[PATCH 01/17] x86: spec_ctrl: fix SPEC_CTRL initialization after kexec

[Andrea Arcangeli <aarcange@redhat.com>]

We can't assume the SPEC_CTRL msr is zero at boot because it could be
left enabled by a previous kernel booted with
spec_store_bypass_disable=on.

Without this fix a boot with spec_store_bypass_disable=on followed by
a kexec boot with spec_store_bypass_disable=off would erroneously and
unexpectedly leave bit 2 set in SPEC_CTRL.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 arch/x86/include/asm/msr-index.h |  2 ++
 arch/x86/kernel/cpu/bugs.c       | 20 +++++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 20ce682a2540..3ba95728a6fe 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -47,6 +47,8 @@
 #define SPEC_CTRL_STIBP			BIT(SPEC_CTRL_STIBP_SHIFT)	/* STIBP mask */
 #define SPEC_CTRL_SSBD_SHIFT		2	   /* Speculative Store Bypass Disable bit */
 #define SPEC_CTRL_SSBD			BIT(SPEC_CTRL_SSBD_SHIFT)	/* Speculative Store Bypass Disable */
+#define SPEC_CTRL_ALL			(SPEC_CTRL_IBRS|SPEC_CTRL_STIBP| \
+					 SPEC_CTRL_SSBD) /* all known bits */
 
 #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
 #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 91c2561b905f..e3922dcf252f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -92,8 +92,26 @@ void __init check_bugs(void)
 	 * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
 	 * init code as it is not enumerated and depends on the family.
 	 */
-	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
+	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
 		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		/*
+		 * Clear the non reserved bits from x86_spec_ctrl_base
+		 * to fix kexec. Otherwise for example SSBD could be
+		 * left enabled despite booting with
+		 * spec_store_bypass_disable=off because SSBD would be
+		 * erroneously mistaken as a reserved bit set by the
+		 * BIOS when in fact it was set by a previous kernel
+		 * booted with spec_store_bypass_disable=on. Careful
+		 * however not to write SPEC_CTRL unnecessarily to
+		 * keep the virt MSR intercept enabled as long as
+		 * possible.
+		 */
+		if (x86_spec_ctrl_base & SPEC_CTRL_ALL) {
+			/* all known bits must not be set at boot, clear it */
+			x86_spec_ctrl_base &= ~SPEC_CTRL_ALL;
+			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		}
+	}
 
 	/* Allow STIBP in MSR_SPEC_CTRL if supported */
 	if (boot_cpu_has(X86_FEATURE_STIBP))