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Re: [PATCH v2 07/23] x86/boot: Use RSTORSSP to establish SSP
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 1 Sep 2025 17:41:28 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 01 Sep 2025 15:41:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01.09.2025 17:33, Andrew Cooper wrote:
> On 01/09/2025 10:28 am, Jan Beulich wrote:
>> On 28.08.2025 17:03, Andrew Cooper wrote:
>>> @@ -908,7 +909,29 @@ static void __init noreturn reinit_bsp_stack(void)
>>> if ( cpu_has_xen_shstk )
>>> {
>>> wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
>>> - asm volatile ("setssbsy" ::: "memory");
>>> +
>>> + /*
>>> + * IDT and FRED differ by a Supervisor Token on the shadow stack,
>>> and
>>> + * therefore by the value in MSR_PL0_SSP.
>> Beside not being overly relevant here afaict, is this last part of the
>> sentence
>> actually correct? Patch 06 doesn't write different values into the MSR.
>
> It is correct, but also well hidden.
>
> #define MSR_FRED_SSP_SL0 MSR_PL0_SSP
>
> I suppose I should should write MSR_PL0_SSP/MSR_FRED_SSP_SL0 here to
> highlight the logically different names for the two modes.
But the code following the comment doesn't access any MSR. That's what
first tripped me up. It was only then that I wasn't able to spot the two
different writes. Now that you point out the aliasing it becomes clear
that until patch 14 it is simply impossible to find that other write.
Jan
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