[PATCH v3 3/8] KVM: SVM: Hold SRCU while reloading guest-owned VMSAs

["Jörg Rödel" <joro@8bytes.org>]

From: Joerg Roedel <joerg.roedel@amd.com>

Take a dedicated KVM SRCU read lock around the guest-owned VMSA memslot
and guest_memfd lookup.  This makes the reload path self-contained and
prevents it from racing a userspace memslot replacement.

Keep the lock through VMCB installation, and release it on all lookup
failure paths.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
---
 arch/x86/kvm/svm/sev.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f690edaf8b69..9681ff0aa5d3 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4059,6 +4059,7 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	gfn_t gfn = gpa_to_gfn(gpa);
 	unsigned long mmu_seq;
 	kvm_pfn_t pfn;
+	int idx;
 
 	lockdep_assert_held(&svm->sev_es.snp_vmsa_mutex);
 
@@ -4090,9 +4091,10 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	if (!VALID_PAGE(gpa))
 		return;
 
-	slot = gfn_to_memslot(vcpu->kvm, gfn);
+	idx = srcu_read_lock(&kvm->srcu);
+	slot = gfn_to_memslot(kvm, gfn);
 	if (!slot)
-		return;
+		goto out_unlock;
 
 	mmu_seq = kvm->mmu_invalidate_seq;
 	smp_rmb();
@@ -4101,8 +4103,8 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	 * The new VMSA will be private memory guest memory, so retrieve the
 	 * PFN from the gmem backend.
 	 */
-	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, NULL))
-		return;
+	if (kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, NULL))
+		goto out_unlock;
 
 	read_lock(&kvm->mmu_lock);
 	/*
@@ -4117,6 +4119,9 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	else
 		svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
 	read_unlock(&kvm->mmu_lock);
+
+out_unlock:
+	srcu_read_unlock(&kvm->srcu, idx);
 }
 
 /*
-- 
2.53.0