com%7C559dd742543741e4bc7608d7c51fa1d5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637194611958586378&sdata=k%2F3WUFrqvibbf%2FEaCFgOIhUWMZ%2BqHjawmmy1GII7KgA%3D&reserved=0
>
>
> On Tue, Mar 10, 2020 at 01:41:18PM +0100, Greg Kroah-Hartman wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> commit 52918ed5fcf05d97d257f4131e19479da18f5d16 upstream.
>>
>> The KVM MMIO support uses bit 51 as the reserved bit to cause nested page
>> faults when a guest performs MMIO. The AMD memory encryption support uses
>> a CPUID function to define the encryption bit position. Given this, it is
>> possible that these bits can conflict.
>>
>> Use svm_hardware_setup() to override the MMIO mask if memory encryption
>> support is enabled. Various checks are performed to ensure that the mask
>> is properly defined and rsvd_bits() is used to generate the new mask (as
>> was done prior to the change that necessitated this patch).
>>
>> Fixes: 28a1f3ac1d0c ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
>> Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>
>> ---
>> arch/x86/kvm/svm.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 43 insertions(+)
>>
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -1088,6 +1088,47 @@ static int avic_ga_log_notifier(u32 ga_t
>> return 0;
>> }
>>
>> +/*
>> + * The default MMIO mask is a single bit (excluding the present bit),
>> + * which could conflict with the memory encryption bit. Check for
>> + * memory encryption support and override the default MMIO mask if
>> + * memory encryption is enabled.
>> + */
>> +static __init void svm_adjust_mmio_mask(void)
>> +{
>> + unsigned int enc_bit, mask_bit;
>> + u64 msr, mask;
>> +
>> + /* If there is no memory encryption support, use existing mask */
>> + if (cpuid_eax(0x80000000) < 0x8000001f)
>> + return;
>> +
>> + /* If memory encryption is not enabled, use existing mask */
>> + rdmsrl(MSR_K8_SYSCFG, msr);
>> + if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
>> + return;
>> +
>> + enc_bit = cpuid_ebx(0x8000001f) & 0x3f;
>> + mask_bit = boot_cpu_data.x86_phys_bits;
>> +
>> + /* Increment the mask bit if it is the same as the encryption bit */
>> + if (enc_bit == mask_bit)
>> + mask_bit++;
>> +
>> + /*
>> + * If the mask bit location is below 52, then some bits above the
>> + * physical addressing limit will always be reserved, so use the
>> + * rsvd_bits() function to generate the mask. This mask, along with
>> + * the present bit, will be used to generate a page fault with
>> + * PFER.RSV = 1.
>> + *
>> + * If the mask bit location is 52 (or above), then clear the mask.
>> + */
>> + mask = (mask_bit < 52) ? rsvd_bits(mask_bit, 51) | PT_PRESENT_MASK : 0;
>> +
>> + kvm_mmu_set_mmio_spte_mask(mask, PT_WRITABLE_MASK | PT_USER_MASK);
>> +}
>> +
>> static __init int svm_hardware_setup(void)
>> {
>> int cpu;
>> @@ -1123,6 +1164,8 @@ static __init int svm_hardware_setup(voi
>> kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
>> }
>>
>> + svm_adjust_mmio_mask();
>> +
>> for_each_possible_cpu(cpu) {
>> r = svm_cpu_init(cpu);
>> if (r)
>>
>>

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