Re: [Xen-devel] [PATCH for-4.9] livepatch: Declare live patching as a supported feature

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/06/17 16:36, Ross Lagerwall wrote:
> Xen Live Patching has been available as tech preview feature since Xen
> 4.7 and has now had a couple of releases to stabilize. Xen Live patching
> has been used by multiple vendors to fix several real-world security
> issues without any severe bugs encountered. Additionally, there are now
> tests in OSSTest that test live patching to ensure that no regressions
> are introduced.
>
> Based on the amount of testing and usage it has had, we are ready to
> declare live patching as a 'Supported' feature.
>
> Live patching is slightly peculiar when it comes to support because it
> allows the host administrator to break their system rather easily
> depending on the content of the live patch.
> Because of this, it is worth detailing out the scope of security
> support:
>
> * Unprivileged access to live patching operations:
>     Live patching operations should only be accessible to privileged
>     guests and it shall be treated as a security issue if this is not
>     the case.
>
> * Bugs in the patch-application code such that vulnerabilities exist
>   after application:
>     If a correct live patch is loaded but it is not applied correctly
>     such that it might result in an insecure system (e.g. not all
>     functions are patched), it shall be treated as a security issue.
>
> * Bugs in livepatch-build-tools creating incorrect live patch that
>   results in an insecure host:
>     If livepatch-build-tools creates an incorrect live patch that
>     results in an insecure host, this shall not be considered a security
>     issue. There are too many OSes and toolchains to consider supporting
>     this. A live patch should be checked to verify that it is valid
>     before loading.
>
> * Loading an incorrect live patch that results in an insecure host or
>   host crash:
>     If a live patch (whether created using livepatch-build-tools or some
>     alternative) is loaded and it results in an insecure host or host
>     crash due to the content of the live patch being incorrect or the
>     issue being inappropriate to live patch, this is not considered as a
>     security issue.
>
> * Bugs in the live patch parsing code (the ELF loader):
>     Bugs in the live patch parsing code such as out-of-bounds reads
>     caused by invalid ELF files are not considered to be security issues
>     because the it can only be triggered by a privileged domain.

For these last points, I think it is worth stating that people using
livepatching are expected to test their patches in a test environment first.

>
> * Bugs which allow a guest to prevent the application of a livepatch:
>     A guest should not be able to prevent the application of a live
>     patch. If an unprivileged guest can prevent the application of a
>     live patch, it shall be treated as a security issue.

This one is harder to say.  We know that enough concurrent live
migrations can, which extends to "lots of activity in the guest".  Its
perhaps worth noting the potential workaround of `xl pause $DOM;
xen-livepatch ...; xl unpause`.

I'd prefer that we excluded situations like this from being within
security support.  "guest having heavy workloads" is normal for end
users, so shouldn't constitute a security vulnerability, as there is
nothing we can do about it.

>
> There are also some generic security questions which it is worth asking:
>
> 1) Is guest->host privilege escalation possible?
>
> The new live patching sysctl subops are only accessible to privileged
> domains and this is tested by OSSTest with an XTF test.
> There is a caveat -- an incorrect live patch can introduce a guest->host
> privilege escalation.
>
> 2) Is guest user->guest kernel escalation possible?
>
> No, although an incorrect live patch can introduce a guest user->guest
> kernel privilege escalation.
>
> 3) Is there any information leakage?
>
> The new live patching sysctl subops are only accessible to privileged
> domains so it is not possible for an unprivileged guest to access the
> list of loaded live patches. This is tested by OSSTest with an XTF test.
> There is a caveat -- an incorrect live patch can introduce an
> information leakage.
>
> 4) Can a Denial-of-Service be triggered?
>
> There are no known ways that an unprivileged guest can prevent a live
> patch from being loaded.
> Once again, there is a caveat that an incorrect live patch can introduce
> an arbitrary denial of service.
>
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

This is all good, but this information needs to be in a file in
docs/features/, most probably livepatching.pandoc

> ---
>  xen/common/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
> index dc8e876..876086c 100644
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -226,7 +226,7 @@ config CRYPTO
>       bool
>  
>  config LIVEPATCH
> -     bool "Live patching support (TECH PREVIEW)"
> +     bool "Live patching support"
>       default n

This default should flip as well.

~Andrew

>       depends on HAS_BUILD_ID = "y"
>       ---help---


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