linux-kernel - [PATCH AUTOSEL 4.9 2/6] x86/bugs: Keep a per-CPU IA32_SPEC

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Message-Id: <20220720011858.1025523-2-sashal@kernel.org>
Date:   Tue, 19 Jul 2022 21:18:53 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        kim.phillips@....com, sblbir@...zon.com,
        alexandre.chartre@...cle.com, chang.seok.bae@...el.com,
        keescook@...omium.org, ebiederm@...ssion.com,
        zhengqi.arch@...edance.com
Subject: [PATCH AUTOSEL 4.9 2/6] x86/bugs: Keep a per-CPU IA32_SPEC_CTRL value

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit caa0ff24d5d0e02abce5e65c3d2b7f20a6617be5 ]

Due to TIF_SSBD and TIF_SPEC_IB the actual IA32_SPEC_CTRL value can
differ from x86_spec_ctrl_base. As such, keep a per-CPU value
reflecting the current task's MSR content.

  [jpoimboe: rename]

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/include/asm/nospec-branch.h |  1 +
 arch/x86/kernel/cpu/bugs.c           | 28 +++++++++++++++++++++++-----
 arch/x86/kernel/process.c            |  2 +-
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 8a618fbf569f..6bc5a324dd65 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -291,6 +291,7 @@ static inline void indirect_branch_prediction_barrier(void)
 
 /* The Intel SPEC CTRL MSR base value cache */
 extern u64 x86_spec_ctrl_base;
+extern void write_spec_ctrl_current(u64 val);
 
 /*
  * With retpoline, we must use IBRS to restrict branch prediction
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b4416df41d63..b4f62f8b47d6 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -45,11 +45,29 @@ static void __init taa_select_mitigation(void);
 static void __init mmio_select_mitigation(void);
 static void __init srbds_select_mitigation(void);
 
-/* The base value of the SPEC_CTRL MSR that always has to be preserved. */
+/* The base value of the SPEC_CTRL MSR without task-specific bits set */
 u64 x86_spec_ctrl_base;
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
+
+/* The current value of the SPEC_CTRL MSR with task-specific bits set */
+DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
+EXPORT_SYMBOL_GPL(x86_spec_ctrl_current);
+
 static DEFINE_MUTEX(spec_ctrl_mutex);
 
+/*
+ * Keep track of the SPEC_CTRL MSR value for the current task, which may differ
+ * from x86_spec_ctrl_base due to STIBP/SSB in __speculation_ctrl_update().
+ */
+void write_spec_ctrl_current(u64 val)
+{
+	if (this_cpu_read(x86_spec_ctrl_current) == val)
+		return;
+
+	this_cpu_write(x86_spec_ctrl_current, val);
+	wrmsrl(MSR_IA32_SPEC_CTRL, val);
+}
+
 /*
  * The vendor and possibly platform specific bits which can be modified in
  * x86_spec_ctrl_base.
@@ -1101,7 +1119,7 @@ static void __init spectre_v2_select_mitigation(void)
 	if (spectre_v2_in_eibrs_mode(mode)) {
 		/* Force it so VMEXIT will restore correctly */
 		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		write_spec_ctrl_current(x86_spec_ctrl_base);
 	}
 
 	switch (mode) {
@@ -1156,7 +1174,7 @@ static void __init spectre_v2_select_mitigation(void)
 
 static void update_stibp_msr(void * __unused)
 {
-	wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+	write_spec_ctrl_current(x86_spec_ctrl_base);
 }
 
 /* Update x86_spec_ctrl_base in case SMT state changed. */
@@ -1399,7 +1417,7 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
 			x86_amd_ssb_disable();
 		} else {
 			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
-			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+			write_spec_ctrl_current(x86_spec_ctrl_base);
 		}
 	}
 
@@ -1604,7 +1622,7 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
 void x86_spec_ctrl_setup_ap(void)
 {
 	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
-		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		write_spec_ctrl_current(x86_spec_ctrl_base);
 
 	if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
 		x86_amd_ssb_disable();
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index f58e4cc20c1c..f4c7fc3e5721 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -358,7 +358,7 @@ static __always_inline void __speculation_ctrl_update(unsigned long tifp,
 	}
 
 	if (updmsr)
-		wrmsrl(MSR_IA32_SPEC_CTRL, msr);
+		write_spec_ctrl_current(msr);
 }
 
 static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)
-- 
2.35.1