x86_64: Do not execute sysret with a non-canonical return address

x86_64: Do not execute sysret with a non-canonical return address

Check for non-canonical guest RIP before attempting to execute sysret.
If sysret is executed with a non-canonical value in RCX, Intel CPUs
take the fault in ring0, but we will necessarily already have switched
to the the user's stack pointer.

This is a security vulnerability, XSA-7 / CVE-2012-0217.

Signed-off-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Keir Fraser <keir.xen@gmail.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>