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

[Qihang <q.h.hack.winter@gmail.com>]

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>
---
 arch/arm64/kvm/vgic/vgic-v3.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

v2:
- Add Cc: stable@vger.kernel.org as requested by Marc Zyngier

diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index 9e841e7afd4a..c3a5e2f1d09a 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -605,47 +605,53 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
 	}
 
 	xa_for_each(&dist->lpi_xa, index, irq) {
 		int byte_offset, bit_nr;
 		struct kvm_vcpu *vcpu;
 		gpa_t pendbase, ptr;
 		bool is_pending;
 		bool stored;
 
+		irq = vgic_get_irq(kvm, index);
+		if (!irq)
+			continue;
+
 		vcpu = irq->target_vcpu;
 		if (!vcpu)
-			continue;
+			goto put_irq;
 
 		pendbase = GICR_PENDBASER_ADDRESS(vcpu->arch.vgic_cpu.pendbaser);
 
 		byte_offset = irq->intid / BITS_PER_BYTE;
 		bit_nr = irq->intid % BITS_PER_BYTE;
 		ptr = pendbase + byte_offset;
 
 		if (ptr != last_ptr) {
 			ret = kvm_read_guest_lock(kvm, ptr, &val, 1);
 			if (ret)
-				goto out;
+				goto put_irq;
 			last_ptr = ptr;
 		}
 
 		stored = val & (1U << bit_nr);
 
 		is_pending = irq->pending_latch;
 
 		if (irq->hw && vlpi_avail)
 			vgic_v4_get_vlpi_state(irq, &is_pending);
 
 		if (stored == is_pending)
-			continue;
+			goto put_irq;
 
 		if (is_pending)
 			val |= 1 << bit_nr;
 		else
 			val &= ~(1 << bit_nr);
 
 		ret = vgic_write_guest_lock(kvm, ptr, &val, 1);
+put_irq:
+		vgic_put_irq(kvm, irq);
 		if (ret)
 			goto out;
 	}
 
 out:
-- 
2.50.1 (Apple Git-155)