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From:   Sasha Levin <sashal@kernel.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Sasha Levin <sashal@kernel.org>, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, kvm@vger.kernel.org
Subject: [PATCH MANUALSEL 5.17 5/7] KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs
Date:   Wed, 27 Apr 2022 11:54:01 -0400
Message-Id: <20220427155408.19352-5-sashal@kernel.org>
In-Reply-To: <20220427155408.19352-1-sashal@kernel.org>
References: <20220427155408.19352-1-sashal@kernel.org>
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From: Paolo Bonzini <pbonzini@redhat.com>

[ Upstream commit 9191b8f0745e63edf519e4a54a4aaae1d3d46fbd ]

WARN and bail if KVM attempts to free a root that isn't backed by a shadow
page.  KVM allocates a bare page for "special" roots, e.g. when using PAE
paging or shadowing 2/3/4-level page tables with 4/5-level, and so root_hpa
will be valid but won't be backed by a shadow page.  It's all too easy to
blindly call mmu_free_root_page() on root_hpa, be nice and WARN instead of
crashing KVM and possibly the kernel.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/mmu/mmu.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 7f009ebb319a..e7cd16e1e0a0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3239,6 +3239,8 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
 		return;
 
 	sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
+	if (WARN_ON(!sp))
+		return;
 
 	if (is_tdp_mmu_page(sp))
 		kvm_tdp_mmu_put_root(kvm, sp, false);
-- 
2.35.1