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 2:03 PM CEST, Andrew Cooper wrote:
> On 11/09/2025 12:53 pm, 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...")
>> Reported-by: Grygorii Strashko <grygorii_strashko@epam.com>
>> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
>> ---
>> Half RFC. Not thoroughly untested. Pipeline is green, but none of this is tested
>> there.
>>
>> v2:
>>   * New patch with the general fix for HVM too. Turns out the correction
>>     logic was buggy after all.
>
> Hmm, this does sound rather more serious.  I have a nagging feeling that
> until recently we always wrote 128 MADT entries.

If so, I don't see where. It used to be 16, waaaaaaaaaaaaaaaaaaaaay back when.
Then it got extended to whatever it needed to be.

I have the nagging feeling that rather opaque "some OSs (cough Windows cough)
don't like more than 16 CPUs was actually this bug in action. Making the DSDTs
with exactly 16 CPUs a particular kind of silly.

>
> So, while this looks like a good fix, I think we might want a second
> Fixes tag.

Happy to add it, but I really don't see anything like that in the git log.

Cheers,
Alejandro