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

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Message-ID: <f6ae3dbb-cfa5-4d8b-26bf-92db6fc9eab1@redhat.com>
Date:   Thu, 8 Apr 2021 14:09:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 07/17] KVM: x86/mmu: Check PDPTRs before allocating PAE
 roots

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