[PATCH v4 2/5] KVM: SEV: Drop page refcount early during RMP fault handling

[Ackerley Tng]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Rework KVM's handling of RMP faults to rely on MMU invalidation logic for
safety, instead of the current approach of holding onto a folio reference
until the RMP operations are complete.  I.e. drop the reference gifted by
guest_memfd immediately after getting the PFN, and instead do RMP updates
under mmu_lock, after checking for relevant MMU invalidations.

This will allow dropping guest_memfd's reference gifting entirely, which is
ideally how KVM would operate for all "follow PFN" operations (GUP has many
more complications, which is why KVM holds a reference across page
faults *on top* of the standard MMU invalidation logic).

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Michael Roth <michael.roth@xxxxxxx>
Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 arch/x86/kvm/svm/sev.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index e198469eb074c..0d027cff734cf 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -5019,6 +5019,7 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	struct kvm_memory_slot *slot;
 	struct kvm *kvm = vcpu->kvm;
 	int order, rmp_level, ret;
+	unsigned long mmu_seq;
 	struct page *page;
 	bool assigned;
 	kvm_pfn_t pfn;
@@ -5046,18 +5047,22 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 		return;
 	}
 
+	mmu_seq = kvm->mmu_invalidate_seq;
+	smp_rmb();
+
 	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &order);
 	if (ret) {
 		pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
 				    gpa);
 		return;
 	}
+	kvm_release_page_unused(page);
 
 	ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
 	if (ret || !assigned) {
 		pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
 				    gpa, pfn, ret);
-		goto out_no_trace;
+		return;
 	}
 
 	/*
@@ -5085,27 +5090,31 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	if (rmp_level == PG_LEVEL_4K)
 		goto out;
 
-	ret = snp_rmptable_psmash(pfn);
-	if (ret) {
-		/*
-		 * Look it up again. If it's 4K now then the PSMASH may have
-		 * raced with another process and the issue has already resolved
-		 * itself. If it's not assigned, then this must have raced with
-		 * another process that made this page shared.
-		 */
-		if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
-		    ((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
+	scoped_guard(read_lock, &kvm->mmu_lock) {
+		if (mmu_invalidate_retry_gfn(kvm, mmu_seq, gfn))
 			goto out;
 
-		pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
-				    gpa, pfn, ret);
+		ret = snp_rmptable_psmash(pfn);
+		if (ret) {
+			/*
+			 * Look it up again. If it's 4K now then the PSMASH may
+			 * have raced with another process and the issue has
+			 * already resolved itself. If it's not assigned, then
+			 * this must have raced with another process that made
+			 * this page shared.
+			 */
+			if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
+			    ((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
+				goto out;
+
+			pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
+					    gpa, pfn, ret);
+		}
 	}
 
 	kvm_zap_gfn_range(kvm, gfn, gfn + PTRS_PER_PMD);
 out:
 	trace_kvm_rmp_fault(vcpu, gpa, pfn, error_code, rmp_level, ret);
-out_no_trace:
-	kvm_release_page_unused(page);
 }
 
 static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)

-- 
2.55.0.887.g758fc8c411-goog