spi: Fix use-after-free with devm_spi_alloc_*

We can't rely on the contents of the devres list during
spi_unregister_controller(), as the list is already torn down at the
time we perform devres_find() for devm_spi_release_controller. This
causes devices registered with devm_spi_alloc_{master,slave}() to be
mistakenly identified as legacy, non-devm managed devices and have their
reference counters decremented below 0.

------------[ cut here ]------------
WARNING: CPU: 1 PID: 660 at lib/refcount.c:28 refcount_warn_saturate+0x108/0x174
[<b0396f04>] (refcount_warn_saturate) from [<b03c56a4>] (kobject_put+0x90/0x98)
[<b03c5614>] (kobject_put) from [<b0447b4c>] (put_device+0x20/0x24)
 r4:b6700140
[<b0447b2c>] (put_device) from [<b07515e8>] (devm_spi_release_controller+0x3c/0x40)
[<b07515ac>] (devm_spi_release_controller) from [<b045343c>] (release_nodes+0x84/0xc4)
 r5:b6700180 r4:b6700100
[<b04533b8>] (release_nodes) from [<b0454160>] (devres_release_all+0x5c/0x60)
 r8:b1638c54 r7:b117ad94 r6:b1638c10 r5:b117ad94 r4:b163dc10
[<b0454104>] (devres_release_all) from [<b044e41c>] (__device_release_driver+0x144/0x1ec)
 r5:b117ad94 r4:b163dc10
[<b044e2d8>] (__device_release_driver) from [<b044f70c>] (device_driver_detach+0x84/0xa0)
 r9:00000000 r8:00000000 r7:b117ad94 r6:b163dc54 r5:b1638c10 r4:b163dc10
[<b044f688>] (device_driver_detach) from [<b044d274>] (unbind_store+0xe4/0xf8)

Instead, determine the devm allocation state as a flag on the
controller which is guaranteed to be stable during cleanup.

Fixes: 5e844cc3 ("spi: Introduce device-managed SPI controller allocation")
Signed-off-by: William A. Kennington III <wak@google.com>
Link: https://lore.kernel.org/r/20210407095527.2771582-1-wak@google.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2496,6 +2496,7 @@ struct spi_controller *__devm_spi_alloc_controller(struct device *dev,
 
 	ctlr = __spi_alloc_controller(dev, size, slave);
 	if (ctlr) {
+		ctlr->devm_allocated = true;
 		*ptr = ctlr;
 		devres_add(dev, ptr);
 	} else {
@@ -2842,11 +2843,6 @@ int devm_spi_register_controller(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_spi_register_controller);
 
-static int devm_spi_match_controller(struct device *dev, void *res, void *ctlr)
-{
-	return *(struct spi_controller **)res == ctlr;
-}
-
 static int __unregister(struct device *dev, void *null)
 {
 	spi_unregister_device(to_spi_device(dev));
@@ -2893,8 +2889,7 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 	/* Release the last reference on the controller if its driver
 	 * has not yet been converted to devm_spi_alloc_master/slave().
 	 */
-	if (!devres_find(ctlr->dev.parent, devm_spi_release_controller,
-			 devm_spi_match_controller, ctlr))
+	if (!ctlr->devm_allocated)
 		put_device(&ctlr->dev);
 
 	/* free bus id */

include/linux/spi/spi.h

@@ -510,6 +510,9 @@ struct spi_controller {
 
 #define SPI_MASTER_GPIO_SS		BIT(5)	/* GPIO CS must select slave */
 
+	/* flag indicating this is a non-devres managed controller */
+	bool			devm_allocated;
+
 	/* flag indicating this is an SPI slave controller */
 	bool			slave;