Re: [PATCH v2 07/17] KVM: x86/mmu: Check PDPTRs before allocating PAE roots

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 08, 2021, Paolo Bonzini wrote:
> On 08/04/21 17:48, Sean Christopherson wrote:
> > Freaking PDPTRs.  I was really hoping we could keep the lock and pages_available()
> > logic outside of the helpers.  What if kvm_mmu_load() reads the PDPTRs and
> > passes them into mmu_alloc_shadow_roots()?  Or is that too ugly?
> 
> The patch I have posted (though untested) tries to do that in a slightly
> less ugly way by pushing make_mmu_pages_available down to mmu_alloc_*_roots.

Yeah, I agree it's less ugly.  It would be nice to not duplicate that code, but
it's probably not worth the ugliness.  :-/

For your approach, can we put the out label after the success path?  Setting
mmu->root_pgd isn't wrong per se, but doing so might mislead future readers into
thinking that it's functionally necessary. 


diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index efb41f31e80a..93f97d0a9e2e 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3244,6 +3244,13 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
        u8 shadow_root_level = mmu->shadow_root_level;
        hpa_t root;
        unsigned i;
+       int r;
+
+       write_lock(&vcpu->kvm->mmu_lock);
+
+       r = make_mmu_pages_available(vcpu);
+       if (r)
+               goto out_unlock;

        if (is_tdp_mmu_enabled(vcpu->kvm)) {
                root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
@@ -3252,8 +3259,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
                root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level, true);
                mmu->root_hpa = root;
        } else if (shadow_root_level == PT32E_ROOT_LEVEL) {
-               if (WARN_ON_ONCE(!mmu->pae_root))
-                       return -EIO;
+               if (WARN_ON_ONCE(!mmu->pae_root)) {
+                       r = -EIO;
+                       goto out_unlock;
+               }

                for (i = 0; i < 4; ++i) {
                        WARN_ON_ONCE(IS_VALID_PAE_ROOT(mmu->pae_root[i]));
@@ -3266,13 +3275,15 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
                mmu->root_hpa = __pa(mmu->pae_root);
        } else {
                WARN_ONCE(1, "Bad TDP root level = %d\n", shadow_root_level);
-               return -EIO;
+               r = -EIO;
+               goto out_unlock;
        }

        /* root_pgd is ignored for direct MMUs. */
        mmu->root_pgd = 0;
-
-       return 0;
+out_unlock:
+       write_unlock(&vcpu->kvm->mmu_lock);
+       return r;
 }

 static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
@@ -3281,7 +3292,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        u64 pdptrs[4], pm_mask;
        gfn_t root_gfn, root_pgd;
        hpa_t root;
-       int i;
+       int i, r;

        root_pgd = mmu->get_guest_pgd(vcpu);
        root_gfn = root_pgd >> PAGE_SHIFT;
@@ -3289,6 +3300,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        if (mmu_check_root(vcpu, root_gfn))
                return 1;

+       /*
+        * On SVM, reading PDPTRs might access guest memory, which might fault
+        * and thus might sleep.  Grab the PDPTRs before acquiring mmu_lock.
+        */
        if (mmu->root_level == PT32E_ROOT_LEVEL) {
                for (i = 0; i < 4; ++i) {
                        pdptrs[i] = mmu->get_pdptr(vcpu, i);
@@ -3300,6 +3315,12 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
                }
        }

+       write_lock(&vcpu->kvm->mmu_lock);
+
+       r = make_mmu_pages_available(vcpu);
+       if (r)
+               goto out_unlock;
+
        /*
         * Do we shadow a long mode page table? If so we need to
         * write-protect the guests page table root.
@@ -3311,8 +3332,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
                goto set_root_pgd;
        }

-       if (WARN_ON_ONCE(!mmu->pae_root))
-               return -EIO;
+       if (WARN_ON_ONCE(!mmu->pae_root)) {
+               r = -EIO;
+               goto out_unlock;
+       }

        /*
         * We shadow a 32 bit page table. This may be a legacy 2-level
@@ -3323,8 +3346,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        if (mmu->shadow_root_level == PT64_ROOT_4LEVEL) {
                pm_mask |= PT_ACCESSED_MASK | PT_WRITABLE_MASK | PT_USER_MASK;

-               if (WARN_ON_ONCE(!mmu->lm_root))
-                       return -EIO;
+               if (WARN_ON_ONCE(!mmu->lm_root)) {
+                       r = -EIO;
+                       goto out_unlock;
+               }

                mmu->lm_root[0] = __pa(mmu->pae_root) | pm_mask;
        }
@@ -3352,8 +3377,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)

 set_root_pgd:
        mmu->root_pgd = root_pgd;
-
-       return 0;
+out_unlock:
+       write_unlock(&vcpu->kvm->mmu_lock);
+       return r;
 }

 static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
@@ -4852,14 +4878,10 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
        r = mmu_alloc_special_roots(vcpu);
        if (r)
                goto out;
-       write_lock(&vcpu->kvm->mmu_lock);
-       if (make_mmu_pages_available(vcpu))
-               r = -ENOSPC;
-       else if (vcpu->arch.mmu->direct_map)
+       if (vcpu->arch.mmu->direct_map)
                r = mmu_alloc_direct_roots(vcpu);
        else
                r = mmu_alloc_shadow_roots(vcpu);
-       write_unlock(&vcpu->kvm->mmu_lock);
        if (r)
                goto out;