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

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

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.

* 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.

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>
---
 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
        depends on HAS_BUILD_ID = "y"
        ---help---
-- 
2.9.4


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