[PATCH v3 2/2] KVM: arm64: vgic: Mitigate potential LPI registration failure

[Carlos López]

Mitigate a potential failure when inserting a new LPI into the VGIC LPI
xarray.

When vgic_add_lpi() is preparing to register a new LPI, it pre-allocates
an xarray entry using xa_reserve_irq(), so that it can later perform the
insertion under the xarray lock without allocating.

However, since xa_reserve_irq() is called before acquiring such lock,
there is a potential race where xa_reserve_irq() observes a populated
entry, thus not performing the allocation, and another CPU removes that
entry before the xarray lock is grabbed to perform the insertion.

  CPU0 (Adding new LPI)                      CPU1 (Releasing LPI)
  =====================                      ===================
  vgic_add_lpi()
      /* Entry populated, does not allocate */
      xa_reserve_irq(.., intid, ..)
                                            vgic_release_deleted_lpis()
                                                xa_lock_irqsave()
                                                vgic_release_lpi_locked()
                          xarray node freed --> __xa_erase(.., intid)
                                                xa_unlock_irqrestore()
      xa_lock_irqsave()
      xa_load(.., intid) == NULL
      vgic_try_get_irq_ref(NULL) == false
      __xa_store(.., intid, irq, 0) <-- xarray node was freed, gfp=0
                                        cannot allocate, returns -ENOMEM

This can happen e.g. if the guest issues a DISCARD while the LPI is
still referenced from a vCPU's active-pending list (ap_list), and the
same INTID is re-mapped via MAPTI.

Mitigate this by passing GFP_NOWAIT to __xa_store(), so that the
allocation can happen under the lock in the rare case that this
condition is hit.

Reported-by: Sashiko <sashiko-bot at kernel.org>
Fixes: 1d6f83f60f79 ("KVM: arm64: vgic: Store LPIs in an xarray")
Signed-off-by: Carlos López <clopez at suse.de>
---
 arch/arm64/kvm/vgic/vgic-its.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 577286069368..ace3e59fff97 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -121,7 +121,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 		 * from the deferred release path, pending cleanup by
 		 * vgic_release_deleted_lpis(). Evict and free it if present.
 		 */
-		oldirq = __xa_store(&dist->lpi_xa, intid, irq, 0);
+		oldirq = __xa_store(&dist->lpi_xa, intid, irq, GFP_NOWAIT);
 		ret = xa_err(oldirq);
 		if (!ret && oldirq)
 			kfree_rcu(oldirq, rcu);
-- 
2.51.0