Re: [Qemu-devel] [qemu-web PATCH] refine spectre blog post

[Dr. David Alan Gilbert]

* Paolo Bonzini (address@hidden) wrote:
> People were confused about the level of protection provided by host kernel
> updates (which do not exist yet, but I digress).  They were also asking
> whether it will be possible to live migrate from old to new QEMU and get
> the fixes.  Clarify both aspects.
> 
> Suggested-by: Dr. David Alan Gilbert <address@hidden>
> Suggested-by: Juan Quintela <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  _posts/2018-01-04-spectre.md | 32 +++++++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/_posts/2018-01-04-spectre.md b/_posts/2018-01-04-spectre.md
> index 1be86d0..5bbc7ed 100644
> --- a/_posts/2018-01-04-spectre.md
> +++ b/_posts/2018-01-04-spectre.md
> @@ -2,6 +2,7 @@
>  layout: post
>  title:  "QEMU and the Spectre and Meltdown attacks"
>  date:   2018-01-04 18:00:00 +0000
> +last_modified_at: 2018-01-05 10:30:00 +0000
>  author: Paolo Bonzini and Eduardo Habkost
>  categories: [meltdown, spectre, security, x86]
>  ---
> @@ -21,17 +22,19 @@ especially on 
> [CVE-2017-5715](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE
>  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:
> +patches to provide the required functionality.
> +
> +Among the three vulnerabilities, CVE-2017-5715 is notable because
> +it allows guests to read potentially sensitive data from hypervisor
> +memory.  Patching the host kernel is sufficient to block attacks from
> +guests to the host.  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:

OK, that looks clearer to me.

>  * 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
> @@ -49,6 +52,10 @@ 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.
>  
> +Once updates are provided, __live migration will not be enough to protect
> +guest kernel from guest userspace__.  Because the virtual CPU has to be
> +changed to one with the new CPUID bits, the guest will have to be restarted.

OK.


Reviewed-by: Dr. David Alan Gilbert <address@hidden>

>  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.
> @@ -58,3 +65,6 @@ 
> Blog](https://security.googleblog.com/2018/01/todays-cpu-vulnerability-what-you-
>  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).
> +
> +__5 Jan 2018__: clarified the level of protection provided by the host kernel
> +update; added a note on live migration.
> -- 
> 2.14.3
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK