[PATCH v2] KVM: Move kvm_io_bus_get_dev() locking responsibilities to callers

[Marc Zyngier <maz@kernel.org>]

kvm_io_bus_get_dev() returns a device that is only matched by the
address, and nothing else. This can cause a lifetime issue if
the matched device is not the expected type, as by the time
the caller can introspect the object, it might be gone (the srcu
lock having been dropped).

Given that there is only a single user of this helper, the simplest
option is to move the locking responsibility to the caller, which
can keep the srcu lock held for as long as it wants.

Note that this aligns with other kvm_io_bus*() helpers, which
already require the srcu lock to be held by the callers.

Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Fixes: 8a39d00670f07 ("KVM: kvm_io_bus: Add kvm_io_bus_get_dev() call")
Link: https://lore.kernel.org/all/20260626111344.802555-1-maz@kernel.org
Cc: stable@vger.kernel.org
---

Notes:
    v2: Drop the previous filtering approach, and move the locking into
        the only caller, similar to kvm_io_bus_{read,write}().

 arch/arm64/kvm/vgic/vgic-its.c |  2 ++
 virt/kvm/kvm_main.c            | 16 +++++-----------
 2 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 4477f870c7b36..740b39875728d 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -508,6 +508,8 @@ static struct vgic_its *__vgic_doorbell_to_its(struct kvm *kvm, gpa_t db)
 	struct kvm_io_device *kvm_io_dev;
 	struct vgic_io_device *iodev;
 
+	guard(srcu)(&kvm->srcu);
+
 	kvm_io_dev = kvm_io_bus_get_dev(kvm, KVM_MMIO_BUS, db);
 	if (!kvm_io_dev)
 		return ERR_PTR(-EINVAL);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 881f92d7a469e..1a529098eec98 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -6069,25 +6069,19 @@ struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 					 gpa_t addr)
 {
 	struct kvm_io_bus *bus;
-	int dev_idx, srcu_idx;
-	struct kvm_io_device *iodev = NULL;
+	int dev_idx;
 
-	srcu_idx = srcu_read_lock(&kvm->srcu);
+	lockdep_assert_held(&kvm->srcu);
 
 	bus = kvm_get_bus_srcu(kvm, bus_idx);
 	if (!bus)
-		goto out_unlock;
+		return NULL;
 
 	dev_idx = kvm_io_bus_get_first_dev(bus, addr, 1);
 	if (dev_idx < 0)
-		goto out_unlock;
-
-	iodev = bus->range[dev_idx].dev;
-
-out_unlock:
-	srcu_read_unlock(&kvm->srcu, srcu_idx);
+		return NULL;
 
-	return iodev;
+	return bus->range[dev_idx].dev;
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_io_bus_get_dev);
 
-- 
2.47.3