KVM: arm64: vgic: Mitigate potential LPI registration failure

KVM: arm64: vgic: Mitigate potential LPI registration failure

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. Add __GFP_ACCOUNT as well to match xa_reserve_irq()'s
flags.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: 1d6f83f60f79 ("KVM: arm64: vgic: Store LPIs in an xarray")
Signed-off-by: Carlos López <clopez@suse.de>
Link: https://patch.msgid.link/20260715105137.3973823-5-clopez@suse.de
Signed-off-by: Marc Zyngier <maz@kernel.org>