Re: [PATCH 4.14 057/126] KVM: SVM: Override default MMIO mask if memory encryption is enabled

[Tom Lendacky]

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@xxxxxxx>
>>
>> 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@xxxxxxxxx>
>> Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>
>> ---
>>  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)
>>
>>