Re: [PATCH v2 0/2] KVM: arm64: Fix spurious warn, null ptr deref on S2 teardown race

[Marc Zyngier]

On Sat, 22 Aug 2026 18:46:52 +0100,
"Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx> wrote:
> 
> When GFNs are invalidated in L0 an MMU notifier triggers
> kvm_unmap_gfn_range() which tears down all of the stage 2 shadow page
> tables for nested guests via kvm_nested_s2_unmap().
> 
> To avoid lockup, the kvm->mmu_lock is dropped while doing this and the task
> rescheduled once for each block of physical address space (32 MiB for 16
> KiB page size), with the lock being reacquired once the task is scheduled
> again.
> 
> This results in a potential race between this L0 tear down and tear down of
> the guest itself in kvm_flush_shadow_all(), a race which has been observed
> on real hardware.
> 
> When this race occurs it causes an invalid kernel warning when the PGT of a
> nested MMU is cleared by kvm_flush_shadow_all() ->
> kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd().
> 
> Patch 1 fixes this by having stage2_apply_range() no longer return an error
> when it has experienced a benign race with pgt teardown when it drops the
> lock.
> 
> Patch 2 addresses something more serious - bad timing can turn this spurious
> warning into a NULL pointer dereference.
> 
> kvm_arch_flush_shadow_all() calls kvm_uninit_stage2_mmu() which calls
> kvm_free_stage2_pgd() on the canonical kvm->arch.mmu for that guest's S2
> mappings, making it NULL.
> 
> This is problematic if it happens before stage2_apply_range() reacquires
> the kvm->mmu_lock, as it ultimately returns to kvm_nested_s2_unmap() which
> dereferences kvm->arch.mmu.pgt with the mmu lock held under the incorrect
> assumption that it means it's valid, resulting in a NULL pointer
> dereference.
> 
> Fix that by checking if kvm->arch.mmu.pgt is NULL before dereferencing it
> in kvm_nested_s2_unmap() and kvm_nested_s2_wp().

With the commit message for patch #1 trimmed to something that fits on
a couple of standard terminal screens ;-) :

Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>

	M.

-- 
Jazz isn't dead. It just smells funny.