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AJvYcCVc5EMq1DrLXss/WhWabAJdvbretz9pWzRrTfMpZBer3dWfQ/+QgbeKq+A6uVTk9qxNdjvWJQQmO0vMn+TbLUyQ0XECLC8VGzIhQ+hD X-Gm-Message-State: AOJu0YwXQixYshK8e+zqd4UuCJ7LfCZG2GQU9Sk6cHTrcH5rxF86MxZE gRlpXJIRElSIi4XJpxIK9woh+heJyBUHlZprpZ7OjVEbUi5UVeJXo3N1bZk8mw== X-Received: by 2002:a05:6358:6f12:b0:17b:c624:b0a1 with SMTP id r18-20020a0563586f1200b0017bc624b0a1mr1060447rwn.23.1709175491359; Wed, 28 Feb 2024 18:58:11 -0800 (PST) Received: from localhost ([2401:fa00:8f:203:f51:e79e:9056:77ea]) by smtp.gmail.com with UTF8SMTPSA id r37-20020a632065000000b005dcc075d5edsm190825pgm.60.2024.02.28.18.58.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 Feb 2024 18:58:10 -0800 (PST) From: David Stevens X-Google-Original-From: David Stevens To: Sean Christopherson , Paolo Bonzini Cc: Yu Zhang , Isaku Yamahata , Zhi Wang , Maxim Levitsky , kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: [PATCH v11 1/8] KVM: Assert that a page's refcount is elevated when marking accessed/dirty Date: Thu, 29 Feb 2024 11:57:52 +0900 Message-ID: <20240229025759.1187910-2-stevensd@google.com> X-Mailer: git-send-email 2.44.0.rc1.240.g4c46232300-goog In-Reply-To: <20240229025759.1187910-1-stevensd@google.com> References: <20240229025759.1187910-1-stevensd@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1792200440361033191 X-GMAIL-MSGID: 1792200440361033191 From: Sean Christopherson Assert that a page's refcount is elevated, i.e. that _something_ holds a reference to the page, when KVM marks a page as accessed and/or dirty. KVM typically doesn't hold a reference to pages that are mapped into the guest, e.g. to allow page migration, compaction, swap, etc., and instead relies on mmu_notifiers to react to changes in the primary MMU. Incorrect handling of mmu_notifier events (or similar mechanisms) can result in KVM keeping a mapping beyond the lifetime of the backing page, i.e. can (and often does) result in use-after-free. Yelling if KVM marks a freed page as accessed/dirty doesn't prevent badness as KVM usually only does A/D updates when unmapping memory from the guest, i.e. the assertion fires well after an underlying bug has occurred, but yelling does help detect, triage, and debug use-after-free bugs. Note, the assertion must use page_count(), NOT page_ref_count()! For hugepages, the returned struct page may be a tailpage and thus not have its own refcount. Signed-off-by: Sean Christopherson --- virt/kvm/kvm_main.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 10bfc88a69f7..c5e4bf7c48f9 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -3204,6 +3204,19 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_unmap); static bool kvm_is_ad_tracked_page(struct page *page) { + /* + * Assert that KVM isn't attempting to mark a freed page as Accessed or + * Dirty, i.e. that KVM's MMU doesn't have a use-after-free bug. KVM + * (typically) doesn't pin pages that are mapped in KVM's MMU, and + * instead relies on mmu_notifiers to know when a mapping needs to be + * zapped/invalidated. Unmapping from KVM's MMU must happen _before_ + * KVM returns from its mmu_notifier, i.e. the page should have an + * elevated refcount at this point even though KVM doesn't hold a + * reference of its own. + */ + if (WARN_ON_ONCE(!page_count(page))) + return false; + /* * Per page-flags.h, pages tagged PG_reserved "should in general not be * touched (e.g. set dirty) except by its owner".