linux-kernel - Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

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Message-ID: <20221128233739.dd3dzx6qkermsfri@desk>
Date:   Mon, 28 Nov 2022 15:37:39 -0800
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Breno Leitao <leitao@...ian.org>, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com,
        jpoimboe@...nel.org, peterz@...radead.org, x86@...nel.org,
        cascardo@...onical.com, leit@...a.com, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

On Mon, Nov 28, 2022 at 03:02:21PM -0800, Pawan Gupta wrote:
>On Mon, Nov 28, 2022 at 11:40:19PM +0100, Borislav Petkov wrote:
>>On Mon, Nov 28, 2022 at 02:03:58PM -0800, Pawan Gupta wrote:
>>>diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>>>index 3e3230cccaa7..cfc2ed2661fc 100644
>>>--- a/arch/x86/kernel/cpu/bugs.c
>>>+++ b/arch/x86/kernel/cpu/bugs.c
>>>@@ -66,7 +66,7 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
>>>  */
>>> void write_spec_ctrl_current(u64 val, bool force)
>>> {
>>>-	if (this_cpu_read(x86_spec_ctrl_current) == val)
>>>+	if (!force && this_cpu_read(x86_spec_ctrl_current) == val)
>>> 		return;
>>> 	this_cpu_write(x86_spec_ctrl_current, val);
>>
>>Still looks hacky to me.
>>
>>I think it would be a lot cleaner if MSR_IA32_SPEC_CTRL gets cleaned of
>>the speculation bits in init_speculation_control() which gets run on
>>*every* CPU.
>>
>>So by the time check_bugs() gets to setup stuff, the MSR will be ready
>>to go regardless.
>>
>>I.e., something like this (not supposed to work - just to show what I
>>mean):
>>
>>diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>index 73cc546e024d..367732c92942 100644
>>--- a/arch/x86/kernel/cpu/common.c
>>+++ b/arch/x86/kernel/cpu/common.c
>>@@ -993,9 +993,19 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
>>	 * Intel CPUs, for finer-grained selection of what's available.
>>	 */
>>	if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {
>>+		u64 msr;
>>+
>>		set_cpu_cap(c, X86_FEATURE_IBRS);
>>		set_cpu_cap(c, X86_FEATURE_IBPB);
>>		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
>>+
>>+		/*
>>+		 * Clear speculation control settings from a previous kernel
>>+		 * run, i.e., kexec.
>>+		 */
>>+		rdmsrl(MSR_IA32_SPEC_CTRL, msr);
>>+		if (msr & SPEC_CTRL_MASK)
>>+			wrmsr (MSR_IA32_SPEC_CTRL, msr & ~SPEC_CTRL_MASK);
>
>Yes thats a cleaner approach, except that the late microcode load will
>ruin the MSR:

Root of the original problem is x86_spec_ctrl_current is not the current
value of MSR at bootup.

How about we update x86_spec_ctrl_current before any writes to the MSR?:

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3e3230cccaa7..68ed52394fd9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -137,8 +137,18 @@ void __init check_bugs(void)
  	 * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
  	 * init code as it is not enumerated and depends on the family.
  	 */
-	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
+	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
  		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		/*
+		 * Previously running software, like kexec for example, may
+		 * have some controls turned ON.
+		 * Clear them and let the mitigations setup below set them
+		 * based on configuration.
+		 */
+		this_cpu_write(x86_spec_ctrl_current, x86_spec_ctrl_base);
+		x86_spec_ctrl_base &= ~SPEC_CTRL_MITIGATIONS_MASK;
+		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+	}
  
  	/* Select the proper CPU mitigations before patching alternatives: */
  	spectre_v1_select_mitigation();
@@ -2047,8 +2057,13 @@ 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))
+	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
+		u64 msr;
+
+		rdmsrl(MSR_IA32_SPEC_CTRL, msr);
+		this_cpu_write(x86_spec_ctrl_current, msr);
  		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+	}
  
  	if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
  		x86_amd_ssb_disable();