Re: [PATCH 1/4] KVM MMU: fix race in invlpg code

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On 04/30/2010 12:00 PM, Xiao Guangrong wrote:
It has race in invlpg code, like below sequences:

A: hold mmu_lock and get 'sp'
B: release mmu_lock and do other things
C: hold mmu_lock and continue use 'sp'

if other path freed 'sp' in stage B, then kernel will crash

This patch checks 'sp' whether lived before use 'sp' in stage C

Signed-off-by: Xiao Guangrong<xiaoguangrong@xxxxxxxxxxxxxx>
---
  arch/x86/kvm/paging_tmpl.h |   18 +++++++++++++++++-
  1 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 624b38f..641d844 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -462,11 +462,15 @@ out_unlock:

  static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva)
  {
-	struct kvm_mmu_page *sp = NULL;
+	struct kvm_mmu_page *sp = NULL, *s;
  	struct kvm_shadow_walk_iterator iterator;
+	struct hlist_head *bucket;
+	struct hlist_node *node, *tmp;
  	gfn_t gfn = -1;
  	u64 *sptep = NULL, gentry;
  	int invlpg_counter, level, offset = 0, need_flush = 0;
+	unsigned index;
+	bool live = false;

  	spin_lock(&vcpu->kvm->mmu_lock);

@@ -519,10 +523,22 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva)

  	mmu_guess_page_from_pte_write(vcpu, gfn_to_gpa(gfn) + offset, gentry);
  	spin_lock(&vcpu->kvm->mmu_lock);
+	index = kvm_page_table_hashfn(gfn);
+	bucket =&vcpu->kvm->arch.mmu_page_hash[index];
+	hlist_for_each_entry_safe(s, node, tmp, bucket, hash_link)
+		if (s == sp) {

At this point, sp might have been freed and re-allocated, now pointing at something completely different. So need to check role etc.

Alternatively, increase root_count. Then sp is guaranteed to be live (though it may have role.invalid set).

+			live = true;
+			break;
+		}
+
+	if (!live)
+		goto unlock_exit;
+
  	if (atomic_read(&vcpu->kvm->arch.invlpg_counter) == invlpg_counter) {
  		++vcpu->kvm->stat.mmu_pte_updated;
  		FNAME(update_pte)(vcpu, sp, sptep,&gentry);
  	}
+unlock_exit:
  	spin_unlock(&vcpu->kvm->mmu_lock);
  	mmu_release_page_from_pte_write(vcpu);
  }


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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