Re: [PATCH v2] KVM: arm64: vgic-v3: take an LPI reference in vgic_v3_save_pending_tables

[Marc Zyngier <maz@kernel.org>]

On Fri, 07 Aug 2026 03:55:34 +0100,
Qihang <q.h.hack.winter@gmail.com> wrote:
> 
> vgic_v3_save_pending_tables() iterates dist->lpi_xa using xa_for_each()
> and dereferences the returned struct vgic_irq in the loop body without
> holding a reference on the LPI.
> 
> The xarray iterator only provides temporary RCU coverage while looking up
> the current entry. That is not sufficient for this loop body, which reads
> fields from struct vgic_irq and performs guest memory accesses before the
> iteration completes.
> 
> A concurrent path can trigger this race: the irqfd cached injection path
> (vgic_its_inject_cached_translation) obtains a transient LPI reference
> via vgic_its_check_cache() without holding kvm->lock, vcpu->mutex,
> config_lock, or its_lock. If guest ITS DISCARD then drops the cache and
> ITE references under its_lock, the transient inject reference may become
> the final one. When vgic_put_irq() drops it, the LPI is erased from
> lpi_xa and freed via kfree_rcu(). Meanwhile, vgic_v3_save_pending_tables()
> may still hold a stale pointer obtained from the xarray iterator and
> dereference it after the RCU grace period completes.
> 
> Fix this by re-fetching each iterated LPI via vgic_get_irq(), which takes
> a stable reference, and dropping it with vgic_put_irq() on all paths.
> This matches the pattern already used by other lpi_xa iterators in the
> vgic ITS code.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Qihang <q.h.hack.winter@gmail.com>

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.