[PATCH 3.16 010/131] x86/bugs: Whitelist allowed SPEC_CTRL MSR values

[Ben Hutchings]

3.16.59-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

commit 1115a859f33276fe8afb31c60cf9d8e657872558 upstream.

Intel and AMD SPEC_CTRL (0x48) MSR semantics may differ in the
future (or in fact use different MSRs for the same functionality).

As such a run-time mechanism is required to whitelist the appropriate MSR
values.

[ tglx: Made the variable __ro_after_init ]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16:
 - We don't have __ro_after_init
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -34,6 +34,12 @@ static void __init ssb_select_mitigation
  */
 static u64 x86_spec_ctrl_base;
 
+/*
+ * The vendor and possibly platform specific bits which can be modified in
+ * x86_spec_ctrl_base.
+ */
+static u64 x86_spec_ctrl_mask = ~SPEC_CTRL_IBRS;
+
 #ifdef CONFIG_X86_32
 
 static double __initdata x = 4195835.0;
@@ -179,7 +185,7 @@ static enum spectre_v2_mitigation spectr
 
 void x86_spec_ctrl_set(u64 val)
 {
-	if (val & ~(SPEC_CTRL_IBRS | SPEC_CTRL_RDS))
+	if (val & x86_spec_ctrl_mask)
 		WARN_ONCE(1, "SPEC_CTRL MSR value 0x%16llx is unknown.\n", val);
 	else
 		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | val);
@@ -497,6 +503,7 @@ static enum ssb_mitigation_cmd __init __
 		switch (boot_cpu_data.x86_vendor) {
 		case X86_VENDOR_INTEL:
 			x86_spec_ctrl_base |= SPEC_CTRL_RDS;
+			x86_spec_ctrl_mask &= ~SPEC_CTRL_RDS;
 			x86_spec_ctrl_set(SPEC_CTRL_RDS);
 			break;
 		case X86_VENDOR_AMD:
@@ -520,7 +527,7 @@ static void ssb_select_mitigation()
 void x86_spec_ctrl_setup_ap(void)
 {
 	if (boot_cpu_has(X86_FEATURE_IBRS))
-		x86_spec_ctrl_set(x86_spec_ctrl_base & (SPEC_CTRL_IBRS | SPEC_CTRL_RDS));
+		x86_spec_ctrl_set(x86_spec_ctrl_base & ~x86_spec_ctrl_mask);
 }
 
 #ifdef CONFIG_SYSFS