[PATCH v4 11/11] x86/vmscape: Add cmdline vmscape=on to override attack vector controls

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

In general, individual mitigation controls can be used to override the
attack vector controls. But, nothing exists to select BHB clearing
mitigation for VMSCAPE. The =force option comes close, but with a
side-effect of also forcibly setting the bug, hence deploying the
mitigation on unaffected parts too.

Add a new cmdline option vmscape=on to enable the mitigation based on the
VMSCAPE variant the CPU is affected by.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 Documentation/admin-guide/hw-vuln/vmscape.rst   | 4 ++++
 Documentation/admin-guide/kernel-parameters.txt | 4 +++-
 arch/x86/kernel/cpu/bugs.c                      | 2 ++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst
index dc63a0bac03d43d1e295de0791dd6497d101f986..580f288ae8bfc601ff000d6d95d711bb9084459e 100644
--- a/Documentation/admin-guide/hw-vuln/vmscape.rst
+++ b/Documentation/admin-guide/hw-vuln/vmscape.rst
@@ -112,3 +112,7 @@ The mitigation can be controlled via the ``vmscape=`` command line parameter:
 
    Force vulnerability detection and mitigation even on processors that are
    not known to be affected.
+
+ * ``vmscape=on``:
+
+   Choose the mitigation based on the VMSCAPE variant the CPU is affected by.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6c42061ca20e581b5192b66c6f25aba38d4f8ff8..4b4711ced5e187495476b5365cd7b3df81db893b 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8104,9 +8104,11 @@
 
 			off		- disable the mitigation
 			ibpb		- use Indirect Branch Prediction Barrier
-					  (IBPB) mitigation (default)
+					  (IBPB) mitigation
 			force		- force vulnerability detection even on
 					  unaffected processors
+			on		- (default) automatically select IBPB
+			                  or BHB clear mitigation based on CPU
 
 	vsyscall=	[X86-64,EARLY]
 			Controls the behavior of vsyscalls (i.e. calls to
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 81b0db27f4094c90ebf4704c74f5e7e6b809560f..b4a21434869fcc01c40a2973f986a3f275f92ef2 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3227,6 +3227,8 @@ static int __init vmscape_parse_cmdline(char *str)
 	} else if (!strcmp(str, "force")) {
 		setup_force_cpu_bug(X86_BUG_VMSCAPE);
 		vmscape_mitigation = VMSCAPE_MITIGATION_ON;
+	} else if (!strcmp(str, "on")) {
+		vmscape_mitigation = VMSCAPE_MITIGATION_ON;
 	} else {
 		pr_err("Ignoring unknown vmscape=%s option.\n", str);
 	}

-- 
2.34.1