Re: [PATCH v5 28/34] x86/fred: fixup fault on ERETU by jumping to fred_entrypoint_user

H. Peter Anvin
2023-03-17T21:01:18Z

On March 17, 2023 2:55:44 AM PDT, andrew.cooper3@citrix.com wrote:
>On 17/03/2023 9:39 am, Lai Jiangshan wrote:
>>> +#ifdef CONFIG_X86_FRED
>>> +static bool ex_handler_eretu(const struct exception_table_entry *fixup,
>>> + struct pt_regs *regs, unsigned long error_code)
>>> +{
>>> + struct pt_regs *uregs = (struct pt_regs *)(regs->sp - offsetof(struct pt_regs, ip));
>>> + unsigned short ss = uregs->ss;
>>> + unsigned short cs = uregs->cs;
>>> +
>>> + fred_info(uregs)->edata = fred_event_data(regs);
>>> + uregs->ssx = regs->ssx;
>>> + uregs->ss = ss;
>>> + uregs->csx = regs->csx;
>>> + uregs->current_stack_level = 0;
>>> + uregs->cs = cs;
>> Hello
>>
>> If the ERETU instruction had tried to return from NMI to ring3 and just faulted,
>> is NMI still blocked?
>>
>> We know that IRET unconditionally enables NMI, but I can't find any clue in the
>> FRED's manual.
>>
>> In the pseudocode of ERETU in the manual, it seems that NMI is only enabled when
>> ERETU succeeds with bit28 in csx set. If so, this code will fail to reenable
>> NMI if bit28 is not explicitly re-set in csx.
>
>IRET clearing NMI blocking is the source of an immense amount of grief,
>and ultimately the reason why Linux and others can't use supervisor
>shadow stacks at the moment.
>
>Changing this property, so NMIs only get unblocked on successful
>execution of an ERET{S,U}, was a key demand of the FRED spec.
>
>i.e. until you have successfully ERET*'d, you're still logically in the
>NMI handler and NMIs need to remain blocked even when handling the #GP
>from a bad ERET.
>
>~Andrew

This is correct.