linux-kernel - [PATCH 1/2] KVM: x86/tdp_mmu: Use rcu_dereference() to protect sptep for dereferencing

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Message-ID: <20241104084229.29882-1-yan.y.zhao@intel.com>
Date: Mon,  4 Nov 2024 16:42:29 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	kernel test robot <lkp@...el.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: [PATCH 1/2] KVM: x86/tdp_mmu: Use rcu_dereference() to protect sptep for dereferencing

From: Rick Edgecombe <rick.p.edgecombe@...el.com>

Use rcu_dereference() to copy the RCU-protected pointer sptep into a local
variable for later dereferencing. This also checks that the dereferencing
occurs within the RCU read-side critical section.

Change is_mirror_sptep()'s input type from "u64 *" to "tdp_ptep_t" (typedef
as "u64 __rcu *") to centralize the call of rcu_dereference().

Opportunistically, since try_cmpxchg64() is now the only place in
__tdp_mmu_set_spte_atomic() that dereferences the local variable, move
the rcu_dereference() call closer to its point of use.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410121644.Eq7zRGPO-lkp@intel.com
Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
 arch/x86/kvm/mmu/spte.h    | 4 ++--
 arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 8496a2abbde2..ef322f972948 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -267,9 +267,9 @@ static inline struct kvm_mmu_page *root_to_sp(hpa_t root)
 	return spte_to_child_sp(root);
 }
 
-static inline bool is_mirror_sptep(u64 *sptep)
+static inline bool is_mirror_sptep(tdp_ptep_t sptep)
 {
-	return is_mirror_sp(sptep_to_sp(sptep));
+	return is_mirror_sp(sptep_to_sp(rcu_dereference(sptep)));
 }
 
 static inline bool is_mmio_spte(struct kvm *kvm, u64 spte)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index b0e1c4cb3004..2741b6587ec9 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -511,7 +511,7 @@ static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t sp
 	 * page table has been modified. Use FROZEN_SPTE similar to
 	 * the zapping case.
 	 */
-	if (!try_cmpxchg64(sptep, &old_spte, FROZEN_SPTE))
+	if (!try_cmpxchg64(rcu_dereference(sptep), &old_spte, FROZEN_SPTE))
 		return -EBUSY;
 
 	/*
@@ -637,8 +637,6 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
 							 struct tdp_iter *iter,
 							 u64 new_spte)
 {
-	u64 *sptep = rcu_dereference(iter->sptep);
-
 	/*
 	 * The caller is responsible for ensuring the old SPTE is not a FROZEN
 	 * SPTE.  KVM should never attempt to zap or manipulate a FROZEN SPTE,
@@ -662,6 +660,8 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
 		if (ret)
 			return ret;
 	} else {
+		u64 *sptep = rcu_dereference(iter->sptep);
+
 		/*
 		 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs
 		 * and does not hold the mmu_lock.  On failure, i.e. if a
-- 
2.43.2