PCI: endpoint: pci-ep-msi: Fix error unwind and prevent double alloc

[ Upstream commit 1cba96c ]

pci_epf_alloc_doorbell() stores the allocated doorbell message array in
epf->db_msg/epf->num_db before requesting MSI vectors. If MSI allocation
fails, the array is freed but the EPF state may still point to freed
memory.

Clear epf->db_msg and epf->num_db on the MSI allocation failure path so
that later cleanup cannot double-free the array and callers can retry
allocation.

Also return -EBUSY when doorbells have already been allocated to prevent
leaking or overwriting an existing allocation.

Fixes: 1c3b002 ("PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller")
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260217063856.3759713-4-den@valinux.co.jp
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Koichiro Den

drivers/pci/endpoint/pci-ep-msi.c

@@ -50,6 +50,9 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
 		return -EINVAL;
 	}
 
+	if (epf->db_msg)
+		return -EBUSY;
+
 	domain = of_msi_map_get_device_domain(epc->dev.parent, 0,
 					      DOMAIN_BUS_PLATFORM_MSI);
 	if (!domain) {
@@ -79,6 +82,8 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
 	if (ret) {
 		dev_err(dev, "Failed to allocate MSI\n");
 		kfree(msg);
+		epf->db_msg = NULL;
+		epf->num_db = 0;
 		return ret;
 	}