retpoline/module: fall back to another spectre mitigation when disabling retpoline

retpoline/module: fall back to another spectre mitigation when disabling retpoline

Commit ("retpoline/module: Taint kernel for missing retpoline in module")
calls the new function disable_retpoline() when check_modinfo() determines
that a LKM being loaded was not compiled with retpoline.

This commit adds code to disable_retpoline() that attempts to fall back to
the Spectre v2 mitigations IBRS/IBPB when disabling retpoline.

Pseudocode for disable_retpoline():

if retpoline is not enabled
	No messages/changes
	return

if we are allowed to fall back to another mitigation
	if IBRS is not in use
		if we enabled it
			spectre_v2_enabled = SPECTRE_V2_IBRS
			pr_err("Spectre v2 mitigation set to IBRS.\n")
			if we enabled IBPB mitigation
				pr_err("Spectre v2 mitigation IBPB enabled.\n")
		else
			pr_err("Could not enable IBRS.\n")
			spectre_v2_enabled = SPECTRE_V2_NONE
			pr_err("No Spectre v2 mitigation to fall back to.\n")
	else
		spectre_v2_enabled = SPECTRE_V2_IBRS;
		pr_err("Spectre v2 mitigation IBRS is set.\n")
else
	spectre_v2_enabled = SPECTRE_V2_NONE;
	pr_err("Cannot choose another Spectre v2 mitigation because retpoline_fallback is off.\n")

if spectre_v2_enabled == SPECTRE_V2_NONE
	pr_err("system may be vulnerable to spectre\n")

The attempt to fall back can be disabled with the new kernel boot parameter
	spectre_v2_heuristics=[retpoline_fallback=off]
Disabling retpoline fallback can also be done through debugfs
"retpoline_fallback".

Orabug: 27457549

Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Reviewed-by: Todd Vierling <todd.vierling@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	Documentation/kernel-parameters.txt
	arch/x86/include/asm/spec_ctrl.h
	arch/x86/kernel/cpu/spec_ctrl.c