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

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

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>
---
v2:
  * Added early returns in AML rather than dubious And() statements
---
 tools/libacpi/mk_dsdt.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/libacpi/mk_dsdt.c b/tools/libacpi/mk_dsdt.c
index 8ac4f9d0b4..aeeb71dfe6 100644
--- 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");
+            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);
-- 
2.43.0