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

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/10/20 1:42 PM, Tom Lendacky wrote:
> 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);

Greg,

I should have asked in the earlier email...  how do you want to address this?

Thanks,
Tom

> 
> 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)
>>>
>>>