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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.09.2025 18:23, 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: 087543338924("hvmloader: limit CPUs exposed to guests")
> Reported-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>

In principle:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

However, ...

> --- a/tools/libacpi/mk_dsdt.c
> +++ b/tools/libacpi/mk_dsdt.c
> @@ -231,6 +231,20 @@ int main(int argc, char **argv)
>      stmt("Store", "ToBuffer(PRS), Local0");
>      for ( cpu = 0; cpu < max_cpus; cpu++ )
>      {
> +        if ( cpu )
> +        {
> +            /*
> +             * Check if we're still within the MADT bounds
> +             *
> +             * LLess() takes one byte, but LLessEqual() takes two. Increase
> +             * `cpu` by 1, so we can avoid it. It does add up once you do it
> +             * 127 times!
> +             */
> +            push_block("If", "LLess(\\_SB.NCPU, %d)", 1 + cpu);
> +            stmt("Return", "One");

... if you already care about size bloat in the conditional, why are the two
bytes per instance that this extra return requires not relevant? They too
add up, and they can be avoided by wrapping the If around the rest of the
code. I didn't count it, but I expect the If encoding to grow by at most one
byte, perhaps none at all.

Jan

> +            pop_block();
> +        }
> +
>          /* Read a byte at a time from the PRST online-CPU bitmask. */
>          if ( (cpu & 7) == 0 )
>              stmt("Store", "DerefOf(Index(Local0, %u)), Local1", cpu/8);