[syzbot] [media?] KASAN: slab-use-after-free Read in media_devnode_unregister

[syzbot] [media?] KASAN: slab-use-after-free Read in media_devnode_unregister

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    65dd046ef558 Add linux-next specific files for 20250909
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17ca4b12580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d3b38340153b83d4
dashboard link: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f73562580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14212642580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/442c9b800ac9/disk-65dd046e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ca7d60439397/vmlinux-65dd046e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9d386003fad0/bzImage-65dd046e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com

em28xx 1-1:0.132: Closing input extension
==================================================================
BUG: KASAN: slab-use-after-free in media_devnode_unregister+0xe2/0xf0 drivers/media/mc/mc-devnode.c:284
Read of size 4 at addr ffff888077b7f4f0 by task kworker/0:3/5945

CPU: 0 UID: 0 PID: 5945 Comm: kworker/0:3 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue: usb_hub_wq hub_event
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xca/0x240 mm/kasan/report.c:482
 kasan_report+0x118/0x150 mm/kasan/report.c:595
 media_devnode_unregister+0xe2/0xf0 drivers/media/mc/mc-devnode.c:284
 media_device_unregister+0x37c/0x400 drivers/media/mc/mc-device.c:828
 em28xx_unregister_media_device drivers/media/usb/em28xx/em28xx-cards.c:3511 [inline]
 em28xx_release_resources+0xac/0x240 drivers/media/usb/em28xx/em28xx-cards.c:3532
 em28xx_usb_disconnect+0x19f/0x2f0 drivers/media/usb/em28xx/em28xx-cards.c:4201
 usb_unbind_interface+0x26e/0x910 drivers/usb/core/driver.c:458
 device_remove drivers/base/dd.c:571 [inline]
 __device_release_driver drivers/base/dd.c:1274 [inline]
 device_release_driver_internal+0x4d6/0x800 drivers/base/dd.c:1297
 bus_remove_device+0x34d/0x410 drivers/base/bus.c:579
 device_del+0x511/0x8e0 drivers/base/core.c:3878
 usb_disable_device+0x3e9/0x8a0 drivers/usb/core/message.c:1418
 usb_disconnect+0x330/0x950 drivers/usb/core/hub.c:2344
 hub_port_connect drivers/usb/core/hub.c:5406 [inline]
 hub_port_connect_change drivers/usb/core/hub.c:5706 [inline]
 port_event drivers/usb/core/hub.c:5870 [inline]
 hub_event+0x1cf5/0x4a20 drivers/usb/core/hub.c:5952
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x47c/0x820 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

Allocated by task 5945:
 kasan_save_stack mm/kasan/common.c:56 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
 poison_kmalloc_redzone mm/kasan/common.c:397 [inline]
 __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:414
 kasan_kmalloc include/linux/kasan.h:260 [inline]
 __kmalloc_cache_noprof+0x3d5/0x6f0 mm/slub.c:5397
 kmalloc_noprof include/linux/slab.h:956 [inline]
 kzalloc_noprof include/linux/slab.h:1090 [inline]
 __media_device_register+0x58/0x280 drivers/media/mc/mc-device.c:720
 em28xx_usb_probe+0x1764/0x2a20 drivers/media/usb/em28xx/em28xx-cards.c:4153
 usb_probe_interface+0x665/0xc30 drivers/usb/core/driver.c:396
 call_driver_probe drivers/base/dd.c:-1 [inline]
 really_probe+0x26a/0x9e0 drivers/base/dd.c:659
 __driver_probe_device+0x18c/0x2f0 drivers/base/dd.c:801
 driver_probe_device+0x4f/0x430 drivers/base/dd.c:831
 __device_attach_driver+0x2ce/0x530 drivers/base/dd.c:959
 bus_for_each_drv+0x251/0x2e0 drivers/base/bus.c:462
 __device_attach+0x2b8/0x400 drivers/base/dd.c:1031
 bus_probe_device+0x185/0x260 drivers/base/bus.c:537
 device_add+0x7b6/0xb50 drivers/base/core.c:3689
 usb_set_configuration+0x1a87/0x20e0 drivers/usb/core/message.c:2210
 usb_generic_driver_probe+0x8d/0x150 drivers/usb/core/generic.c:250
 usb_probe_device+0x1c1/0x390 drivers/usb/core/driver.c:291
 call_driver_probe drivers/base/dd.c:-1 [inline]
 really_probe+0x26a/0x9e0 drivers/base/dd.c:659
 __driver_probe_device+0x18c/0x2f0 drivers/base/dd.c:801
 driver_probe_device+0x4f/0x430 drivers/base/dd.c:831
 __device_attach_driver+0x2ce/0x530 drivers/base/dd.c:959
 bus_for_each_drv+0x251/0x2e0 drivers/base/bus.c:462
 __device_attach+0x2b8/0x400 drivers/base/dd.c:1031
 bus_probe_device+0x185/0x260 drivers/base/bus.c:537
 device_add+0x7b6/0xb50 drivers/base/core.c:3689
 usb_new_device+0xa39/0x16f0 drivers/usb/core/hub.c:2694
 hub_port_connect drivers/usb/core/hub.c:5566 [inline]
 hub_port_connect_change drivers/usb/core/hub.c:5706 [inline]
 port_event drivers/usb/core/hub.c:5870 [inline]
 hub_event+0x2958/0x4a20 drivers/usb/core/hub.c:5952
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x47c/0x820 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Freed by task 5945:
 kasan_save_stack mm/kasan/common.c:56 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
 __kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:587
 kasan_save_free_info mm/kasan/kasan.h:406 [inline]
 poison_slab_object mm/kasan/common.c:252 [inline]
 __kasan_slab_free+0x5b/0x80 mm/kasan/common.c:284
 kasan_slab_free include/linux/kasan.h:233 [inline]
 slab_free_hook mm/slub.c:2484 [inline]
 slab_free mm/slub.c:6121 [inline]
 kfree+0x199/0x6d0 mm/slub.c:6329
 media_devnode_release+0x61/0xa0 drivers/media/mc/mc-devnode.c:57
 device_release+0x9c/0x1c0 drivers/base/core.c:-1
 kobject_cleanup lib/kobject.c:689 [inline]
 kobject_release lib/kobject.c:720 [inline]
 kref_put include/linux/kref.h:65 [inline]
 kobject_put+0x228/0x480 lib/kobject.c:737
 media_devnode_unregister+0x6d/0xf0 drivers/media/mc/mc-devnode.c:281
 media_device_unregister+0x37c/0x400 drivers/media/mc/mc-device.c:828
 em28xx_unregister_media_device drivers/media/usb/em28xx/em28xx-cards.c:3511 [inline]
 em28xx_release_resources+0xac/0x240 drivers/media/usb/em28xx/em28xx-cards.c:3532
 em28xx_usb_disconnect+0x19f/0x2f0 drivers/media/usb/em28xx/em28xx-cards.c:4201
 usb_unbind_interface+0x26e/0x910 drivers/usb/core/driver.c:458
 device_remove drivers/base/dd.c:571 [inline]
 __device_release_driver drivers/base/dd.c:1274 [inline]
 device_release_driver_internal+0x4d6/0x800 drivers/base/dd.c:1297
 bus_remove_device+0x34d/0x410 drivers/base/bus.c:579
 device_del+0x511/0x8e0 drivers/base/core.c:3878
 usb_disable_device+0x3e9/0x8a0 drivers/usb/core/message.c:1418
 usb_disconnect+0x330/0x950 drivers/usb/core/hub.c:2344
 hub_port_connect drivers/usb/core/hub.c:5406 [inline]
 hub_port_connect_change drivers/usb/core/hub.c:5706 [inline]
 port_event drivers/usb/core/hub.c:5870 [inline]
 hub_event+0x1cf5/0x4a20 drivers/usb/core/hub.c:5952
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x47c/0x820 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff888077b7f000
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 1264 bytes inside of
 freed 2048-byte region [ffff888077b7f000, ffff888077b7f800)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x77b78
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801a842000 dead000000000122 0000000000000000
raw: 0000000000000000 0000000000080008 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801a842000 dead000000000122 0000000000000000
head: 0000000000000000 0000000000080008 00000000f5000000 0000000000000000
head: 00fff00000000003 ffffea0001dede01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 6020, tgid 6020 (kworker/0:4), ts 107508329196, free_ts 106053601945
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1850
 prep_new_page mm/page_alloc.c:1858 [inline]
 get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3863
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5153
 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
 alloc_slab_page mm/slub.c:2996 [inline]
 allocate_slab+0x8a/0x330 mm/slub.c:3164
 new_slab mm/slub.c:3218 [inline]
 ___slab_alloc+0xbd1/0x13f0 mm/slub.c:4420
 __slab_alloc+0x55/0xa0 mm/slub.c:4511
 __slab_alloc_node mm/slub.c:4586 [inline]
 slab_alloc_node mm/slub.c:4996 [inline]
 __do_kmalloc_node mm/slub.c:5365 [inline]
 __kmalloc_node_track_caller_noprof+0x5c7/0x800 mm/slub.c:5385
 kmalloc_reserve+0x136/0x290 net/core/skbuff.c:600
 __alloc_skb+0x142/0x2d0 net/core/skbuff.c:669
 alloc_skb include/linux/skbuff.h:1377 [inline]
 mld_newpack+0x13c/0xc40 net/ipv6/mcast.c:1775
 add_grhead+0x5a/0x2a0 net/ipv6/mcast.c:1886
 add_grec+0x1452/0x1740 net/ipv6/mcast.c:2025
 mld_send_initial_cr+0x288/0x550 net/ipv6/mcast.c:2268
 mld_dad_work+0x46/0x490 net/ipv6/mcast.c:2294
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
page last free pid 13 tgid 13 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1394 [inline]
 __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2900
 discard_slab mm/slub.c:3262 [inline]
 __put_partials+0x146/0x170 mm/slub.c:3727
 put_cpu_partial+0x17c/0x250 mm/slub.c:3802
 __slab_free+0x2b9/0x390 mm/slub.c:5555
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x97/0x140 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:349
 kasan_slab_alloc include/linux/kasan.h:250 [inline]
 slab_post_alloc_hook mm/slub.c:4710 [inline]
 slab_alloc_node mm/slub.c:5008 [inline]
 kmem_cache_alloc_node_noprof+0x433/0x710 mm/slub.c:5060
 __alloc_skb+0x112/0x2d0 net/core/skbuff.c:659
 alloc_skb include/linux/skbuff.h:1377 [inline]
 nlmsg_new include/net/netlink.h:1055 [inline]
 rtmsg_ifinfo_build_skb+0x84/0x260 net/core/rtnetlink.c:4392
 unregister_netdevice_many_notify+0x1556/0x1ff0 net/core/dev.c:12218
 ops_exit_rtnl_list net/core/net_namespace.c:186 [inline]
 ops_undo_list+0x3dc/0x990 net/core/net_namespace.c:247
 cleanup_net+0x4c5/0x800 net/core/net_namespace.c:682
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
 kthread+0x711/0x8a0 kernel/kthread.c:463

Memory state around the buggy address:
 ffff888077b7f380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888077b7f400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888077b7f480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                             ^
 ffff888077b7f500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888077b7f580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
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If the report is a duplicate of another one, reply with:
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If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [media?] KASAN: slab-use-after-free Read in media_devnode_unregister

[Edward Adam Davis]

#syz test

diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
index 0d01cbae98f2..6daa7aa99442 100644
--- a/drivers/media/mc/mc-devnode.c
+++ b/drivers/media/mc/mc-devnode.c
@@ -276,13 +276,10 @@ void media_devnode_unregister(struct media_devnode *devnode)
 	/* Delete the cdev on this minor as well */
 	cdev_device_del(&devnode->cdev, &devnode->dev);
 	devnode->media_dev = NULL;
+	clear_bit(devnode->minor, media_devnode_nums);
 	mutex_unlock(&media_devnode_lock);
 
 	put_device(&devnode->dev);
-
-	mutex_lock(&media_devnode_lock);
-	clear_bit(devnode->minor, media_devnode_nums);
-	mutex_unlock(&media_devnode_lock);
 }
 
 /*

Re: [syzbot] [media?] KASAN: slab-use-after-free Read in media_devnode_unregister

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com

Tested on:

commit:         65dd046e Add linux-next specific files for 20250909
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13c4ed62580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d3b38340153b83d4
dashboard link: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=15ff2642580000

Note: testing is done by a robot and is best-effort only.

[PATCH Next] media: mc: Clear minor number before put device

[Edward Adam Davis]

The device minor should not be cleared after the device is released.

Fixes: 9e14868dc952 ("media: mc: Clear minor number reservation at unregistration time")
Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 drivers/media/mc/mc-devnode.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
index 0d01cbae98f2..6daa7aa99442 100644
--- a/drivers/media/mc/mc-devnode.c
+++ b/drivers/media/mc/mc-devnode.c
@@ -276,13 +276,10 @@ void media_devnode_unregister(struct media_devnode *devnode)
 	/* Delete the cdev on this minor as well */
 	cdev_device_del(&devnode->cdev, &devnode->dev);
 	devnode->media_dev = NULL;
+	clear_bit(devnode->minor, media_devnode_nums);
 	mutex_unlock(&media_devnode_lock);
 
 	put_device(&devnode->dev);
-
-	mutex_lock(&media_devnode_lock);
-	clear_bit(devnode->minor, media_devnode_nums);
-	mutex_unlock(&media_devnode_lock);
 }
 
 /*
-- 
2.43.0

Re: [PATCH Next] media: mc: Clear minor number before put device

[Laurent Pinchart]

Hello Edward,

On Wed, Sep 10, 2025 at 09:15:27AM +0800, Edward Adam Davis wrote:
> The device minor should not be cleared after the device is released.

The most important piece of information in a commit message is the
reason for the patch. You need to explain *why* this is needed, not just
state it should be done.

> Fixes: 9e14868dc952 ("media: mc: Clear minor number reservation at unregistration time")
> Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
> Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
>  drivers/media/mc/mc-devnode.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
> index 0d01cbae98f2..6daa7aa99442 100644
> --- a/drivers/media/mc/mc-devnode.c
> +++ b/drivers/media/mc/mc-devnode.c
> @@ -276,13 +276,10 @@ void media_devnode_unregister(struct media_devnode *devnode)
>  	/* Delete the cdev on this minor as well */
>  	cdev_device_del(&devnode->cdev, &devnode->dev);
>  	devnode->media_dev = NULL;
> +	clear_bit(devnode->minor, media_devnode_nums);
>  	mutex_unlock(&media_devnode_lock);
>  
>  	put_device(&devnode->dev);
> -
> -	mutex_lock(&media_devnode_lock);
> -	clear_bit(devnode->minor, media_devnode_nums);
> -	mutex_unlock(&media_devnode_lock);
>  }
>  
>  /*

-- 
Regards,

Laurent Pinchart

[PATCH Next V2] media: mc: Clear minor number before put device

[Edward Adam Davis]

syzbot report a slab-use-after-free in media_devnode_unregister.

The following calltrace shows the entire process of UAF generation:

hub_event()->
  port_event()->
    hub_port_connect_change()->
      hub_port_connect()->
        usb_disconnect()->
	  usb_disable_device()->
	    device_del()->
	      bus_remove_device()->
	        device_release_driver_internal()->
		  __device_release_driver()->
		    device_remove()->
		      usb_unbind_interface()->
		        em28xx_usb_disconnect()->
			  em28xx_release_resources()->
			    em28xx_unregister_media_device()->
			      media_device_unregister()->
			        media_devnode_unregister()->
				  put_device()->
				    media_devnode_release()->
				      kfree(devnode)
				  clear_bit(devnode->minor, media_devnode_nums) 

[1] kfree(devnode), after this code is executed, devnode is released.
[2] clear_bit(devnode->minor, media_devnode_nums), this accesses the
freed devnode, trigger uaf

We clear the device's minor num before freeing devnode to avoid a UAF.

Fixes: 9e14868dc952 ("media: mc: Clear minor number reservation at unregistration time")
Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
V1 -> V2: update comments for rootcause

 drivers/media/mc/mc-devnode.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
index 0d01cbae98f2..6daa7aa99442 100644
--- a/drivers/media/mc/mc-devnode.c
+++ b/drivers/media/mc/mc-devnode.c
@@ -276,13 +276,10 @@ void media_devnode_unregister(struct media_devnode *devnode)
 	/* Delete the cdev on this minor as well */
 	cdev_device_del(&devnode->cdev, &devnode->dev);
 	devnode->media_dev = NULL;
+	clear_bit(devnode->minor, media_devnode_nums);
 	mutex_unlock(&media_devnode_lock);
 
 	put_device(&devnode->dev);
-
-	mutex_lock(&media_devnode_lock);
-	clear_bit(devnode->minor, media_devnode_nums);
-	mutex_unlock(&media_devnode_lock);
 }
 
 /*
-- 
2.43.0

Re: [PATCH Next V2] media: mc: Clear minor number before put device

[Sakari Ailus]

Hi Edward,

On Wed, Sep 10, 2025 at 06:31:45PM +0800, Edward Adam Davis wrote:
> syzbot report a slab-use-after-free in media_devnode_unregister.
> 
> The following calltrace shows the entire process of UAF generation:
> 
> hub_event()->
>   port_event()->
>     hub_port_connect_change()->
>       hub_port_connect()->
>         usb_disconnect()->
> 	  usb_disable_device()->
> 	    device_del()->
> 	      bus_remove_device()->
> 	        device_release_driver_internal()->
> 		  __device_release_driver()->
> 		    device_remove()->
> 		      usb_unbind_interface()->
> 		        em28xx_usb_disconnect()->
> 			  em28xx_release_resources()->
> 			    em28xx_unregister_media_device()->
> 			      media_device_unregister()->
> 			        media_devnode_unregister()->
> 				  put_device()->
> 				    media_devnode_release()->
> 				      kfree(devnode)
> 				  clear_bit(devnode->minor, media_devnode_nums) 
> 
> [1] kfree(devnode), after this code is executed, devnode is released.
> [2] clear_bit(devnode->minor, media_devnode_nums), this accesses the
> freed devnode, trigger uaf
> 
> We clear the device's minor num before freeing devnode to avoid a UAF.
> 
> Fixes: 9e14868dc952 ("media: mc: Clear minor number reservation at unregistration time")
> Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
> Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>

Thanks for the update. However, v1 was already merged. I'll mark this as
"not applicable".

-- 
Kind regards,

Sakari Ailus