[PATCH 5.15.y] KVM: arm64: vgic-v3: Undo assignment on iodev registration failure

[Sasha Levin <sashal@kernel.org>]

From: Karl Mehltretter <kmehltretter@gmail.com>

[ Upstream commit 8a88a46b86ca3dbbd24522760d29edfb3f24ac2a ]

vgic_register_redist_iodev() assigns a redistributor region and base
address to the vCPU before adding its iodev to the MMIO bus. However, the
region's free_index is advanced only after registration succeeds.

If kvm_io_bus_register_dev() fails, the vCPU retains the assignment while
free_index still identifies the same slot as free. A later registration can
therefore reuse a slot that remains assigned to the vCPU.

Reserve the slot before registering its iodev. The caller holds slots_lock,
so a registration failure cannot race with a later assignment. Undo the
reservation and clear the cached assignment on failure.

Fixes: dbd9733ab674 ("KVM: arm/arm64: Replace the single rdist region by a list")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260831081005.41346-2-kmehltretter@gmail.com
[maz: moved helper into the sole caller]
Signed-off-by: Marc Zyngier <maz@kernel.org>
[ sashal: adapted locking to rely on the caller-held kvm->lock instead of config_lock and guard(mutex). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/arm64/kvm/vgic/vgic-mmio-v3.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index fd1fa2561520e..c3c0204b3eb1e 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -739,16 +739,21 @@ int vgic_register_redist_iodev(struct kvm_vcpu *vcpu)
 	rd_dev->nr_regions = ARRAY_SIZE(vgic_v3_rd_registers);
 	rd_dev->redist_vcpu = vcpu;
 
+	/* Protected by kvm->lock */
+	rdreg->free_index++;
+
 	mutex_lock(&kvm->slots_lock);
 	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, rd_base,
 				      2 * SZ_64K, &rd_dev->dev);
 	mutex_unlock(&kvm->slots_lock);
 
-	if (ret)
-		return ret;
+	if (ret) {
+		vgic_cpu->rdreg->free_index--;
+		vgic_cpu->rdreg = NULL;
+		vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
+	}
 
-	rdreg->free_index++;
-	return 0;
+	return ret;
 }
 
 void vgic_unregister_redist_iodev(struct kvm_vcpu *vcpu)
-- 
2.53.0