[PATCH 1/3] KVM: x86: Fix out-of-bounds access in kvm_recalculate_phys_map()

Michal Luczaj
2023-05-25T18:36:20Z

Handle the case of vCPU addition and/or APIC enabling during the APIC map
recalculations. Check the sanity of x2APIC ID in !x2apic_format &&
apic_x2apic_mode() case.

kvm_recalculate_apic_map() creates the APIC map iterating over the list of
vCPUs twice. First to find the max APIC ID and allocate a max-sized buffer,
then again, calling kvm_recalculate_phys_map() for each vCPU. This opens a
race window: value of max APIC ID can increase _after_ the buffer was
allocated.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
arch/x86/kvm/lapic.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index e542cf285b51..39b9a318d04c 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -265,10 +265,14 @@ static int kvm_recalculate_phys_map(struct kvm_apic_map *new,
* mapped, i.e. is aliased to multiple vCPUs. The optimized
* map requires a strict 1:1 mapping between IDs and vCPUs.
*/
- if (apic_x2apic_mode(apic))
+ if (apic_x2apic_mode(apic)) {
+ if (x2apic_id > new->max_apic_id)
+ return -EINVAL;
+
physical_id = x2apic_id;
- else
+ } else {
physical_id = xapic_id;
+ }

if (new->phys_map[physical_id])
return -EINVAL;
--
2.40.1