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

[Paolo Bonzini]

On 08/04/21 13:15, Wanpeng Li wrote:
I saw this splatting:
  BUG: sleeping function called from invalid context at
arch/x86/kvm/kvm_cache_regs.h:115
   kvm_pdptr_read+0x20/0x60 [kvm]
   kvm_mmu_load+0x3bd/0x540 [kvm]

There is a might_sleep() in kvm_pdptr_read(), however, the original
commit didn't explain more. I can send a formal one if the below fix
is acceptable.

I think we can just push make_mmu_pages_available down into
kvm_mmu_load's callees.  This way it's not necessary to hold the lock
until after the PDPTR check:

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0d92a269c5fa..f92c3695bfeb 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3244,6 +3244,12 @@ 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 < 0)
+               goto out_unlock;
 
 	if (is_tdp_mmu_enabled(vcpu->kvm)) {
                root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
@@ -3266,13 +3272,16 @@ 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;
        }
 
+out_unlock:
+       write_unlock(&vcpu->kvm->mmu_lock);
+
        /* root_pgd is ignored for direct MMUs. */
        mmu->root_pgd = 0;
 
-	return 0;
+       return r;
 }
 
 static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
@@ -3282,6 +3291,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        gfn_t root_gfn, root_pgd;
        hpa_t root;
        int i;
+       int r;
 
 	root_pgd = mmu->get_guest_pgd(vcpu);
        root_gfn = root_pgd >> PAGE_SHIFT;
@@ -3300,6 +3310,11 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
                }
        }
 
+	write_lock(&vcpu->kvm->mmu_lock);
+       r = make_mmu_pages_available(vcpu);
+       if (r < 0)
+               goto out_unlock;
+
        /*
         * Do we shadow a long mode page table? If so we need to
         * write-protect the guests page table root.
@@ -3308,7 +3323,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
                root = mmu_alloc_root(vcpu, root_gfn, 0,
                                      mmu->shadow_root_level, false);
                mmu->root_hpa = root;
-               goto set_root_pgd;
+               goto out_unlock;
        }
 
 	if (WARN_ON_ONCE(!mmu->pae_root))
@@ -3350,7 +3365,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        else
                mmu->root_hpa = __pa(mmu->pae_root);
 
-set_root_pgd:
+out_unlock:
+       write_unlock(&vcpu->kvm->mmu_lock);
        mmu->root_pgd = root_pgd;
 
 	return 0;
@@ -4852,14 +4868,10 @@ 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;
 

Paolo