Re: [PATCH v2 1/2] libacpi: Prevent CPU hotplug AML from corrupting memory

[Alejandro Vallejo <alejandro.garciavallejo@amd.com>]

On Thu Sep 11, 2025 at 4:52 PM CEST, Jan Beulich wrote:
> On 11.09.2025 13:53, Alejandro Vallejo wrote:
>> CPU hotplug relies on the online CPU bitmap being provided on PIO 0xaf00
>> by the device model. The GPE handler checks this and compares it against
>> the "online" flag on each MADT LAPIC entry, setting the flag to its
>> related bit in the bitmap and adjusting the table's checksum.
>> 
>> The bytecode doesn't, however, stop at NCPUS. It keeps comparing until it
>> reaches 128, even if that overflows the MADT into some other (hopefully
>> mapped) memory. The reading isn't as problematic as the writing though.
>> 
>> If an "entry" outside the MADT is deemed to disagree with the CPU bitmap
>> then the bit where the "online" flag would be is flipped, thus
>> corrupting that memory. And the MADT checksum gets adjusted for a flip
>> that happened outside its range. It's all terrible.
>> 
>> Note that this corruption happens regardless of the device-model being
>> present or not, because even if the bitmap holds 0s, the overflowed
>> memory might not at the bits corresponding to the "online" flag.
>> 
>> This patch adjusts the DSDT so entries >=NCPUS are skipped.
>> 
>> Fixes: c70ad37a1f7c("HVM vcpu add/remove: setup dsdt infrastructure...")
>
> The code in question originates from e5dc62c4d4f1 ("hvmloader: Fix CPU
> hotplug notify handler in ACPI DSDT"), though. Before that there was a
> different issue (as mentioned in the description).

As you mentioned elsewhere, it probably is 087543338924("hvmloader: limit CPUs
exposed to guests") that matters. Until then the DSDT was correct.

>
>> --- a/tools/libacpi/mk_dsdt.c
>> +++ b/tools/libacpi/mk_dsdt.c
>> @@ -239,7 +239,8 @@ int main(int argc, char **argv)
>>          /* Extract current CPU's status: 0=offline; 1=online. */
>>          stmt("And", "Local1, 1, Local2");
>>          /* Check if status is up-to-date in the relevant MADT LAPIC entry... */
>> -        push_block("If", "LNotEqual(Local2, \\_SB.PR%02X.FLG)", cpu);
>> +        push_block("If", "And(LLess(%d, NCPU), LNotEqual(Local2, \\_SB.PR%02X.FLG))",
>> +                   cpu, cpu);
>
> Don't we need to use \\_SB.NCPU here? From the other two uses it's not
> quite clear; it might also be that the one using this form is actually
> needlessly doing so. Yet here it may be better if only for consistency's
> sake, as the LNotEqual() also operates on an absolute reference.

\SB.PMAT method does the same thing. I'll just change that too while at it.

> The other thing is that I'm not fluent in AML operand evaluation rules.
> We want to avoid even the read access to FLG, and I'm unconvinced And()
> will avoid evaluating its 2nd argument when the first one is 0. IOW this
> may need to become a 2nd "If".

I don't think there are any rules, it's unspecified. While in practice it
wouldn't matter a lot, it's indeed better not to rely on it not blowing up.

After sending this, I wondered about having a separate if with an early return.

>
> I further think that strictly speaking you mean LAnd() here, not And()
> (but the above concern remains; all the ASL spec says is "Source1 and
> Source2 are evaluated as integers" for both And() and LAnd()).

I very definitely did mean LAnd! Nice catch. As for 

>
> Jan

TL;DR: Will s/And/LAnd/ and move it to a separate If

Cheers,
Alejandro