linux-kernel - Re: [PATCH 00/29] x86: Kernel IBT

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Date:   Sat, 19 Feb 2022 10:58:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Poimboe, Josh" <jpoimboe@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "Milburn, Alyssa" <alyssa.milburn@...el.com>
Subject: Re: [PATCH 00/29] x86: Kernel IBT

On Sat, Feb 19, 2022 at 01:29:45AM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-02-18 at 17:49 +0100, Peter Zijlstra wrote:
> > This is an (almost!) complete Kernel IBT implementation. It's been
> > self-hosting
> > for a few days now. That is, it runs on IBT enabled hardware
> > (Tigerlake) and is
> > capable of building the next kernel.
> > 
> > It is also almost clean on allmodconfig using GCC-11.2.
> > 
> > The biggest TODO item at this point is Clang, I've not yet looked at
> > that.
> 
> Do you need to turn this off before kexec?

Probably... :-) I've never looked at that code though; so I'm not
exactly sure where to put things.

I'm assuming kexec does a hot-unplug of all but the boot-cpu which then
leaves only a single CPU with state in machine_kexec() ? Does the below
look reasonable?

--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -638,6 +638,12 @@ static __always_inline void setup_cet(st
 	}
 }
 
+void cet_disable(void)
+{
+	cr4_clear_bits(X86_CR4_CET);
+	wrmsrl(MSR_IA32_S_CET, 0);
+}
+
 /*
  * Some CPU features depend on higher CPUID levels, which may not always
  * be available due to CPUID level capping or broken virtualization
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 33d41e350c79..cf26356db53e 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -72,4 +72,7 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c);
 #else
 static inline void init_ia32_feat_ctl(struct cpuinfo_x86 *c) {}
 #endif
+
+extern void cet_disable(void);
+
 #endif /* _ASM_X86_CPU_H */
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index f5da4a18070a..29a2a1732605 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -310,6 +310,7 @@ void machine_kexec(struct kimage *image)
 	/* Interrupts aren't acceptable while we reboot */
 	local_irq_disable();
 	hw_breakpoint_disable();
+	cet_disable();
 
 	if (image->preserve_context) {
 #ifdef CONFIG_X86_IO_APIC