livepatch: Declare live patching as a supported feature

See docs/features/livepatch.pandoc for the details.

Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

docs/features/livepatch.pandoc

@@ -0,0 +1,106 @@
+% Live Patching
+% Revision 1
+
+\clearpage
+
+# Basics
+
+---------------- ----------------------------------------------------
+         Status: **Supported**
+
+   Architecture: x86
+
+         Status: **Tech Preview/Experimental**
+
+   Architecture: ARM
+
+      Component: Hypervisor, toolstack
+---------------- ----------------------------------------------------
+
+
+# Details
+
+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 on x86.
+
+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 the scope of security support:
+
+1) 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.
+
+2) 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.
+
+3) Bugs in livepatch-build-tools creating an 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. A live patch should be checked to verify that it is valid
+   before loading.
+
+4) 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.
+
+5) 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.
+
+6) 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 somehow prevent the application
+   of a live patch despite pausing it (xl pause ...), it shall be
+   treated as a security issue.
+
+Note: It is expected that live patches are tested in a test environment
+before being used in production to avoid unexpected issues. In
+particular, to avoid the issues described by (3), (4), & (5).
+
+There are also some generic security questions which are 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.

xen/common/Kconfig

@@ -226,8 +226,8 @@ config CRYPTO
 	bool
 
 config LIVEPATCH
-	bool "Live patching support (TECH PREVIEW)"
-	default n
+	bool "Live patching support"
+	default X86
 	depends on HAS_BUILD_ID = "y"
 	---help---
 	  Allows a running Xen hypervisor to be dynamically patched using