linux-kernel - [PATCH 3/5] KVM: x86/mmu: Harden TDP MMU iteration against root w/o shadow page

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Message-ID: <20230722012350.2371049-4-seanjc@google.com>
Date:   Fri, 21 Jul 2023 18:23:48 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Reima Ishii <ishiir@...cc.u-tokyo.ac.jp>
Subject: [PATCH 3/5] KVM: x86/mmu: Harden TDP MMU iteration against root w/o
 shadow page

Explicitly check that tdp_iter_start() is handed a valid shadow page
to harden KVM against bugs where

Opportunistically stop the TDP MMU iteration instead of continuing on
with garbage if the incoming root is bogus.  Attempting to walk a garbage
root is more likely to caused major problems than doing nothing.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_iter.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
index d2eb0d4f8710..bd30ebfb2f2c 100644
--- a/arch/x86/kvm/mmu/tdp_iter.c
+++ b/arch/x86/kvm/mmu/tdp_iter.c
@@ -39,13 +39,14 @@ void tdp_iter_restart(struct tdp_iter *iter)
 void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
 		    int min_level, gfn_t next_last_level_gfn)
 {
-	int root_level = root->role.level;
-
-	WARN_ON(root_level < 1);
-	WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
+	if (WARN_ON_ONCE(!root || (root->role.level < 1) ||
+			 (root->role.level > PT64_ROOT_MAX_LEVEL))) {
+		iter->valid = false;
+		return;
+	}
 
 	iter->next_last_level_gfn = next_last_level_gfn;
-	iter->root_level = root_level;
+	iter->root_level = root->role.level;
 	iter->min_level = min_level;
 	iter->pt_path[iter->root_level - 1] = (tdp_ptep_t)root->spt;
 	iter->as_id = kvm_mmu_page_as_id(root);
-- 
2.41.0.487.g6d72f3e995-goog