BugLink: https://bugs.launchpad.net/bugs/1853142

commit af2e8c68 upstream.

On some systems that are vulnerable to Spectre v2, it is up to
software to flush the link stack (return address stack), in order to
protect against Spectre-RSB.

When exiting from a guest we do some house keeping and then
potentially exit to C code which is several stack frames deep in the
host kernel. We will then execute a series of returns without
preceeding calls, opening up the possiblity that the guest could have
poisoned the link stack, and direct speculative execution of the host
to a gadget of some sort.

To prevent this we add a flush of the link stack on exit from a guest.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[dja: backport to v4.4, drop P9 support]
Signed-off-by: Daniel Axtens <dja@axtens.net>

CVE-2019-18660
Signed-off-by: Benjamin M Romer <benjamin.romer@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Sultan Alsawaf <sultan.alsawaf@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>