Re: [PATCH MANUALSEL 5.15 4/7] KVM: x86/mmu: do not allow readers to acquire references to invalid roots

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On 4/27/22 17:54, Sasha Levin wrote:
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>

[ Upstream commit 614f6970aa70242a3f8a8051b01244c029f77b2a ]

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@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
  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 853780eb033b..7e854313ec3b 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -155,14 +155,15 @@ 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)
#define for_each_tdp_mmu_root(_kvm, _root, _as_id) \
  	list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link,		\
@@ -828,7 +829,7 @@ 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);

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>




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