From 6dbf79e7164e9a86c1e466062c48498142ae6128 Mon Sep 17 00:00:00 2001 From: Takuya Yoshikawa Date: Sun, 5 Feb 2012 20:42:41 +0900 Subject: [PATCH] KVM: Fix write protection race during dirty logging This patch fixes a race introduced by: commit 95d4c16ce78cb6b7549a09159c409d52ddd18dae KVM: Optimize dirty logging by rmap_write_protect() During protecting pages for dirty logging, other threads may also try to protect a page in mmu_sync_children() or kvm_mmu_get_page(). In such a case, because get_dirty_log releases mmu_lock before flushing TLB's, the following race condition can happen: A (get_dirty_log) B (another thread) lock(mmu_lock) clear pte.w unlock(mmu_lock) lock(mmu_lock) pte.w is already cleared unlock(mmu_lock) skip TLB flush return ... TLB flush Though thread B assumes the page has already been protected when it returns, the remaining TLB entry will break that assumption. This patch fixes this problem by making get_dirty_log hold the mmu_lock until it flushes the TLB's. Signed-off-by: Takuya Yoshikawa Signed-off-by: Marcelo Tosatti Signed-off-by: Avi Kivity --- arch/x86/kvm/x86.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index e86f9b22eaca..3df0b7a140b0 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3065,6 +3065,8 @@ static void write_protect_slot(struct kvm *kvm, unsigned long *dirty_bitmap, unsigned long nr_dirty_pages) { + spin_lock(&kvm->mmu_lock); + /* Not many dirty pages compared to # of shadow pages. */ if (nr_dirty_pages < kvm->arch.n_used_mmu_pages) { unsigned long gfn_offset; @@ -3072,16 +3074,13 @@ static void write_protect_slot(struct kvm *kvm, for_each_set_bit(gfn_offset, dirty_bitmap, memslot->npages) { unsigned long gfn = memslot->base_gfn + gfn_offset; - spin_lock(&kvm->mmu_lock); kvm_mmu_rmap_write_protect(kvm, gfn, memslot); - spin_unlock(&kvm->mmu_lock); } kvm_flush_remote_tlbs(kvm); - } else { - spin_lock(&kvm->mmu_lock); + } else kvm_mmu_slot_remove_write_access(kvm, memslot->id); - spin_unlock(&kvm->mmu_lock); - } + + spin_unlock(&kvm->mmu_lock); } /*