[PATCH] ata: pata_parport: Fix use-after-free in new_device_store

[Pei Xiao]

The function new_device_store() calls driver_find() without any
protection against concurrent driver unregistration. This can lead
to a use-after-free (UAF) when a driver is unloaded (via rmmod)
in parallel with a new device addition via sysfs.

The race window exists because driver_find() returns a pointer to
the driver's private data, but does not increase its reference
count. The caller is responsible for ensuring the driver remains
valid, but new_device_store() did not hold any lock or reference
during the lookup and subsequent use.

Concurrently, pata_parport_unregister_driver() releases the
pi_mutex before calling driver_unregister(), allowing new_device_store
to proceed with a stale pointer after the driver has been freed.

Fix this by expanding the critical section protected by pi_mutex
in both functions:

- In new_device_store(): acquire pi_mutex before calling driver_find(),
  and keep it held until all uses of the found driver pointer are
  completed.
- In pata_parport_unregister_driver(): hold pi_mutex throughout the
  entire unregistration process, including the driver_unregister()
  call, so that no concurrent lookup can see a partially removed
  driver.

Thread A (new_device_store)       | Thread B (pata_parport_unregister_driver)
  driver_find("aten")             |
                                  |   mutex_lock(&pi_mutex)
                                  |   idr_remove(&protocols, id)
                                  |   mutex_unlock(&pi_mutex)
                                  |   driver_unregister(&pr->driver)
                                  |   (frees driver_private)
  /* continues using stale driv */|
  -> UAF!                         |

Logs:
BUG: KASAN: slab-use-after-free in driver_find+0xd0/0xd4
Read of size 8 at addr ffffff9f970dc690 by task sh/4737
Call trace:
 show_stack+0x14/0x1c (C)
 dump_stack_lvl+0x70/0x84
 print_report+0xf4/0x5a4
 kasan_report+0xa0/0xe4
 __asan_report_load8_noabort+0x18/0x20
 driver_find+0xd0/0xd4
 new_device_store+0x140/0x30c [pata_parport]
 bus_attr_store+0x5c/0x94
 sysfs_kf_write+0x1c4/0x25c
 ...

Allocated by task 4734:
 kasan_save_stack+0x28/0x4c
 kasan_save_track+0x1c/0x34
 kasan_save_alloc_info+0x3c/0x4c
 __kasan_kmalloc+0x98/0xac
 __kmalloc_cache_noprof+0x158/0x3cc
 bus_add_driver+0x70/0x4d8
 driver_register+0xf0/0x3b0
 pata_parport_register_driver+0xb8/0x1c4 [pata_parport]
 0xffffffc07f5ce014
 do_one_initcall+0xb8/0x36c
 do_init_module+0x230/0x6d4
 ...

Freed by task 4737:
 kasan_save_stack+0x28/0x4c
 kasan_save_track+0x1c/0x34
 kasan_save_free_info+0x48/0x8c
 __kasan_slab_free+0x5c/0x84
 kfree+0x174/0x3e0
 driver_release+0x1c/0x7c
 kobject_put+0x178/0x47c
 driver_find+0x78/0xd4
 new_device_store+0x140/0x30c [pata_parport]
 bus_attr_store+0x5c/0x94
 sysfs_kf_write+0x1c4/0x25c
 ...

Fixes: 72f2b0b21850 ("drivers/block: Move PARIDE protocol modules to drivers/ata/pata_parport")
Reported-by: Shuangpeng Bai <shuangpeng.kernel@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/20260728024015.2014674-1-shuangpeng.kernel@xxxxxxxxx/
Signed-off-by: Pei Xiao <xiaopei01@xxxxxxxxxx>
---
 drivers/ata/pata_parport/pata_parport.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/ata/pata_parport/pata_parport.c b/drivers/ata/pata_parport/pata_parport.c
index 40baeac594a9..34f09192a419 100644
--- a/drivers/ata/pata_parport/pata_parport.c
+++ b/drivers/ata/pata_parport/pata_parport.c
@@ -3,6 +3,7 @@
  * Copyright 2023 Ondrej Zary
  * based on paride.c by Grant R. Guenther <grant@xxxxxxxxxx>
  */
+#include <linux/cleanup.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/parport.h>
@@ -618,8 +619,9 @@ void pata_parport_unregister_driver(struct pi_protocol *pr)
 			break;
 	}
 	idr_remove(&protocols, id);
-	mutex_unlock(&pi_mutex);
 	driver_unregister(&pr->driver);
+	mutex_unlock(&pi_mutex);
+
 }
 EXPORT_SYMBOL_GPL(pata_parport_unregister_driver);
 
@@ -646,6 +648,7 @@ static ssize_t new_device_store(const struct bus_type *bus, const char *buf, siz
 		port_wanted = -1;
 	}
 
+	guard(mutex)(&pi_mutex);
 	drv = driver_find(protocol, &pata_parport_bus_type);
 	if (!drv) {
 		if (strcmp(protocol, "auto")) {
@@ -656,15 +659,12 @@ static ssize_t new_device_store(const struct bus_type *bus, const char *buf, siz
 	} else {
 		pr_wanted = container_of(drv, struct pi_protocol, driver);
 	}
-
-	mutex_lock(&pi_mutex);
 	/* walk all parports */
 	idr_for_each_entry(&parport_list, parport, port_num) {
 		if (port_num == port_wanted || port_wanted == -1) {
 			parport = parport_find_number(port_num);
 			if (!parport) {
 				pr_err("no such port %s\n", port);
-				mutex_unlock(&pi_mutex);
 				return -ENODEV;
 			}
 			/* walk all protocols */
@@ -677,7 +677,6 @@ static ssize_t new_device_store(const struct bus_type *bus, const char *buf, siz
 			parport_put_port(parport);
 		}
 	}
-	mutex_unlock(&pi_mutex);
 	if (!ok)
 		return -ENODEV;
 
-- 
2.25.1