linux-kernel - [PATCH v2 1/4] KVM: x86/mmu: Fix RCU usage in handle_removed_tdp_mmu

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Message-Id: <20210315182643.2437374-2-bgardon@google.com>
Date:   Mon, 15 Mar 2021 11:26:40 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH v2 1/4] KVM: x86/mmu: Fix RCU usage in handle_removed_tdp_mmu_page

The pt passed into handle_removed_tdp_mmu_page does not need RCU
protection, as it is not at any risk of being freed by another thread at
that point. However, the implicit cast from tdp_sptep_t to u64 * dropped
the __rcu annotation without a proper rcu_derefrence. Fix this by
passing the pt as a tdp_ptep_t and then rcu_dereferencing it in
the function.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Ben Gardon <bgardon@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index d78915019b08..db2936cca4bf 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -301,11 +301,16 @@ static void tdp_mmu_unlink_page(struct kvm *kvm, struct kvm_mmu_page *sp,
  *
  * Given a page table that has been removed from the TDP paging structure,
  * iterates through the page table to clear SPTEs and free child page tables.
+ *
+ * Note that pt is passed in as a tdp_ptep_t, but it does not need RCU
+ * protection. Since this thread removed it from the paging structure,
+ * this thread will be responsible for ensuring the page is freed. Hence the
+ * early rcu_dereferences in the function.
  */
-static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
+static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
 					bool shared)
 {
-	struct kvm_mmu_page *sp = sptep_to_sp(pt);
+	struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
 	int level = sp->role.level;
 	gfn_t base_gfn = sp->gfn;
 	u64 old_child_spte;
@@ -318,7 +323,7 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
 	tdp_mmu_unlink_page(kvm, sp, shared);
 
 	for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
-		sptep = pt + i;
+		sptep = rcu_dereference(pt) + i;
 		gfn = base_gfn + (i * KVM_PAGES_PER_HPAGE(level - 1));
 
 		if (shared) {
-- 
2.31.0.rc2.261.g7f71774620-goog