linux-kernel - [Patch v7 05/18] x86/speculation: Disable STIBP when enhanced IBRS is in use

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Date:   Tue, 20 Nov 2018 16:08:44 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Jiri Kosina <jikos@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        stable@...r.kernel.org
Subject: [Patch v7 05/18] x86/speculation: Disable STIBP when enhanced IBRS is in use

If enhanced IBRS is engaged, STIBP is redundant in mitigating Spectre
v2 user space exploits from hyperthread sibling.

Disable STIBP when enhanced IBRS is used.

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3a6f13b..199f27e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -323,11 +323,16 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	return cmd;
 }
 
+/* Determine if STIBP should be always on. */
 static bool stibp_needed(void)
 {
 	if (spectre_v2_enabled == SPECTRE_V2_NONE)
 		return false;
 
+	/* Using enhanced IBRS makes using STIBP unnecessary. */
+	if (static_cpu_has(X86_FEATURE_USE_IBRS_ENHANCED))
+		return false;
+
 	if (!boot_cpu_has(X86_FEATURE_STIBP))
 		return false;
 
@@ -856,6 +861,9 @@ static ssize_t l1tf_show_state(char *buf)
 
 static char *stibp_state(void)
 {
+	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+		return "";
+
 	if (x86_spec_ctrl_base & SPEC_CTRL_STIBP)
 		return ", STIBP";
 	else
-- 
2.9.4