Thanks Paolo ! Do we need to update guest kernel too, if qemu use cpumodel=qemu64 ?
(For example, I have some very old guests where kernel update is not possible) Regards, Alexandre ----- Mail original ----- De: "pbonzini" <pbonz...@redhat.com> À: "qemu-devel" <qemu-devel@nongnu.org> Cc: "ehabkost" <ehabk...@redhat.com> Envoyé: Jeudi 4 Janvier 2018 18:56:09 Objet: [Qemu-devel] [qemu-web PATCH] add a blog post about "Spectre" --- _posts/2018-01-04-spectre.md | 60 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 60 insertions(+) create mode 100644 _posts/2018-01-04-spectre.md diff --git a/_posts/2018-01-04-spectre.md b/_posts/2018-01-04-spectre.md new file mode 100644 index 0000000..1be86d0 --- /dev/null +++ b/_posts/2018-01-04-spectre.md @@ -0,0 +1,60 @@ +--- +layout: post +title: "QEMU and the Spectre and Meltdown attacks" +date: 2018-01-04 18:00:00 +0000 +author: Paolo Bonzini and Eduardo Habkost +categories: [meltdown, spectre, security, x86] +--- +As you probably know by now, three critical architectural flaws in CPUs have +been recently disclosed that allow user processes to read kernel or hypervisor +memory through cache side-channel attacks. These flaws, collectively +named _Meltdown_ and _Spectre_, affect in one way or another almost +all processors that perform out-of-order execution, including x86 (from +Intel and AMD), POWER, s390 and ARM processors. + +No microcode updates are required to block the _Meltdown_ attack; it is +enough to update the guest operating system to a version that separates +the user and kernel address spaces (known as _page table isolation_ for +the Linux kernel). Therefore, this post will focus on _Spectre_, and +especially on [CVE-2017-5715](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-5715). + +Fixing or mitigating _Spectre_ in general, and CVE-2017-5715 in particular, +requires cooperation between the processor and the operating system kernel or +hypervisor; the processor can be updated through microcode or millicode +patches to provide the required functionality. CVE-2017-5715 allows guests +to read potentially sensitive data from hypervisor memory; however, __patching +the host kernel is sufficient to block this attack__. + +On the other hand, in order to protect the guest kernel from a malicious +userspace, updates are also needed to the guest kernel and, depending on +the processor architecture, to QEMU. Just like on bare-metal, the guest +kernel will use the new functionality provided by the microcode or millicode +updates. When running under a hypervisor, processor emulation is mostly out of +QEMU's scope, so QEMU's role in the fix is small, but nevertheless important. +In the case of KVM: + +* QEMU configures the hypervisor to emulate a specific processor model. +For x86, QEMU has to be aware of new CPUID bits introduced by the microcode +update, and it must provide them to guests depending on how the guest is +configured. + +* upon virtual machine migration, QEMU reads the CPU state on the source +and transmits it to the destination. For x86, QEMU has to be aware of new +model specific registers (MSRs). + +Right now, there are no public patches to KVM that expose the new CPUID bits +and MSRs to the virtual machines, therefore there is no urgent need to update +QEMU; remember that __updating the host kernel is enough to protect the +host from malicious guests__. Nevertheless, updates will be posted to the +qemu-devel mailing list in the next few days, and a 2.11.1 patch release +will be released with the fix. + +As of today, the QEMU project is not aware of whether similar changes will +be required for non-x86 processors. If so, they will also posted to the +mailing list and backported to recent stable releases. + +For more information on the vulnerabilities, please refer to the [Google Security +Blog](https://security.googleblog.com/2018/01/todays-cpu-vulnerability-what-you-need.html) +and [Google Project +Zero](https://googleprojectzero.blogspot.it/2018/01/reading-privileged-memory-with-side.html) +posts on the topic, as well as the [Spectre and Meltdown FAQ](https://meltdownattack.com/#faq). -- 2.14.3
