Re: [PATCH] KVM: nVMX: Prevent vmlaunch with EPTP pointing outside assigned memory area

From: Yuan Yao
Date: Fri Jun 30 2023 - 01:02:18 EST

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In reply to: Sean Christopherson: "Re: [PATCH] KVM: nVMX: Prevent vmlaunch with EPTP pointing outside assigned memory area"
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On Thu, Jun 29, 2023 at 01:30:31PM -0700, Sean Christopherson wrote:
> On Thu, Jun 29, 2023, Sean Christopherson wrote:
> > On Thu, Jun 29, 2023, Yan Zhao wrote:
> > > On Wed, Jun 28, 2023 at 08:37:45AM -0700, Sean Christopherson wrote:
> > > ...
> > > > So I think we should try this:
> > > >
> > > > ---
> > > > arch/x86/kvm/mmu/mmu.c | 19 -------------------
> > > > include/linux/kvm_host.h | 1 -
> > > > virt/kvm/kvm_main.c | 13 ++-----------
> > > > 3 files changed, 2 insertions(+), 31 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > index 60397a1beda3..e305737edf84 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -3671,19 +3671,6 @@ void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> > > > }
> > > > EXPORT_SYMBOL_GPL(kvm_mmu_free_guest_mode_roots);
> > > >
> > > > -
> > > > -static int mmu_check_root(struct kvm_vcpu *vcpu, gfn_t root_gfn)
> > > > -{
> > > > - int ret = 0;
> > > > -
> > > > - if (!kvm_vcpu_is_visible_gfn(vcpu, root_gfn)) {
> > > > - kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> > > > - ret = 1;
> > > > - }
> > > > -
> > > > - return ret;
> > > > -}
> > > > -
> > > > static hpa_t mmu_alloc_root(struct kvm_vcpu *vcpu, gfn_t gfn, int quadrant,
> > > > u8 level)
> > > > {
> > > > @@ -3821,9 +3808,6 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> > > > root_pgd = kvm_mmu_get_guest_pgd(vcpu, mmu);
> > > > root_gfn = root_pgd >> PAGE_SHIFT;
> > > >
> > > > - if (mmu_check_root(vcpu, root_gfn))
> > > > - return 1;
> > > > -
> > > Hi Sean,
> > > The checking of existence of memslot is still useful,
> > > Otherwise NULL pointer dereference would be met as in below call trace.
> > >
> > > mmu_alloc_shadow_roots
> > > |->mmu_alloc_root
> > > |->mmu_alloc_root(root_gfn)
> > > |->kvm_mmu_get_shadow_page
> > > |->__kvm_mmu_get_shadow_page
> > > |->kvm_mmu_alloc_shadow_page
> > > |->account_shadowed
> > > |->slot = __gfn_to_memslot(slots, gfn); ==>NULL pointer
> > > | kvm_slot_page_track_add_page(kvm, slot, gfn,KVM_PAGE_TRACK_WRITE);
> > > |->update_gfn_track(slot, gfn, mode, 1);
> > > |->index = gfn_to_index(gfn, slot->base_gfn, PG_LEVEL_4K); ==>NULL pointer dereference
> >
> > Argh, right, the internal memslot might "work", but the no memslot case will not.
> > The non-root path effectively injects a page fault if there's no memslot.
> >
> > Oof, and simply skipping the accounting for the no-slot case would result in an
> > imbalanced world if userspace creates a valid memslot before unaccount_shadowed()
> > is called.
> >
> > As much as it pains me to propagate KVM's arbitrary behavior, doing the right from
> > an architectural perspective is really gross for KVM, e.g. would need all kinds of
> > dedicated code in the MMU.
>
> Turns out, not _that_ gross. Exempting triple fault "works", but as I called out
> earlier, it generates the incorrect exit reason in KVM-Unit-Tests, e.g. when KUT
> stuffs a bad GUEST_RFLAGS to trigger a VM-Exit consistency check, because the
> consistency check has higher priority than anything that can lead to triple fault.
> And I couldn't bring myself to propagate the hack into testing code.
>
> Lightly tested, but if this pans out, I'll post a series to (a) exempt triple
> fault so that there's a fix for stable@, (b) effect the below over several patches,
> and (c) revert the triple fault hack so that HEAD is architecturally less wrong.
>
> ---
> arch/x86/kvm/kvm_onhyperv.c | 5 ++
> arch/x86/kvm/mmu.h | 7 +--
> arch/x86/kvm/mmu/mmu.c | 92 ++++++++++++++++-----------------
> arch/x86/kvm/mmu/mmu_internal.h | 10 ++++
> arch/x86/kvm/mmu/spte.h | 12 +++++
> arch/x86/kvm/mmu/tdp_iter.c | 7 ++-
> arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
> 7 files changed, 79 insertions(+), 56 deletions(-)
>
> diff --git a/arch/x86/kvm/kvm_onhyperv.c b/arch/x86/kvm/kvm_onhyperv.c
> index ded0bd688c65..77add2afc92b 100644
> --- a/arch/x86/kvm/kvm_onhyperv.c
> +++ b/arch/x86/kvm/kvm_onhyperv.c
> @@ -113,6 +113,11 @@ void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp)
> {
> struct kvm_arch *kvm_arch = &vcpu->kvm->arch;
>
> + if (!kvm_mmu_is_usable_root(root_tdp)) {
> + vcpu->arch.hv_root_tdp = INVALID_PAGE;
> + return;
> + }
> +
> if (kvm_x86_ops.flush_remote_tlbs == hv_flush_remote_tlbs) {
> spin_lock(&kvm_arch->hv_root_tdp_lock);
> vcpu->arch.hv_root_tdp = root_tdp;
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 92d5a1924fc1..6dcc81582afa 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -146,12 +146,7 @@ static inline unsigned long kvm_get_active_pcid(struct kvm_vcpu *vcpu)
>
> static inline void kvm_mmu_load_pgd(struct kvm_vcpu *vcpu)
> {
> - u64 root_hpa = vcpu->arch.mmu->root.hpa;
> -
> - if (!VALID_PAGE(root_hpa))
> - return;
> -
> - static_call(kvm_x86_load_mmu_pgd)(vcpu, root_hpa,
> + static_call(kvm_x86_load_mmu_pgd)(vcpu, vcpu->arch.mmu->root.hpa,
> vcpu->arch.mmu->root_role.level);
> }
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6e6078194ec7..64a918d89472 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3574,11 +3574,7 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> if (!VALID_PAGE(*root_hpa))
> return;
>
> - /*
> - * The "root" may be a special root, e.g. a PAE entry, treat it as a
> - * SPTE to ensure any non-PA bits are dropped.
> - */
> - sp = spte_to_child_sp(*root_hpa);
> + sp = root_to_sp(*root_hpa);
> if (WARN_ON(!sp))
> return;
>
> @@ -3624,7 +3620,9 @@ void kvm_mmu_free_roots(struct kvm *kvm, struct kvm_mmu *mmu,
> &invalid_list);
>
> if (free_active_root) {
> - if (to_shadow_page(mmu->root.hpa)) {
> + if (kvm_mmu_is_dummy_root(mmu->root.hpa)) {
> + /* Nothing to cleanup for dummy roots. */
> + } else if (root_to_sp(mmu->root.hpa)) {
> mmu_free_root_page(kvm, &mmu->root.hpa, &invalid_list);
> } else if (mmu->pae_root) {
> for (i = 0; i < 4; ++i) {
> @@ -3648,6 +3646,7 @@ EXPORT_SYMBOL_GPL(kvm_mmu_free_roots);
> void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> {
> unsigned long roots_to_free = 0;
> + struct kvm_mmu_page *sp;
> hpa_t root_hpa;
> int i;
>
> @@ -3662,8 +3661,8 @@ void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> if (!VALID_PAGE(root_hpa))
> continue;
>
> - if (!to_shadow_page(root_hpa) ||
> - to_shadow_page(root_hpa)->role.guest_mode)
> + sp = root_to_sp(root_hpa);
> + if (!sp || sp->role.guest_mode)
> roots_to_free |= KVM_MMU_ROOT_PREVIOUS(i);
> }
>
> @@ -3671,19 +3670,6 @@ void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> }
> EXPORT_SYMBOL_GPL(kvm_mmu_free_guest_mode_roots);
>
> -
> -static int mmu_check_root(struct kvm_vcpu *vcpu, gfn_t root_gfn)
> -{
> - int ret = 0;
> -
> - if (!kvm_vcpu_is_visible_gfn(vcpu, root_gfn)) {
> - kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> - ret = 1;
> - }
> -
> - return ret;
> -}
> -
> static hpa_t mmu_alloc_root(struct kvm_vcpu *vcpu, gfn_t gfn, int quadrant,
> u8 level)
> {
> @@ -3821,8 +3807,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> root_pgd = kvm_mmu_get_guest_pgd(vcpu, mmu);
> root_gfn = root_pgd >> PAGE_SHIFT;
>
> - if (mmu_check_root(vcpu, root_gfn))
> - return 1;
> + if (!kvm_vcpu_is_visible_gfn(vcpu, root_gfn)) {
> + mmu->root.hpa = kvm_mmu_get_dummy_root();
> + return 0;
> + }
>
> /*
> * On SVM, reading PDPTRs might access guest memory, which might fault
> @@ -3834,8 +3822,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> if (!(pdptrs[i] & PT_PRESENT_MASK))
> continue;
>
> - if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
> - return 1;
> + if (kvm_vcpu_is_visible_gfn(vcpu, pdptrs[i] >> PAGE_SHIFT))
> + pdptrs[i] = 0;

Hi Sean,

Should this be "!kvm_vcpu_is_visible_gfn(vcpu, pdptrs[i] >> PAGE_SHIFT)" and
turn the pae_root[i] to dummy root yet ? IIRC the fault will be injected to
the guest/L2 when meets out of range GFN/L1 GPA while walking guest page
table/vmcs12 EPT table, without further update on the dummy root and other
middle level shadow page table entries.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 60397a1beda3..4c3465ecfc15 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3834,8 +3834,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
if (!(pdptrs[i] & PT_PRESENT_MASK))
continue;

- if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
- return 1;
+ if (!kvm_vcpu_is_visible_gfn(vcpu, pdptrs[i] >> PAGE_SHIFT))
+ pdptrs[i] = INVALID_GPA;
}
}

@@ -3892,6 +3892,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
WARN_ON_ONCE(IS_VALID_PAE_ROOT(mmu->pae_root[i]));

if (mmu->cpu_role.base.level == PT32E_ROOT_LEVEL) {
+ if (pdptrs[i] == INVALID_GPA) {
+ mmu->pae_root[i] = kvm_mmu_get_dummy_root();
+ continue;
+ }
if (!(pdptrs[i] & PT_PRESENT_MASK)) {
mmu->pae_root[i] = INVALID_PAE_ROOT;
continue;

> }
> }
>
> @@ -4002,7 +3990,7 @@ static bool is_unsync_root(hpa_t root)
> {
> struct kvm_mmu_page *sp;
>
> - if (!VALID_PAGE(root))
> + if (!VALID_PAGE(root) || kvm_mmu_is_dummy_root(root))
> return false;
>
> /*
> @@ -4018,7 +4006,7 @@ static bool is_unsync_root(hpa_t root)
> * requirement isn't satisfied.
> */
> smp_rmb();
> - sp = to_shadow_page(root);
> + sp = root_to_sp(root);
>
> /*
> * PAE roots (somewhat arbitrarily) aren't backed by shadow pages, the
> @@ -4035,8 +4023,9 @@ static bool is_unsync_root(hpa_t root)
>
> void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> {
> - int i;
> struct kvm_mmu_page *sp;
> + hpa_t root;
> + int i;
>
> if (vcpu->arch.mmu->root_role.direct)
> return;
> @@ -4047,12 +4036,12 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> vcpu_clear_mmio_info(vcpu, MMIO_GVA_ANY);
>
> if (vcpu->arch.mmu->cpu_role.base.level >= PT64_ROOT_4LEVEL) {
> - hpa_t root = vcpu->arch.mmu->root.hpa;
> - sp = to_shadow_page(root);
> -
> + root = vcpu->arch.mmu->root.hpa;
> if (!is_unsync_root(root))
> return;
>
> + sp = root_to_sp(root);
> +
> write_lock(&vcpu->kvm->mmu_lock);
> mmu_sync_children(vcpu, sp, true);
> write_unlock(&vcpu->kvm->mmu_lock);
> @@ -4062,8 +4051,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> write_lock(&vcpu->kvm->mmu_lock);
>
> for (i = 0; i < 4; ++i) {
> - hpa_t root = vcpu->arch.mmu->pae_root[i];
> -
> + root = vcpu->arch.mmu->pae_root[i];
> if (IS_VALID_PAE_ROOT(root)) {
> sp = spte_to_child_sp(root);
> mmu_sync_children(vcpu, sp, true);
> @@ -4382,7 +4370,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
> struct kvm_page_fault *fault)
> {
> - struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root.hpa);
> + struct kvm_mmu_page *sp = root_to_sp(vcpu->arch.mmu->root.hpa);
>
> /* Special roots, e.g. pae_root, are not backed by shadow pages. */
> if (sp && is_obsolete_sp(vcpu->kvm, sp))
> @@ -4562,9 +4550,16 @@ static void nonpaging_init_context(struct kvm_mmu *context)
> static inline bool is_root_usable(struct kvm_mmu_root_info *root, gpa_t pgd,
> union kvm_mmu_page_role role)
> {
> - return (role.direct || pgd == root->pgd) &&
> - VALID_PAGE(root->hpa) &&
> - role.word == to_shadow_page(root->hpa)->role.word;
> + struct kvm_mmu_page *sp;
> +
> + if (!VALID_PAGE(root->hpa))
> + return false;
> +
> + if (!role.direct && pgd != root->pgd)
> + return false;
> +
> + sp = root_to_sp(root->hpa);
> + return sp && role.word == sp->role.word;
> }
>
> /*
> @@ -4634,11 +4629,10 @@ static bool fast_pgd_switch(struct kvm *kvm, struct kvm_mmu *mmu,
> gpa_t new_pgd, union kvm_mmu_page_role new_role)
> {
> /*
> - * For now, limit the caching to 64-bit hosts+VMs in order to avoid
> - * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
> - * later if necessary.
> + * Limit reuse to 64-bit hosts+VMs without "special" roots in order to
> + * avoid having to deal with PDPTEs and other complexities.
> */
> - if (VALID_PAGE(mmu->root.hpa) && !to_shadow_page(mmu->root.hpa))
> + if (VALID_PAGE(mmu->root.hpa) && !root_to_sp(mmu->root.hpa))
> kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
>
> if (VALID_PAGE(mmu->root.hpa))
> @@ -4684,9 +4678,12 @@ void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd)
> * If this is a direct root page, it doesn't have a write flooding
> * count. Otherwise, clear the write flooding count.
> */
> - if (!new_role.direct)
> - __clear_sp_write_flooding_count(
> - to_shadow_page(vcpu->arch.mmu->root.hpa));
> + if (!new_role.direct) {
> + struct kvm_mmu_page *sp = root_to_sp(vcpu->arch.mmu->root.hpa);
> +
> + if (!WARN_ON_ONCE(!sp))
> + __clear_sp_write_flooding_count(sp);
> + }
> }
> EXPORT_SYMBOL_GPL(kvm_mmu_new_pgd);
>
> @@ -5536,12 +5533,13 @@ static bool is_obsolete_root(struct kvm *kvm, hpa_t root_hpa)
> * positives and free roots that don't strictly need to be freed, but
> * such false positives are relatively rare:
> *
> - * (a) only PAE paging and nested NPT has roots without shadow pages
> + * (a) only PAE paging and nested NPT have roots without shadow pages
> + * (or any shadow paging flavor with a dummy root)
> * (b) remote reloads due to a memslot update obsoletes _all_ roots
> * (c) KVM doesn't track previous roots for PAE paging, and the guest
> * is unlikely to zap an in-use PGD.
> */
> - sp = to_shadow_page(root_hpa);
> + sp = root_to_sp(root_hpa);
> return !sp || is_obsolete_sp(kvm, sp);
> }
>
> @@ -5728,7 +5726,7 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> int r, emulation_type = EMULTYPE_PF;
> bool direct = vcpu->arch.mmu->root_role.direct;
>
> - if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> + if (WARN_ON_ONCE(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> return RET_PF_RETRY;
>
> r = RET_PF_INVALID;
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index d39af5639ce9..3ca986450393 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -44,6 +44,16 @@ extern bool dbg;
> #define INVALID_PAE_ROOT 0
> #define IS_VALID_PAE_ROOT(x) (!!(x))
>
> +static inline hpa_t kvm_mmu_get_dummy_root(void)
> +{
> + return my_zero_pfn(0) << PAGE_SHIFT;
> +}
> +
> +static inline bool kvm_mmu_is_dummy_root(hpa_t shadow_page)
> +{
> + return is_zero_pfn(shadow_page >> PAGE_SHIFT);
> +}
> +
> typedef u64 __rcu *tdp_ptep_t;
>
> struct kvm_mmu_page {
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 1279db2eab44..ac8ad12f9698 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -236,6 +236,18 @@ static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
> return to_shadow_page(__pa(sptep));
> }
>
> +static inline struct kvm_mmu_page *root_to_sp(hpa_t root)
> +{
> + if (kvm_mmu_is_dummy_root(root))
> + return NULL;
> +
> + /*
> + * The "root" may be a special root, e.g. a PAE entry, treat it as a
> + * SPTE to ensure any non-PA bits are dropped.
> + */
> + return spte_to_child_sp(root);
> +}
> +
> static inline bool is_mmio_spte(u64 spte)
> {
> return (spte & shadow_mmio_mask) == shadow_mmio_value &&
> diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> index d2eb0d4f8710..eda82a0e7fdb 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.c
> +++ b/arch/x86/kvm/mmu/tdp_iter.c
> @@ -41,8 +41,11 @@ void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
> {
> int root_level = root->role.level;
>
> - WARN_ON(root_level < 1);
> - WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
> + if (WARN_ON_ONCE(!root || (root_level < 1) ||
> + (root_level > PT64_ROOT_MAX_LEVEL))) {
> + iter->valid = false;
> + return;
> + }
>
> iter->next_last_level_gfn = next_last_level_gfn;
> iter->root_level = root_level;
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 512163d52194..046ac2589611 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -689,7 +689,7 @@ static inline void tdp_mmu_iter_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> else
>
> #define tdp_mmu_for_each_pte(_iter, _mmu, _start, _end) \
> - for_each_tdp_pte(_iter, to_shadow_page(_mmu->root.hpa), _start, _end)
> + for_each_tdp_pte(_iter, root_to_sp(_mmu->root.hpa), _start, _end)
>
> /*
> * Yield if the MMU lock is contended or this thread needs to return control
>
> base-commit: 9bee9f1881ecd4eb68ba1ca88b56bff88e50fc8a
> --
>

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