PCI/bwctrl: Fix NULL pointer deref on unbind and bind

The interrupt handler for bandwidth notifications, pcie_bwnotif_irq(),
dereferences a "data" pointer.

On unbind, that pointer is set to NULL by pcie_bwnotif_remove().  However
the interrupt handler may still be invoked afterwards and will dereference
that NULL pointer.

That's because the interrupt is requested using a devm_*() helper and the
driver core releases devm_*() resources *after* calling ->remove().

pcie_bwnotif_remove() does clear the Link Bandwidth Management Interrupt
Enable and Link Autonomous Bandwidth Interrupt Enable bits in the Link
Control Register, but that won't prevent execution of pcie_bwnotif_irq():
The interrupt for bandwidth notifications may be shared with AER, DPC,
PME, and hotplug.  So pcie_bwnotif_irq() may be executed as long as the
interrupt is requested.

There's a similar race on bind:  pcie_bwnotif_probe() requests the
interrupt when the "data" pointer still points to NULL.  A NULL pointer
deref may thus likewise occur if AER, DPC, PME or hotplug raise an
interrupt in-between the bandwidth controller's call to devm_request_irq()
and assignment of the "data" pointer.

Drop the devm_*() usage and reorder requesting of the interrupt to fix the
issue.

While at it, drop a stray but harmless no_free_ptr() invocation when
assigning the "data" pointer in pcie_bwnotif_probe().

Ilpo points out that the locking on unbind and bind needs to be symmetric,
so move the call to pcie_bwnotif_disable() inside the critical section
protected by pcie_bwctrl_setspeed_rwsem and pcie_bwctrl_lbms_rwsem.

Evert reports a hang on shutdown of an ASUS ROG Strix SCAR 17 G733PYV.
The issue is no longer reproducible with the present commit.

Evert found that attaching a USB-C monitor prevented the hang.  The
machine contains an ASMedia USB 3.2 controller below a hotplug-capable
Root Port.  So one possible explanation is that the controller gets
hot-removed on shutdown unless something is connected.  And the ensuing
hotplug interrupt occurs exactly when the bandwidth controller is
unregistering.  The precise cause could not be determined because the
screen had already turned black when the hang occurred.

Link: https://lore.kernel.org/r/ae2b02c9cfbefff475b6e132b0aa962aaccbd7b2.1736162539.git.lukas@wunner.de
Fixes: 665745f ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
Reported-by: Evert Vorster <[email protected]>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219629
Signed-off-by: Lukas Wunner <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Tested-by: Evert Vorster <[email protected]>
Reviewed-by: Ilpo Järvinen <[email protected]>

Author: l1k

drivers/pci/pcie/bwctrl.c

@@ -303,14 +303,17 @@ static int pcie_bwnotif_probe(struct pcie_device *srv)
 	if (ret)
 		return ret;
 
-	ret = devm_request_irq(&srv->device, srv->irq, pcie_bwnotif_irq,
-			       IRQF_SHARED, "PCIe bwctrl", srv);
-	if (ret)
-		return ret;
-
 	scoped_guard(rwsem_write, &pcie_bwctrl_setspeed_rwsem) {
 		scoped_guard(rwsem_write, &pcie_bwctrl_lbms_rwsem) {
-			port->link_bwctrl = no_free_ptr(data);
+			port->link_bwctrl = data;
+
+			ret = request_irq(srv->irq, pcie_bwnotif_irq,
+					  IRQF_SHARED, "PCIe bwctrl", srv);
+			if (ret) {
+				port->link_bwctrl = NULL;
+				return ret;
+			}
+
 			pcie_bwnotif_enable(srv);
 		}
 	}
@@ -331,11 +334,15 @@ static void pcie_bwnotif_remove(struct pcie_device *srv)
 
 	pcie_cooling_device_unregister(data->cdev);
 
-	pcie_bwnotif_disable(srv->port);
+	scoped_guard(rwsem_write, &pcie_bwctrl_setspeed_rwsem) {
+		scoped_guard(rwsem_write, &pcie_bwctrl_lbms_rwsem) {
+			pcie_bwnotif_disable(srv->port);
+
+			free_irq(srv->irq, srv);
 
-	scoped_guard(rwsem_write, &pcie_bwctrl_setspeed_rwsem)
-		scoped_guard(rwsem_write, &pcie_bwctrl_lbms_rwsem)
 			srv->port->link_bwctrl = NULL;
+		}
+	}
 }
 
 static int pcie_bwnotif_suspend(struct pcie_device *srv)