diff mbox series

[5/5] KVM: x86/mmu: Use dummy root, backed by zero page, for !visible guest roots

Message ID 20230722012350.2371049-6-seanjc@google.com
State New
Headers
Series KVM: x86/mmu: Don't synthesize triple fault on bad root |

Commit Message

Sean Christopherson July 22, 2023, 1:23 a.m. UTC 
  When attempting to allocate a shadow root for a !visible guest root gfn,
e.g. that resides in MMIO space, load a dummy root that is backed by the
zero page instead of immediately synthesizing a triple fault shutdown
(using the zero page ensures any attempt to translate memory will generate
a !PRESENT fault and thus VM-Exit).

Unless the vCPU is racing with memslot activity, KVM will inject a page
fault due to not finding a visible slot in FNAME(walk_addr_generic), i.e.
the end result is mostly same, but critically KVM will inject a fault only
*after* KVM runs the vCPU with the bogus root.

Waiting to inject a fault until after running the vCPU fixes a bug where
KVM would bail from nested VM-Enter if L1 tried to run L2 with TDP enabled
and a !visible root.  Even though a bad root will *probably* lead to
shutdown, (a) it's not guaranteed and (b) the CPU won't read the
underlying memory until after VM-Enter succeeds.  E.g. if L1 runs L2 with
a VMX preemption timer value of '0', then architecturally the preemption
timer VM-Exit is guaranteed to occur before the CPU executes any
instruction, i.e. before the CPU needs to translate a GPA to a HPA (so
long as there are no injected events with higher priority than the
preemption timer).

If KVM manages to get to FNAME(fetch) with a dummy root, e.g. because
userspace created a memslot between installing the dummy root and handling
the page fault, simply unload the MMU to allocate a new root and retry the
instruction.

Reported-by: Reima Ishii <ishiir@g.ecc.u-tokyo.ac.jp>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c          | 41 +++++++++++++++------------------
 arch/x86/kvm/mmu/mmu_internal.h | 10 ++++++++
 arch/x86/kvm/mmu/paging_tmpl.h  | 11 +++++++++
 arch/x86/kvm/mmu/spte.h         |  3 +++
 4 files changed, 42 insertions(+), 23 deletions(-)

Comments

Yu Zhang July 25, 2023, 11:36 a.m. UTC | #1 
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 122bfc0124d3..e9d4d7b66111 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -646,6 +646,17 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
>  		goto out_gpte_changed;
>  
> +	/*
> +	 * Load a new root and retry the faulting instruction in the extremely
> +	 * unlikely scenario that the guest root gfn became visible between
> +	 * loading a dummy root and handling the resulting page fault, e.g. if
> +	 * userspace create a memslot in the interim.
> +	 */
> +	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
> +		kvm_mmu_unload(vcpu);

Do we really need a kvm_mmu_unload()? Could we just set
vcpu->arch.mmu->root.hpa to INVALID_PAGE here? 

> +		goto out_gpte_changed;
> +	}
> +
>  	for_each_shadow_entry(vcpu, fault->addr, it) {
>  		gfn_t table_gfn;

B.R.
Yu
Sean Christopherson July 25, 2023, 3:53 p.m. UTC | #2 
On Tue, Jul 25, 2023, Yu Zhang wrote:
> > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > index 122bfc0124d3..e9d4d7b66111 100644
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -646,6 +646,17 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> >  	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> >  		goto out_gpte_changed;
> >  
> > +	/*
> > +	 * Load a new root and retry the faulting instruction in the extremely
> > +	 * unlikely scenario that the guest root gfn became visible between
> > +	 * loading a dummy root and handling the resulting page fault, e.g. if
> > +	 * userspace create a memslot in the interim.
> > +	 */
> > +	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
> > +		kvm_mmu_unload(vcpu);
> 
> Do we really need a kvm_mmu_unload()? Could we just set
> vcpu->arch.mmu->root.hpa to INVALID_PAGE here?

Oof, yeah.  Not only is a full unload overkill, if this code were hit it would
lead to deadlock because kvm_mmu_free_roots() expects to be called *without*
mmu_lock held.

Hmm, but I don't love the idea of open coding a free/reset of the current root.
I'm leaning toward

		kvm_make_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu);

since it's conceptually similar to KVM unloading roots when a memslot is deleted
or moved, just reversed.  That would obviously tie this code to KVM's handling of
the dummy root just as much as manually invalidating root.hpa (probably more so),
but that might actually be a good thing because then the rule for the dummy root
is that it's always considered obsolete (when checked), and that could be
explicitly documented in is_obsolete_root().
Yu Zhang July 26, 2023, 9:20 a.m. UTC | #3 
On Tue, Jul 25, 2023 at 08:53:19AM -0700, Sean Christopherson wrote:
> On Tue, Jul 25, 2023, Yu Zhang wrote:
> > > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > > index 122bfc0124d3..e9d4d7b66111 100644
> > > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > > @@ -646,6 +646,17 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > >  	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> > >  		goto out_gpte_changed;
> > >  
> > > +	/*
> > > +	 * Load a new root and retry the faulting instruction in the extremely
> > > +	 * unlikely scenario that the guest root gfn became visible between
> > > +	 * loading a dummy root and handling the resulting page fault, e.g. if
> > > +	 * userspace create a memslot in the interim.
> > > +	 */
> > > +	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
> > > +		kvm_mmu_unload(vcpu);
> > 
> > Do we really need a kvm_mmu_unload()? Could we just set
> > vcpu->arch.mmu->root.hpa to INVALID_PAGE here?
> 
> Oof, yeah.  Not only is a full unload overkill, if this code were hit it would
> lead to deadlock because kvm_mmu_free_roots() expects to be called *without*
> mmu_lock held.
> 
> Hmm, but I don't love the idea of open coding a free/reset of the current root.
> I'm leaning toward
> 
> 		kvm_make_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu);
> 
> since it's conceptually similar to KVM unloading roots when a memslot is deleted
> or moved, just reversed.  That would obviously tie this code to KVM's handling of
> the dummy root just as much as manually invalidating root.hpa (probably more so),
> but that might actually be a good thing because then the rule for the dummy root
> is that it's always considered obsolete (when checked), and that could be
> explicitly documented in is_obsolete_root().
> 

Oh, right. KVM_REQ_MMU_FREE_OBSOLETE_ROOTS should work. Thanks!

B.R.
Yu
diff mbox series

Patch

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index dd8cc46551b2..20e289e872eb 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3620,7 +3620,9 @@  void kvm_mmu_free_roots(struct kvm *kvm, struct kvm_mmu *mmu,
 					   &invalid_list);
 
 	if (free_active_root) {
-		if (root_to_sp(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) {
@@ -3668,19 +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)
 {
@@ -3818,8 +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
@@ -3831,8 +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;
 		}
 	}
 
@@ -3999,7 +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;
 
 	/*
@@ -4405,6 +4396,10 @@  static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 {
 	int r;
 
+	/* Dummy roots are used only for shadowing bad guest roots. */
+	if (WARN_ON_ONCE(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa)))
+		return RET_PF_RETRY;
+
 	if (page_fault_handle_page_track(vcpu, fault))
 		return RET_PF_EMULATE;
 
@@ -4642,9 +4637,8 @@  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) && !root_to_sp(mmu->root.hpa))
 		kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
@@ -5561,7 +5555,8 @@  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.
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/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 122bfc0124d3..e9d4d7b66111 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -646,6 +646,17 @@  static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
 		goto out_gpte_changed;
 
+	/*
+	 * Load a new root and retry the faulting instruction in the extremely
+	 * unlikely scenario that the guest root gfn became visible between
+	 * loading a dummy root and handling the resulting page fault, e.g. if
+	 * userspace create a memslot in the interim.
+	 */
+	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
+		kvm_mmu_unload(vcpu);
+		goto out_gpte_changed;
+	}
+
 	for_each_shadow_entry(vcpu, fault->addr, it) {
 		gfn_t table_gfn;
 
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 9f8e8cda89e8..ac8ad12f9698 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -238,6 +238,9 @@  static inline struct kvm_mmu_page *sptep_to_sp(u64 *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.