linux-kernel - Re: [PATCH] x86/cpu: Avoid writing MSR_IA32_TSX

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Message-ID: <20220908010329.rhetlp6plqlaiegf@desk>
Date:   Wed, 7 Sep 2022 18:03:29 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Hans de Goede <hdegoede@...hat.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] x86/cpu: Avoid writing MSR_IA32_TSX_CTRL when writing it
 is not supported

On Tue, Sep 06, 2022 at 11:00:08PM +0000, Andrew Cooper wrote:
> On 06/09/2022 22:00, Peter Zijlstra wrote:
> > On Tue, Sep 06, 2022 at 10:56:47PM +0200, Hans de Goede wrote:
> >> Hi,
> >>
> >> On 9/6/22 22:43, Peter Zijlstra wrote:
> >>> On Tue, Sep 06, 2022 at 10:17:43PM +0200, Hans de Goede wrote:
> >>>> On an Intel Atom N2600 (and presumable other Cedar Trail models)
> >>>> MSR_IA32_TSX_CTRL can be read, causing saved_msr.valid to be set for it
> >>>> by msr_build_context().
> >>>>
> >>>> This causes restore_processor_state() to try and restore it, but writing
> >>>> this MSR is not allowed on the Intel Atom N2600 leading to:
> >>> FWIW, virt tends to do this same thing a lot. They'll allow reading
> >>> random MSRs and only fail on write.
> >> Right. So I guess I should send a v2 with an updated commit
> >> message mentioning this ?
> > Nah, just saying this is a somewhat common pattern with MSRs.
> >
> > The best ones are the one where writing the value read is invalid :/ or
> > those who also silently eat a 0 write just for giggles. Luckily that
> > doesn't happen often.
> 
> Several comments.  First of all, MSR_TSX_CTRL is a fully read/write
> MSR.  If virt is doing this wrong, fix the hypervisor.  But this doesn't
> look virt related?
> 
> More importantly, MSR_TSX_CTRL does not plausibly exist on an Atom
> N2600, as it is more than a decade old.
> 
> MSR_TSX_CTRL was retrofitted in microcode to the MDS_NO, TAA-vulnerable
> CPUs which is a very narrow range from about 1 quarter of 2019 which
> includes Cascade Lake, and then included architecturally on subsequent
> parts which support TSX.
> 
> pm_save_spec_msr() is totally broken.  It's poking MSRs blindly without
> checking the enumeration of the capability first.

pm_save_spec_msr() relies on valid-msr-check in build_msr_context(), but
obviously it is not working in this particular case.

Does adding the enumeration check as below looks okay:

(I am not sure if I got the enumeration right for MSR_AMD64_LS_CFG).

---
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 8cbf623f0ecf..a750c1a1964b 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -76,6 +76,8 @@ static inline void init_ia32_feat_ctl(struct cpuinfo_x86 *c) {}
 
 extern __noendbr void cet_disable(void);
 
+extern bool spec_msr_valid(u32 msr_id);
+
 struct ucode_cpu_info;
 
 int intel_cpu_collect_info(struct ucode_cpu_info *uci);
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3e508f239098..7430a36fd7ae 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1278,6 +1278,26 @@ static bool __init cpu_matches(const struct x86_cpu_id *table, unsigned long whi
 	return m && !!(m->driver_data & which);
 }
 
+bool spec_msr_valid(u32 msr_id)
+{
+	u64 ia32_cap = x86_read_arch_cap_msr();
+
+	switch (msr_id) {
+	case MSR_IA32_SPEC_CTRL:
+		return boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL);
+	case MSR_IA32_TSX_CTRL:
+		return !!(ia32_cap & ARCH_CAP_TSX_CTRL_MSR);
+	case MSR_TSX_FORCE_ABORT:
+		return boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT);
+	case MSR_IA32_MCU_OPT_CTRL:
+		return boot_cpu_has(X86_FEATURE_SRBDS_CTRL);
+	case MSR_AMD64_LS_CFG:
+		return boot_cpu_has(X86_FEATURE_LS_CFG_SSBD);
+	}
+
+	return false;
+}
+
 u64 x86_read_arch_cap_msr(void)
 {
 	u64 ia32_cap = 0;
diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index bb176c72891c..8db73f7982c7 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -520,8 +520,12 @@ static void pm_save_spec_msr(void)
 		MSR_IA32_MCU_OPT_CTRL,
 		MSR_AMD64_LS_CFG,
 	};
+	int i;
 
-	msr_build_context(spec_msr_id, ARRAY_SIZE(spec_msr_id));
+	for (i=0; i < ARRAY_SIZE(spec_msr_id); i++) {
+		if (spec_msr_valid(spec_msr_id[i]))
+			msr_build_context(&spec_msr_id[i], 1);
+	}
 }
 
 static int pm_check_save_msr(void)