[PATCH v4 6/9] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE

Sean Christopherson
2022-08-30T23:56:23Z

Set nx_huge_page_disallowed in TDP MMU shadow pages before making the SP
visible to other readers, i.e. before setting its SPTE. This will allow
KVM to query the flag when determining if a shadow page can be replaced
by a NX huge page without violating the rules of the mitigation.

Note, the shadow/legacy MMU holds mmu_lock for write, so it's impossible
for another CPU to see a shadow page without an up-to-date
nx_huge_page_disallowed, i.e. only the TDP MMU needs the complicated
dance.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
---
arch/x86/kvm/mmu/mmu.c | 28 ++++++++++++++++++----------
arch/x86/kvm/mmu/mmu_internal.h | 5 ++---
arch/x86/kvm/mmu/tdp_mmu.c | 31 ++++++++++++++++++-------------
3 files changed, 38 insertions(+), 26 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 04eb87f5a39d..de06c1f87635 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -802,22 +802,25 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
}

-void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
- bool nx_huge_page_possible)
+void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
{
if (KVM_BUG_ON(!list_empty(&sp->possible_nx_huge_page_link), kvm))
return;

- sp->nx_huge_page_disallowed = true;
-
- if (!nx_huge_page_possible)
- return;
-
++kvm->stat.nx_lpage_splits;
list_add_tail(&sp->possible_nx_huge_page_link,
&kvm->arch.possible_nx_huge_pages);
}

+static void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+ bool nx_huge_page_possible)
+{
+ sp->nx_huge_page_disallowed = true;
+
+ if (nx_huge_page_possible)
+ track_possible_nx_huge_page(kvm, sp);
+}
+
static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
{
struct kvm_memslots *slots;
@@ -835,10 +838,8 @@ static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
kvm_mmu_gfn_allow_lpage(slot, gfn);
}

-void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
{
- sp->nx_huge_page_disallowed = false;
-
if (list_empty(&sp->possible_nx_huge_page_link))
return;

@@ -846,6 +847,13 @@ void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
list_del_init(&sp->possible_nx_huge_page_link);
}

+static void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+{
+ sp->nx_huge_page_disallowed = false;
+
+ untrack_possible_nx_huge_page(kvm, sp);
+}
+
static struct kvm_memory_slot *
gfn_to_memslot_dirty_bitmap(struct kvm_vcpu *vcpu, gfn_t gfn,
bool no_dirty_log)
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 67879459a25c..22152241bd29 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -328,8 +328,7 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_

void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);

-void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
- bool nx_huge_page_possible);
-void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);

#endif /* __KVM_X86_MMU_INTERNAL_H */
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index d1079fabe14c..fd38465aee9e 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -403,8 +403,11 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
lockdep_assert_held_write(&kvm->mmu_lock);

list_del(&sp->link);
- if (sp->nx_huge_page_disallowed)
- unaccount_nx_huge_page(kvm, sp);
+
+ if (sp->nx_huge_page_disallowed) {
+ sp->nx_huge_page_disallowed = false;
+ untrack_possible_nx_huge_page(kvm, sp);
+ }

if (shared)
spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
@@ -1123,16 +1126,13 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
* @kvm: kvm instance
* @iter: a tdp_iter instance currently on the SPTE that should be set
* @sp: The new TDP page table to install.
- * @account_nx: True if this page table is being installed to split a
- * non-executable huge page.
* @shared: This operation is running under the MMU lock in read mode.
*
* Returns: 0 if the new page table was installed. Non-0 if the page table
* could not be installed (e.g. the atomic compare-exchange failed).
*/
static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
- struct kvm_mmu_page *sp, bool account_nx,
- bool shared)
+ struct kvm_mmu_page *sp, bool shared)
{
u64 spte = make_nonleaf_spte(sp->spt, !kvm_ad_enabled());
int ret = 0;
@@ -1147,8 +1147,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,

spin_lock(&kvm->arch.tdp_mmu_pages_lock);
list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
- if (account_nx)
- account_nx_huge_page(kvm, sp, true);
spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
tdp_account_mmu_page(kvm, sp);

@@ -1162,6 +1160,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
{
struct kvm_mmu *mmu = vcpu->arch.mmu;
+ struct kvm *kvm = vcpu->kvm;
struct tdp_iter iter;
struct kvm_mmu_page *sp;
int ret;
@@ -1198,9 +1197,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
}

if (!is_shadow_present_pte(iter.old_spte)) {
- bool account_nx = fault->huge_page_disallowed &&
- fault->req_level >= iter.level;
-
/*
* If SPTE has been frozen by another thread, just
* give up and retry, avoiding unnecessary page table
@@ -1212,10 +1208,19 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
sp = tdp_mmu_alloc_sp(vcpu);
tdp_mmu_init_child_sp(sp, &iter);

- if (tdp_mmu_link_sp(vcpu->kvm, &iter, sp, account_nx, true)) {
+ sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
+
+ if (tdp_mmu_link_sp(kvm, &iter, sp, true)) {
tdp_mmu_free_sp(sp);
break;
}
+
+ if (fault->huge_page_disallowed &&
+ fault->req_level >= iter.level) {
+ spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+ track_possible_nx_huge_page(kvm, sp);
+ spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
+ }
}
}

@@ -1503,7 +1508,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
* correctness standpoint since the translation will be the same either
* way.
*/
- ret = tdp_mmu_link_sp(kvm, iter, sp, false, shared);
+ ret = tdp_mmu_link_sp(kvm, iter, sp, shared);
if (ret)
goto out;

--
2.37.2.672.g94769d06f0-goog