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

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Message-ID: <1C202B85-FCB1-4282-9CA6-ED9115F203BB@chromium.org>
Date:   Mon, 21 Feb 2022 00:42:25 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        "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>,
        "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 February 19, 2022 1:58:27 AM PST, Peter Zijlstra <peterz@...radead.org> wrote:
>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);

I'd rather keep the pinning...

>+	wrmsrl(MSR_IA32_S_CET, 0);
>+}

Eh, why not just require kexec to be IBT safe? That seems a reasonable exercise if we ever expect UEFI to enforce IBT when starting the kernel on a normal boot...

-Kees

>+
> /*
>  * 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

-- 
Kees Cook