[PATCH 5.4 120/152] KVM: SVM: Override default MMIO mask if memory encryption is enabled

Greg Kroah-Hartman
2020-03-03T17:57:17Z

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
@@ -1298,6 +1298,47 @@ static void shrink_ple_window(struct kvm
}
}

+/*
+ * 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, mask, PT_WRITABLE_MASK | PT_USER_MASK);
+}
+
static __init int svm_hardware_setup(void)
{
int cpu;
@@ -1352,6 +1393,8 @@ static __init int svm_hardware_setup(voi
}
}

+ svm_adjust_mmio_mask();
+
for_each_possible_cpu(cpu) {
r = svm_cpu_init(cpu);
if (r)