[SRU OEM-5.14 06/11] x86/speculation/mmio: Enable CPU Fill buffer clearing on idle

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Wed Jun 15 12:17:20 UTC 2022

From: Pawan Gupta <pawan.kumar.gupta at linux.intel.com>

commit 99a83db5a605137424e1efe29dc0573d6a5b6316 upstream

When the CPU is affected by Processor MMIO Stale Data vulnerabilities,
Fill Buffer Stale Data Propagator (FBSDP) can propagate stale data out
of Fill buffer to uncore buffer when CPU goes idle. Stale data can then
be exploited with other variants using MMIO operations.

Mitigate it by clearing the Fill buffer before entering idle state.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta at linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Co-developed-by: Josh Poimboeuf <jpoimboe at redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe at redhat.com>
Signed-off-by: Borislav Petkov <bp at suse.de>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
 arch/x86/kernel/cpu/bugs.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dc3b8b434fdc..a39019760d9e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -424,6 +424,14 @@ static void __init mmio_select_mitigation(void)
+	/*
+	 * If Processor-MMIO-Stale-Data bug is present and Fill Buffer data can
+	 * be propagated to uncore buffers, clearing the Fill buffers on idle
+	 * is required irrespective of SMT state.
+	 */
+	if (!(ia32_cap & ARCH_CAP_FBSDP_NO))
+		static_branch_enable(&mds_idle_clear);
 	 * Check if the system has the right microcode.
@@ -1188,6 +1196,8 @@ static void update_indir_branch_cond(void)
 /* Update the static key controlling the MDS CPU buffer clear in idle */
 static void update_mds_branch_idle(void)
+	u64 ia32_cap = x86_read_arch_cap_msr();
 	 * Enable the idle clearing if SMT is active on CPUs which are
 	 * affected only by MSBDS and not any other MDS variant.
@@ -1199,10 +1209,12 @@ static void update_mds_branch_idle(void)
 	if (!boot_cpu_has_bug(X86_BUG_MSBDS_ONLY))
-	if (sched_smt_active())
+	if (sched_smt_active()) {
-	else
+	} else if (mmio_mitigation == MMIO_MITIGATION_OFF ||
+		   (ia32_cap & ARCH_CAP_FBSDP_NO)) {
+	}
 #define MDS_MSG_SMT "MDS CPU bug present and SMT on, data leak possible. See https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html for more details.\n"

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