linux-kernel - [PATCH v4 07/30] KVM: x86/mmu: do not allow readers to acquire references to invalid roots

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Date:   Thu,  3 Mar 2022 14:38:19 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: [PATCH v4 07/30] KVM: x86/mmu: do not allow readers to acquire references to invalid roots

Remove the "shared" argument of for_each_tdp_mmu_root_yield_safe, thus ensuring
that readers do not ever acquire a reference to an invalid root.  After this
patch, all readers except kvm_tdp_mmu_zap_invalidated_roots() treat
refcount=0/valid, refcount=0/invalid and refcount=1/invalid in exactly the
same way.  kvm_tdp_mmu_zap_invalidated_roots() is different but it also
does not acquire a reference to the invalid root, and it cannot see
refcount=0/invalid because it is guaranteed to run after
kvm_tdp_mmu_invalidate_all_roots().

Opportunistically add a lockdep assertion to the yield-safe iterator.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index d39593b9ac9e..79bc48ddb69d 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -166,14 +166,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 	for (_root = tdp_mmu_next_root(_kvm, NULL, _shared, _only_valid);	\
 	     _root;								\
 	     _root = tdp_mmu_next_root(_kvm, _root, _shared, _only_valid))	\
-		if (kvm_mmu_page_as_id(_root) != _as_id) {			\
+		if (kvm_lockdep_assert_mmu_lock_held(_kvm, _shared) &&		\
+		    kvm_mmu_page_as_id(_root) != _as_id) {			\
 		} else
 
 #define for_each_valid_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared)	\
 	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, true)
 
-#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared)		\
-	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, false)
+#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id)			\
+	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, false, false)
 
 /*
  * Iterate over all TDP MMU roots.  Requires that mmu_lock be held for write,
@@ -808,7 +809,7 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
 {
 	struct kvm_mmu_page *root;
 
-	for_each_tdp_mmu_root_yield_safe(kvm, root, as_id, false)
+	for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
 		flush = zap_gfn_range(kvm, root, start, end, can_yield, flush,
 				      false);
 
-- 
2.31.1