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 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.

We don't want to drop mmu_lock, even temporarily.  The reason for holding it
across the entire sequence is to ensure kvm_mmu_available_pages() isn't violated.

> 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:

...

> @@ -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;

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?

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index efb41f31e80a..e3c4938cd665 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3275,11 +3275,11 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
        return 0;
 }

-static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
+static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu, u64 pdptrs[4])
 {
        struct kvm_mmu *mmu = vcpu->arch.mmu;
-       u64 pdptrs[4], pm_mask;
        gfn_t root_gfn, root_pgd;
+       u64 pm_mask;
        hpa_t root;
        int i;

@@ -3291,11 +3291,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)

        if (mmu->root_level == PT32E_ROOT_LEVEL) {
                for (i = 0; i < 4; ++i) {
-                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
-                       if (!(pdptrs[i] & PT_PRESENT_MASK))
-                               continue;
-
-                       if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
+                       if ((pdptrs[i] & PT_PRESENT_MASK) &&
+                           mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
                                return 1;
                }
        }
@@ -4844,21 +4841,33 @@ EXPORT_SYMBOL_GPL(kvm_mmu_reset_context);

 int kvm_mmu_load(struct kvm_vcpu *vcpu)
 {
-       int r;
+       struct kvm_mmu *mmu = vcpu->arch.mmu;
+       u64 pdptrs[4];
+       int r, i;

-       r = mmu_topup_memory_caches(vcpu, !vcpu->arch.mmu->direct_map);
+       r = mmu_topup_memory_caches(vcpu, !mmu->direct_map);
        if (r)
                goto out;
        r = mmu_alloc_special_roots(vcpu);
        if (r)
                goto out;
+
+       /*
+        * On SVM, reading PDPTRs might access guest memory, which might fault
+        * and thus might sleep.  Grab the PDPTRs before acquiring mmu_lock.
+        */
+       if (!mmu->direct_map && mmu->root_level == PT32E_ROOT_LEVEL) {
+               for (i = 0; i < 4; ++i)
+                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
+       }
+
        write_lock(&vcpu->kvm->mmu_lock);
        if (make_mmu_pages_available(vcpu))
                r = -ENOSPC;
        else if (vcpu->arch.mmu->direct_map)
                r = mmu_alloc_direct_roots(vcpu);
        else
-               r = mmu_alloc_shadow_roots(vcpu);
+               r = mmu_alloc_shadow_roots(vcpu, pdptrs);
        write_unlock(&vcpu->kvm->mmu_lock);
        if (r)
                goto out;