[PATCH v4 1/9] gpio: aggregator: protect driver attr handlers against module unload

[Koichiro Den <koichiro.den@xxxxxxxxxxxxx>]

Both new_device_store and delete_device_store touch module global
resources (e.g. gpio_aggregator_lock). To prevent race conditions with
module unload, a reference needs to be held.

Add try_module_get() in these handlers.

For new_device_store, this eliminates what appears to be the most dangerous
scenario: if an id is allocated from gpio_aggregator_idr but
platform_device_register has not yet been called or completed, a concurrent
module unload could fail to unregister/delete the device, leaving behind a
dangling platform device/GPIO forwarder. This can result in various issues.
The following simple reproducer demonstrates these problems:

  #!/bin/bash
  while :; do
    # note: whether 'gpiochip0 0' exists or not does not matter.
    echo 'gpiochip0 0' > /sys/bus/platform/drivers/gpio-aggregator/new_device
  done &
  while :; do
    modprobe gpio-aggregator
    modprobe -r gpio-aggregator
  done &
  wait

  Starting with the following warning, several kinds of warnings will appear
  and the system may become unstable:

  ------------[ cut here ]------------
  list_del corruption, ffff888103e2e980->next is LIST_POISON1 (dead000000000100)
  WARNING: CPU: 1 PID: 1327 at lib/list_debug.c:56 __list_del_entry_valid_or_report+0xa3/0x120
  [...]
  RIP: 0010:__list_del_entry_valid_or_report+0xa3/0x120
  [...]
  Call Trace:
   <TASK>
   ? __list_del_entry_valid_or_report+0xa3/0x120
   ? __warn.cold+0x93/0xf2
   ? __list_del_entry_valid_or_report+0xa3/0x120
   ? report_bug+0xe6/0x170
   ? __irq_work_queue_local+0x39/0xe0
   ? handle_bug+0x58/0x90
   ? exc_invalid_op+0x13/0x60
   ? asm_exc_invalid_op+0x16/0x20
   ? __list_del_entry_valid_or_report+0xa3/0x120
   gpiod_remove_lookup_table+0x22/0x60
   new_device_store+0x315/0x350 [gpio_aggregator]
   kernfs_fop_write_iter+0x137/0x1f0
   vfs_write+0x262/0x430
   ksys_write+0x60/0xd0
   do_syscall_64+0x6c/0x180
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
   [...]
   </TASK>
  ---[ end trace 0000000000000000 ]---

Fixes: 828546e24280 ("gpio: Add GPIO Aggregator")
Signed-off-by: Koichiro Den <koichiro.den@xxxxxxxxxxxxx>
---
 drivers/gpio/gpio-aggregator.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpio-aggregator.c b/drivers/gpio/gpio-aggregator.c
index 65f41cc3eafc..d668ddb2e81d 100644
--- a/drivers/gpio/gpio-aggregator.c
+++ b/drivers/gpio/gpio-aggregator.c
@@ -119,10 +119,15 @@ static ssize_t new_device_store(struct device_driver *driver, const char *buf,
 	struct platform_device *pdev;
 	int res, id;
 
+	if (!try_module_get(THIS_MODULE))
+		return -ENOENT;
+
 	/* kernfs guarantees string termination, so count + 1 is safe */
 	aggr = kzalloc(sizeof(*aggr) + count + 1, GFP_KERNEL);
-	if (!aggr)
-		return -ENOMEM;
+	if (!aggr) {
+		res = -ENOMEM;
+		goto put_module;
+	}
 
 	memcpy(aggr->args, buf, count + 1);
 
@@ -161,6 +166,7 @@ static ssize_t new_device_store(struct device_driver *driver, const char *buf,
 	}
 
 	aggr->pdev = pdev;
+	module_put(THIS_MODULE);
 	return count;
 
 remove_table:
@@ -175,6 +181,8 @@ static ssize_t new_device_store(struct device_driver *driver, const char *buf,
 	kfree(aggr->lookups);
 free_ga:
 	kfree(aggr);
+put_module:
+	module_put(THIS_MODULE);
 	return res;
 }
 
@@ -203,13 +211,19 @@ static ssize_t delete_device_store(struct device_driver *driver,
 	if (error)
 		return error;
 
+	if (!try_module_get(THIS_MODULE))
+		return -ENOENT;
+
 	mutex_lock(&gpio_aggregator_lock);
 	aggr = idr_remove(&gpio_aggregator_idr, id);
 	mutex_unlock(&gpio_aggregator_lock);
-	if (!aggr)
+	if (!aggr) {
+		module_put(THIS_MODULE);
 		return -ENOENT;
+	}
 
 	gpio_aggregator_free(aggr);
+	module_put(THIS_MODULE);
 	return count;
 }
 static DRIVER_ATTR_WO(delete_device);
-- 
2.45.2