linux-kernel - [PATCH 3/3] KVM: x86/mmu: Reword a misleading comment about checking gpte

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240802203900.348808-4-seanjc@google.com>
Date: Fri,  2 Aug 2024 13:39:00 -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
Subject: [PATCH 3/3] KVM: x86/mmu: Reword a misleading comment about checking gpte_changed()

Rewrite the comment in FNAME(fetch) to explain why KVM needs to check that
the gPTE is still fresh before continuing the shadow page walk, even if
KVM already has a linked shadow page for the gPTE in question.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 480c54122991..405bd7ceee2a 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -695,8 +695,14 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 			return RET_PF_RETRY;
 
 		/*
-		 * Verify that the gpte in the page we've just write
-		 * protected is still there.
+		 * Verify that the gpte in the page, which is now either
+		 * write-protected or unsync, wasn't modified between the fault
+		 * and acquiring mmu_lock.  This needs to be done even when
+		 * reusing an existing shadow page to ensure the information
+		 * gathered by the walker matches the information stored in the
+		 * shadow page (which could have been modified by a different
+		 * vCPU even if the page was already linked).  Holding mmu_lock
+		 * prevents the shadow page from changing after this point.
 		 */
 		if (FNAME(gpte_changed)(vcpu, gw, it.level - 1))
 			return RET_PF_RETRY;
-- 
2.46.0.rc2.264.g509ed76dc8-goog