linux-kernel - Re: [PATCH 0/2] x86/cfi: Fix FineIBT

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Date:   Fri, 16 Jun 2023 16:24:17 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, alyssa.milburn@...ux.intel.com,
        linux-kernel@...r.kernel.org, keescook@...omium.org,
        jpoimboe@...nel.org, joao@...rdrivepizza.com,
        tim.c.chen@...ux.intel.com
Subject: Re: [PATCH 0/2] x86/cfi: Fix FineIBT

On Thu, Jun 15, 2023 at 12:40 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Hi!
>
> Alyssa reported a FineIBT issue; these patches sort that.
>
> Basically the morale of the story is that CALL_NOSPEC and JMP_NOSPEC are
> suspect and likely not what you want. The remaining users are:
>
> arch/x86/crypto/crc32c-pcl-intel-asm_64.S:      JMP_NOSPEC bufp
>
> This is broken on anything IBT afaict.
>
> arch/x86/include/asm/mshyperv.h:                             CALL_NOSPEC
> arch/x86/include/asm/mshyperv.h:        __asm__ __volatile__(CALL_NOSPEC
> arch/x86/include/asm/mshyperv.h:                __asm__ __volatile__(CALL_NOSPEC
> arch/x86/include/asm/mshyperv.h:                __asm__ __volatile__ (CALL_NOSPEC
> arch/x86/include/asm/mshyperv.h:                                     CALL_NOSPEC
> arch/x86/include/asm/mshyperv.h:                __asm__ __volatile__ (CALL_NOSPEC
> arch/x86/include/asm/xen/hypercall.h:   asm volatile(CALL_NOSPEC
>
> These are hypercalls and supposedly the targets are having ENDBR on.
>
> arch/x86/kernel/ftrace_64.S:    CALL_NOSPEC r8
>
> The thing is !DYNAMIC_FTRACE only, which we can fix with a Kconfig
> dependency I suppose.
>
> arch/x86/kvm/emulate.c: asm("push %[flags]; popf; " CALL_NOSPEC
> arch/x86/kvm/emulate.c: asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n"
>
> calls into the magic fastop stuff and should be ok, those have explicit
> ENDBR + IBT_NOSEAL() annotations.
>
> arch/x86/kvm/vmx/vmenter.S:     VMX_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
>
> calls the IDT vectors manually, so should be ok (gross, but not broken
> from this pov).
>
> arch/x86/platform/efi/efi_stub_64.S:    CALL_NOSPEC rdi
>
> effectively a nocfi call, so should be ok.

Thanks, this looks good to me. For the series:

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami