Re: [PATCH 3.17 100/141] x86, microcode: Fix accessing dis_ucode_ldr on 32-bit

[Boris Ostrovsky]

On 11/25/2014 05:18 PM, Borislav Petkov wrote:
> Adding x86 people.
>
> On Tue, Nov 25, 2014 at 04:59:34PM -0500, Boris Ostrovsky wrote:
> > This should be cpuid(0x1, &eax, &ebx, &ecx, &edx). Otherwise we are not
> > getting bits that the hypervisor wants the guest to see (on Xen cpuid()
> > turns into hypercall, on baremetal it's native).
> >
> > With that change it works and
> >
> > Tested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Sigh... I take this back. It breaks 32-bit baremetal. I haven't looked 
any further but it seems to be dying very early. I suspect cpuid pv_op 
is not set up yet. If that's true, perhaps you could check whether it is 
valid in x86_guest()?

I won't be able to do anything tomorrow morning, the best I can hope for 
is evening.


-boris



> Thanks for testing.
>
> > (May be worth adding a comment as to what is_guest() is checking for since
> > 31 is a magic number).
> See below.
>
> > BTW, the crash had nothing to do with accessing dis_ucode_ldr, we are
> > crashing much later, in load_ucode_intel_ap(), trying to access
> > *initrd_start_p. And the reason we didn't crash before was because compiler
> > optimized out whole load_ucode_ap() since check_loader_disabled_ap() was
> > always true.
> Right, and my fix actually uncovered the original issue :-\
>
> Ok, here's a v2 which adds the check to the late loader too, for
> completeness. I'll write a proper commit message tomorrow.
>
> ---
> diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
> index 64dc362506b7..654907db5f09 100644
> --- a/arch/x86/include/asm/microcode.h
> +++ b/arch/x86/include/asm/microcode.h
> @@ -87,4 +87,9 @@ static inline int __init save_microcode_in_initrd(void)
>   }
>   #endif
>   
> +/* Check whether we're running as a guest on a hypervisor. */
> +static inline bool x86_guest(void)
> +{
> +	return !!(cpuid_ecx(1) & BIT(31));
> +}
>   #endif /* _ASM_X86_MICROCODE_H */
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index 2ce9051174e6..0b6db2a97f61 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -557,6 +557,9 @@ static int __init microcode_init(void)
>   	struct cpuinfo_x86 *c = &cpu_data(0);
>   	int error;
>   
> +	if (x86_guest())
> +		return 0;
> +
>   	if (dis_ucode_ldr)
>   		return 0;
>   
> diff --git a/arch/x86/kernel/cpu/microcode/core_early.c b/arch/x86/kernel/cpu/microcode/core_early.c
> index 2c017f242a78..dfa93e74c370 100644
> --- a/arch/x86/kernel/cpu/microcode/core_early.c
> +++ b/arch/x86/kernel/cpu/microcode/core_early.c
> @@ -98,6 +98,9 @@ void __init load_ucode_bsp(void)
>   {
>   	int vendor, x86;
>   
> +	if (x86_guest())
> +		return;
> +
>   	if (check_loader_disabled_bsp())
>   		return;
>   
> @@ -134,6 +137,9 @@ void load_ucode_ap(void)
>   {
>   	int vendor, x86;
>   
> +	if (x86_guest())
> +		return;
> +
>   	if (check_loader_disabled_ap())
>   		return;
>   

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