x86/speculation: Allow guests to use SSBD even if host does not

The bits set in x86_spec_ctrl_mask are used to calculate the guest's value
of SPEC_CTRL that is written to the MSR before VMENTRY, and control which
mitigations the guest can enable.  In the case of SSBD, unless the host has
enabled SSBD always on mode (by passing "spec_store_bypass_disable=on" in
the kernel parameters), the SSBD bit is not set in the mask and the guest
can not properly enable the SSBD always on mitigation mode.

This has been confirmed by running the SSBD PoC on a guest using the SSBD
always on mitigation mode (booted with kernel parameter
"spec_store_bypass_disable=on"), and verifying that the guest is vulnerable
unless the host is also using SSBD always on mode. In addition, the guest
OS incorrectly reports the SSB vulnerability as mitigated.

Always set the SSBD bit in x86_spec_ctrl_mask when the host CPU supports
it, allowing the guest to use SSBD whether or not the host has chosen to
enable the mitigation in any of its modes.

Fixes: be6fcb54 ("x86/bugs: Rework spec_ctrl base and mask logic")
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: bp@alien8.de
Cc: rkrcmar@redhat.com
Cc: kvm@vger.kernel.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1560187210-11054-1-git-send-email-alejandro.j.jimenez@oracle.com