[RFC PATCH v5 09/45] KVM: x86: Rework .free_external_spt() into .reclaim_external_sp()

[Sean Christopherson <seanjc@google.com>]

Massage .free_external_spt() into .reclaim_external_sp() to free up (pun
intended) "free" for actually freeing memory, and to allow TDX to do more
than just "free" the S-EPT entry.  Specifically, nullify external_spt to
leak the S-EPT page if reclaiming the page fails, as that detail and
implementation choice has no business living in the TDP MMU.

Use "sp" instead of "spt" even though "spt" is arguably more accurate, as
"spte" and "spt" are dangerously close in name, and because the key
parameter is a kvm_mmu_page, not a pointer to an S-EPT page table.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm-x86-ops.h |  2 +-
 arch/x86/include/asm/kvm_host.h    |  4 ++--
 arch/x86/kvm/mmu/tdp_mmu.c         | 13 ++-----------
 arch/x86/kvm/vmx/tdx.c             | 27 ++++++++++++---------------
 4 files changed, 17 insertions(+), 29 deletions(-)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 57eb1f4832ae..c17cedc485c9 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -95,8 +95,8 @@ KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
 KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
 KVM_X86_OP(load_mmu_pgd)
 KVM_X86_OP_OPTIONAL_RET0(set_external_spte)
-KVM_X86_OP_OPTIONAL_RET0(free_external_spt)
 KVM_X86_OP_OPTIONAL(remove_external_spte)
+KVM_X86_OP_OPTIONAL(reclaim_external_sp)
 KVM_X86_OP(has_wbinvd_exit)
 KVM_X86_OP(get_l2_tsc_offset)
 KVM_X86_OP(get_l2_tsc_multiplier)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d12ca0f8a348..b35a07ed11fb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1858,8 +1858,8 @@ struct kvm_x86_ops {
 				 u64 mirror_spte);
 
 	/* Update external page tables for page table about to be freed. */
-	int (*free_external_spt)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
-				 void *external_spt);
+	void (*reclaim_external_sp)(struct kvm *kvm, gfn_t gfn,
+				    struct kvm_mmu_page *sp);
 
 	/* Update external page table from spte getting removed, and flush TLB. */
 	void (*remove_external_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 27ac520f2a89..18764dbc97ea 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -456,17 +456,8 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
 				    old_spte, FROZEN_SPTE, level, shared);
 	}
 
-	if (is_mirror_sp(sp) &&
-	    WARN_ON(kvm_x86_call(free_external_spt)(kvm, base_gfn, sp->role.level,
-						    sp->external_spt))) {
-		/*
-		 * Failed to free page table page in mirror page table and
-		 * there is nothing to do further.
-		 * Intentionally leak the page to prevent the kernel from
-		 * accessing the encrypted page.
-		 */
-		sp->external_spt = NULL;
-	}
+	if (is_mirror_sp(sp))
+		kvm_x86_call(reclaim_external_sp)(kvm, base_gfn, sp);
 
 	call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
 }
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 30494f9ceb31..66bc3ceb5e17 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1783,27 +1783,24 @@ static void tdx_track(struct kvm *kvm)
 	kvm_make_all_cpus_request(kvm, KVM_REQ_OUTSIDE_GUEST_MODE);
 }
 
-static int tdx_sept_free_private_spt(struct kvm *kvm, gfn_t gfn,
-				     enum pg_level level, void *private_spt)
+static void tdx_sept_reclaim_private_sp(struct kvm *kvm, gfn_t gfn,
+					struct kvm_mmu_page *sp)
 {
-	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
-
 	/*
-	 * free_external_spt() is only called after hkid is freed when TD is
-	 * tearing down.
 	 * KVM doesn't (yet) zap page table pages in mirror page table while
 	 * TD is active, though guest pages mapped in mirror page table could be
 	 * zapped during TD is active, e.g. for shared <-> private conversion
 	 * and slot move/deletion.
+	 *
+	 * In other words, KVM should only free mirror page tables after the
+	 * TD's hkid is freed, when the TD is being torn down.
+	 *
+	 * If the S-EPT PTE can't be removed for any reason, intentionally leak
+	 * the page to prevent the kernel from accessing the encrypted page.
 	 */
-	if (KVM_BUG_ON(is_hkid_assigned(kvm_tdx), kvm))
-		return -EIO;
-
-	/*
-	 * The HKID assigned to this TD was already freed and cache was
-	 * already flushed. We don't have to flush again.
-	 */
-	return tdx_reclaim_page(virt_to_page(private_spt));
+	if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) ||
+	    tdx_reclaim_page(virt_to_page(sp->external_spt)))
+		sp->external_spt = NULL;
 }
 
 static void tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
@@ -3617,7 +3614,7 @@ void __init tdx_hardware_setup(void)
 	vt_x86_ops.vm_size = max_t(unsigned int, vt_x86_ops.vm_size, sizeof(struct kvm_tdx));
 
 	vt_x86_ops.set_external_spte = tdx_sept_set_private_spte;
-	vt_x86_ops.free_external_spt = tdx_sept_free_private_spt;
+	vt_x86_ops.reclaim_external_sp = tdx_sept_reclaim_private_sp;
 	vt_x86_ops.remove_external_spte = tdx_sept_remove_private_spte;
 	vt_x86_ops.protected_apic_has_interrupt = tdx_protected_apic_has_interrupt;
 }
-- 
2.53.0.rc1.217.geba53bf80e-goog