linux-kernel - [PATCH] KVM: X86: fix a bad_page bug

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Date:   Sat, 14 Oct 2017 05:48:14 +0800
From:   Peng Hao <peng.hao2@....com.cn>
To:     pbonzini@...hat.com, rkrcmar@...hat.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peng Hao <peng.hao2@....com.cn>
Subject: [PATCH]  KVM: X86: fix a bad_page bug_on path

When poweroff L1 Guest with L2 guset on L1, it exists a path to
trigger a bad_page bug_on.
!page_count(pfn_to_page(pfn)) Warning in mmu_spte_clear_track_bits will
appear before,then it may set A/D bit for the freed page and trigger a
bad_page bug_on.

Signed-off-by: Peng Hao <peng.hao2@....com.cn>
---
 arch/x86/kvm/mmu.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index eca30c1..398de96 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -711,12 +711,8 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
 
 	pfn = spte_to_pfn(old_spte);
 
-	/*
-	 * KVM does not hold the refcount of the page used by
-	 * kvm mmu, before reclaiming the page, we should
-	 * unmap it from mmu first.
-	 */
-	WARN_ON(!kvm_is_reserved_pfn(pfn) && !page_count(pfn_to_page(pfn)));
+	if (!page_count(pfn_to_page(pfn)))
+		return 1;
 
 	if (is_accessed_spte(old_spte))
 		kvm_set_pfn_accessed(pfn);
-- 
1.8.3.1