[tip: x86/cpu] x86/mce: WARN once if IA32_FEAT_CTL MSR is left unlocked

[tip-bot2 for Sean Christopherson]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     6d527cebfa04ba4792be9e79e0d7cab22ab6c377
Gitweb:        https://git.kernel.org/tip/6d527cebfa04ba4792be9e79e0d7cab22ab6c377
Author:        Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
AuthorDate:    Fri, 20 Dec 2019 20:44:59 -08:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Mon, 13 Jan 2020 17:47:18 +01:00

x86/mce: WARN once if IA32_FEAT_CTL MSR is left unlocked

WARN if the IA32_FEAT_CTL MSR is somehow left unlocked now that CPU
initialization unconditionally locks the MSR.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20191221044513.21680-6-sean.j.christopherson@xxxxxxxxx
---
 arch/x86/kernel/cpu/mce/intel.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/intel.c b/arch/x86/kernel/cpu/mce/intel.c
index c238518..5627b10 100644
--- a/arch/x86/kernel/cpu/mce/intel.c
+++ b/arch/x86/kernel/cpu/mce/intel.c
@@ -116,14 +116,15 @@ static bool lmce_supported(void)
 	/*
 	 * BIOS should indicate support for LMCE by setting bit 20 in
 	 * IA32_FEAT_CTL without which touching MCG_EXT_CTL will generate a #GP
-	 * fault.
+	 * fault.  The MSR must also be locked for LMCE_ENABLED to take effect.
+	 * WARN if the MSR isn't locked as init_ia32_feat_ctl() unconditionally
+	 * locks the MSR in the event that it wasn't already locked by BIOS.
 	 */
 	rdmsrl(MSR_IA32_FEAT_CTL, tmp);
-	if ((tmp & (FEAT_CTL_LOCKED | FEAT_CTL_LMCE_ENABLED)) ==
-		   (FEAT_CTL_LOCKED | FEAT_CTL_LMCE_ENABLED))
-		return true;
+	if (WARN_ON_ONCE(!(tmp & FEAT_CTL_LOCKED)))
+		return false;
 
-	return false;
+	return tmp & FEAT_CTL_LMCE_ENABLED;
 }
 
 bool mce_intel_cmci_poll(void)