linux-kernel - [PATCH 6.8 1/5] x86/mmio: Disable KVM mitigation when X86_FEATURE_CLEAR_CPU

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Message-ID: <20240313163019.613705-2-sashal@kernel.org>
Date: Wed, 13 Mar 2024 12:30:15 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 6.8 1/5] x86/mmio: Disable KVM mitigation when X86_FEATURE_CLEAR_CPU_BUF is set

From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>

commit e95df4ec0c0c9791941f112db699fae794b9862a upstream.

Currently MMIO Stale Data mitigation for CPUs not affected by MDS/TAA is
to only deploy VERW at VMentry by enabling mmio_stale_data_clear static
branch. No mitigation is needed for kernel->user transitions. If such
CPUs are also affected by RFDS, its mitigation may set
X86_FEATURE_CLEAR_CPU_BUF to deploy VERW at kernel->user and VMentry.
This could result in duplicate VERW at VMentry.

Fix this by disabling mmio_stale_data_clear static branch when
X86_FEATURE_CLEAR_CPU_BUF is enabled.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/kernel/cpu/bugs.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 48d049cd74e71..cd6ac89c1a0df 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -422,6 +422,13 @@ static void __init mmio_select_mitigation(void)
 	if (boot_cpu_has_bug(X86_BUG_MDS) || (boot_cpu_has_bug(X86_BUG_TAA) &&
 					      boot_cpu_has(X86_FEATURE_RTM)))
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+	/*
+	 * X86_FEATURE_CLEAR_CPU_BUF could be enabled by other VERW based
+	 * mitigations, disable KVM-only mitigation in that case.
+	 */
+	if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
+		static_branch_disable(&mmio_stale_data_clear);
 	else
 		static_branch_enable(&mmio_stale_data_clear);
 
@@ -498,8 +505,11 @@ static void __init md_clear_update_mitigation(void)
 		taa_mitigation = TAA_MITIGATION_VERW;
 		taa_select_mitigation();
 	}
-	if (mmio_mitigation == MMIO_MITIGATION_OFF &&
-	    boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
+	/*
+	 * MMIO_MITIGATION_OFF is not checked here so that mmio_stale_data_clear
+	 * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
+	 */
+	if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
 		mmio_mitigation = MMIO_MITIGATION_VERW;
 		mmio_select_mitigation();
 	}
-- 
2.43.0