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Subject[PATCH 4.14 110/115] kvm: x86: Set highest physical address bits in non-present/reserved SPTEs
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Junaid Shahid <junaids@google.com>

commit 28a1f3ac1d0c8558ee4453d9634dad891a6e922e upstream.

Always set the 5 upper-most supported physical address bits to 1 for SPTEs
that are marked as non-present or reserved, to make them unusable for
L1TF attacks from the guest. Currently, this just applies to MMIO SPTEs.
(We do not need to mark PTEs that are completely 0 as physical page 0
is already reserved.)

This allows mitigation of L1TF without disabling hyper-threading by using
shadow paging mode instead of EPT.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/mmu.c | 43 ++++++++++++++++++++++++++++++++++++++-----
arch/x86/kvm/x86.c | 8 ++++++--
2 files changed, 44 insertions(+), 7 deletions(-)

--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -220,6 +220,17 @@ static const u64 shadow_acc_track_saved_
PT64_EPT_EXECUTABLE_MASK;
static const u64 shadow_acc_track_saved_bits_shift = PT64_SECOND_AVAIL_BITS_SHIFT;

+/*
+ * This mask must be set on all non-zero Non-Present or Reserved SPTEs in order
+ * to guard against L1TF attacks.
+ */
+static u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
+
+/*
+ * The number of high-order 1 bits to use in the mask above.
+ */
+static const u64 shadow_nonpresent_or_rsvd_mask_len = 5;
+
static void mmu_spte_set(u64 *sptep, u64 spte);
static void mmu_free_roots(struct kvm_vcpu *vcpu);

@@ -308,9 +319,13 @@ static void mark_mmio_spte(struct kvm_vc
{
unsigned int gen = kvm_current_mmio_generation(vcpu);
u64 mask = generation_mmio_spte_mask(gen);
+ u64 gpa = gfn << PAGE_SHIFT;

access &= ACC_WRITE_MASK | ACC_USER_MASK;
- mask |= shadow_mmio_value | access | gfn << PAGE_SHIFT;
+ mask |= shadow_mmio_value | access;
+ mask |= gpa | shadow_nonpresent_or_rsvd_mask;
+ mask |= (gpa & shadow_nonpresent_or_rsvd_mask)
+ << shadow_nonpresent_or_rsvd_mask_len;

trace_mark_mmio_spte(sptep, gfn, access, gen);
mmu_spte_set(sptep, mask);
@@ -323,8 +338,14 @@ static bool is_mmio_spte(u64 spte)

static gfn_t get_mmio_spte_gfn(u64 spte)
{
- u64 mask = generation_mmio_spte_mask(MMIO_GEN_MASK) | shadow_mmio_mask;
- return (spte & ~mask) >> PAGE_SHIFT;
+ u64 mask = generation_mmio_spte_mask(MMIO_GEN_MASK) | shadow_mmio_mask |
+ shadow_nonpresent_or_rsvd_mask;
+ u64 gpa = spte & ~mask;
+
+ gpa |= (spte >> shadow_nonpresent_or_rsvd_mask_len)
+ & shadow_nonpresent_or_rsvd_mask;
+
+ return gpa >> PAGE_SHIFT;
}

static unsigned get_mmio_spte_access(u64 spte)
@@ -381,7 +402,7 @@ void kvm_mmu_set_mask_ptes(u64 user_mask
}
EXPORT_SYMBOL_GPL(kvm_mmu_set_mask_ptes);

-void kvm_mmu_clear_all_pte_masks(void)
+static void kvm_mmu_reset_all_pte_masks(void)
{
shadow_user_mask = 0;
shadow_accessed_mask = 0;
@@ -391,6 +412,18 @@ void kvm_mmu_clear_all_pte_masks(void)
shadow_mmio_mask = 0;
shadow_present_mask = 0;
shadow_acc_track_mask = 0;
+
+ /*
+ * 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.
+ */
+ if (boot_cpu_data.x86_phys_bits <
+ 52 - shadow_nonpresent_or_rsvd_mask_len)
+ shadow_nonpresent_or_rsvd_mask =
+ rsvd_bits(boot_cpu_data.x86_phys_bits -
+ shadow_nonpresent_or_rsvd_mask_len,
+ boot_cpu_data.x86_phys_bits - 1);
}

static int is_cpuid_PSE36(void)
@@ -5473,7 +5506,7 @@ static void mmu_destroy_caches(void)

int kvm_mmu_module_init(void)
{
- kvm_mmu_clear_all_pte_masks();
+ kvm_mmu_reset_all_pte_masks();

pte_list_desc_cache = kmem_cache_create("pte_list_desc",
sizeof(struct pte_list_desc),
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6194,8 +6194,12 @@ static void kvm_set_mmio_spte_mask(void)
* Set the reserved bits and the present bit of an paging-structure
* entry to generate page fault with PFER.RSV = 1.
*/
- /* Mask the reserved physical address bits. */
- mask = rsvd_bits(maxphyaddr, 51);
+
+ /*
+ * Mask the uppermost physical address bit, which would be reserved as
+ * long as the supported physical address width is less than 52.
+ */
+ mask = 1ull << 51;

/* Set the present bit. */
mask |= 1ull;

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