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SubjectRe: [PATCH 00/29] x86: Kernel IBT


On February 19, 2022 1:58:27 AM PST, Peter Zijlstra <peterz@infradead.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

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