Re: [PATCH] KVM: x86/mmu: Do not create SPTEs for GFNs that exceed host.MAXPHYADDR

Paolo Bonzini
2022-04-29T10:36:28Z

On 4/29/22 01:34, Sean Christopherson wrote:

> +static inline gfn_t kvm_mmu_max_gfn_host(void)
> +{
> + /*
> + * Disallow SPTEs (via memslots or cached MMIO) whose gfn would exceed
> + * host.MAXPHYADDR. Assuming KVM is running on bare metal, guest
> + * accesses beyond host.MAXPHYADDR will hit a #PF(RSVD) and never hit
> + * an EPT Violation/Misconfig / #NPF, and so KVM will never install a
> + * SPTE for such addresses. That doesn't hold true if KVM is running
> + * as a VM itself, e.g. if the MAXPHYADDR KVM sees is less than
> + * hardware's real MAXPHYADDR, but since KVM can't honor such behavior
> + * on bare metal, disallow it entirely to simplify e.g. the TDP MMU.
> + */
> + return (1ULL << (shadow_phys_bits - PAGE_SHIFT)) - 1;

The host.MAXPHYADDR however does not matter if EPT/NPT is not in use, because
the shadow paging fault path can accept any gfn.

> -static inline gfn_t tdp_mmu_max_gfn_host(void)
> +static inline gfn_t tdp_mmu_max_exclusive_gfn_host(void)
> {
> /*
> - * Bound TDP MMU walks at host.MAXPHYADDR, guest accesses beyond that
> - * will hit a #PF(RSVD) and never hit an EPT Violation/Misconfig / #NPF,
> - * and so KVM will never install a SPTE for such addresses.
> + * Bound TDP MMU walks at host.MAXPHYADDR. KVM disallows memslots with
> + * a gpa range that would exceed the max gfn, and KVM does not create
> + * MMIO SPTEs for "impossible" gfns, instead sending such accesses down
> + * the slow emulation path every time.
> */
> - return 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> + return kvm_mmu_max_gfn_host() + 1;
> }

Slightly nicer name, tdp_mmu_max_gfn_exclusive(). It has to be the host
one because EPT/NPT is in use, but it doesn't really matter.

> + * whose gfn is greater than host.MAXPHYADDR, any guest that
> + * generates such gfns is either malicious or in the weeds.
> + * Note, it's possible to observe a gfn > host.MAXPHYADDR if
> + * and only if host.MAXPHYADDR is inaccurate with respect to
> + * hardware behavior, e.g. if KVM itself is running as a VM.

I don't think maliciousness is particularly likely, and "in the weeds" implies
L2 is the buggy one. Slightly more accurate:

* whose gfn is greater than host.MAXPHYADDR, any guest that
* generates such gfns is running nested and is being tricked
* by L0 userspace (you can observe gfn > L1.MAXPHYADDR if
* and only if L1's MAXPHYADDR is inaccurate with respect to
* the hardware's).

Putting everything together and rebasing on top of kvm/master:

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index e6cae6f22683..dba275d323a7 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -65,6 +65,30 @@ static __always_inline u64 rsvd_bits(int s, int e)
return ((2ULL << (e - s)) - 1) << s;
}

+/*
+ * The number of non-reserved physical address bits irrespective of features
+ * that repurpose legal bits, e.g. MKTME.
+ */
+extern u8 __read_mostly shadow_phys_bits;
+
+static inline gfn_t kvm_mmu_max_gfn(void)
+{
+ /*
+ * Note that this uses the host MAXPHYADDR, not the guest's.
+ * EPT/NPT cannot support GPAs that would exceed host.MAXPHYADDR;
+ * assuming KVM is running on bare metal, guest accesses beyond
+ * host.MAXPHYADDR will hit a #PF(RSVD) and never cause a vmexit
+ * (either EPT Violation/Misconfig or #NPF), and so KVM will never
+ * install a SPTE for such addresses. If KVM is running as a VM
+ * itself, on the other hand, it might see a MAXPHYADDR that is less
+ * than hardware's real MAXPHYADDR. Using the host MAXPHYADDR
+ * disallows such SPTEs entirely and simplifies the TDP MMU.
+ */
+ int max_gpa_bits = likely(tdp_enabled) ? shadow_phys_bits : 52;
+
+ return (1ULL << (max_gpa_bits - PAGE_SHIFT)) - 1;
+}
+
void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask);
void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only);

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index af7910a46c12..7b632a4f81cb 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3033,9 +3033,15 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
/*
* If MMIO caching is disabled, emulate immediately without
* touching the shadow page tables as attempting to install an
- * MMIO SPTE will just be an expensive nop.
+ * MMIO SPTE will just be an expensive nop. Do not cache MMIO
+ * whose gfn is greater than host.MAXPHYADDR, any guest that
+ * generates such gfns is running nested and is being tricked
+ * by L0 userspace (you can observe gfn > L1.MAXPHYADDR if
+ * and only if L1's MAXPHYADDR is inaccurate with respect to
+ * the hardware's).
*/
- if (unlikely(!shadow_mmio_value)) {
+ if (unlikely(!shadow_mmio_value) ||
+ unlikely(fault->gfn > kvm_mmu_max_gfn())) {
*ret_val = RET_PF_EMULATE;
return true;
}
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 73f12615416f..e4abeb5df1b1 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -201,12 +201,6 @@ static inline bool is_removed_spte(u64 spte)
*/
extern u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;

-/*
- * The number of non-reserved physical address bits irrespective of features
- * that repurpose legal bits, e.g. MKTME.
- */
-extern u8 __read_mostly shadow_phys_bits;
-
static inline bool is_mmio_spte(u64 spte)
{
return (spte & shadow_mmio_mask) == shadow_mmio_value &&
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c472769e0300..edc68538819b 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -815,14 +815,15 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm,
return iter->yielded;
}

-static inline gfn_t tdp_mmu_max_gfn_host(void)
+static inline gfn_t tdp_mmu_max_gfn_exclusive(void)
{
/*
- * Bound TDP MMU walks at host.MAXPHYADDR, guest accesses beyond that
- * will hit a #PF(RSVD) and never hit an EPT Violation/Misconfig / #NPF,
- * and so KVM will never install a SPTE for such addresses.
+ * Bound TDP MMU walks at host.MAXPHYADDR. KVM disallows memslots with
+ * a gpa range that would exceed the max gfn, and KVM does not create
+ * MMIO SPTEs for "impossible" gfns, instead sending such accesses down
+ * the slow emulation path every time.
*/
- return 1ULL << (shadow_phys_bits - PAGE_SHIFT);
+ return kvm_mmu_max_gfn() + 1;
}

static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
@@ -830,7 +831,7 @@ static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
{
struct tdp_iter iter;

- gfn_t end = tdp_mmu_max_gfn_host();
+ gfn_t end = tdp_mmu_max_gfn_exclusive();
gfn_t start = 0;

for_each_tdp_pte_min_level(iter, root, zap_level, start, end) {
@@ -923,7 +924,7 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
{
struct tdp_iter iter;

- end = min(end, tdp_mmu_max_gfn_host());
+ end = min(end, tdp_mmu_max_gfn_exclusive());

lockdep_assert_held_write(&kvm->mmu_lock);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 278b2fdd3590..015ecc249c2e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11994,8 +11994,12 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
struct kvm_memory_slot *new,
enum kvm_mr_change change)
{
- if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
+ if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
+ if ((new->base_gfn + new->npages - 1) > kvm_mmu_max_gfn())
+ return -EINVAL;
+
return kvm_alloc_memslot_metadata(kvm, new);
+ }

if (change == KVM_MR_FLAGS_ONLY)
memcpy(&new->arch, &old->arch, sizeof(old->arch));