CVE-2025-40016
In the Linux kernel, the following vulnerability has been resolved:

media: uvcvideo: Mark invalid entities with id UVC_INVALID_ENTITY_ID

Per UVC 1.1+ specification 3.7.2, units and terminals must have a non-zero
unique ID.

```
Each Unit and Terminal within the video function is assigned a unique
identification number, the Unit ID (UID) or Terminal ID (TID), contained in
the bUnitID or bTerminalID field of the descriptor. The value 0x00 is
reserved for undefined ID,
```

If we add a new entity with id 0 or a duplicated ID, it will be marked
as UVC_INVALID_ENTITY_ID.

In a previous attempt commit 3dd075fe8ebb ("media: uvcvideo: Require
entities to have a non-zero unique ID"), we ignored all the invalid units,
this broke a lot of non-compatible cameras. Hopefully we are more lucky
this time.

This also prevents some syzkaller reproducers from triggering warnings due
to a chain of entities referring to themselves. In one particular case, an
Output Unit is connected to an Input Unit, both with the same ID of 1. But
when looking up for the source ID of the Output Unit, that same entity is
found instead of the input entity, which leads to such warnings.

In another case, a backward chain was considered finished as the source ID
was 0. Later on, that entity was found, but its pads were not valid.

Here is a sample stack trace for one of those cases.

[   20.650953] usb 1-1: new high-speed USB device number 2 using dummy_hcd
[   20.830206] usb 1-1: Using ep0 maxpacket: 8
[   20.833501] usb 1-1: config 0 descriptor??
[   21.038518] usb 1-1: string descriptor 0 read error: -71
[   21.038893] usb 1-1: Found UVC 0.00 device <unnamed> (2833:0201)
[   21.039299] uvcvideo 1-1:0.0: Entity type for entity Output 1 was not initialized!
[   21.041583] uvcvideo 1-1:0.0: Entity type for entity Input 1 was not initialized!
[   21.042218] ------------[ cut here ]------------
[   21.042536] WARNING: CPU: 0 PID: 9 at drivers/media/mc/mc-entity.c:1147 media_create_pad_link+0x2c4/0x2e0
[   21.043195] Modules linked in:
[   21.043535] CPU: 0 UID: 0 PID: 9 Comm: kworker/0:1 Not tainted 6.11.0-rc7-00030-g3480e43aeccf #444
[   21.044101] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
[   21.044639] Workqueue: usb_hub_wq hub_event
[   21.045100] RIP: 0010:media_create_pad_link+0x2c4/0x2e0
[   21.045508] Code: fe e8 20 01 00 00 b8 f4 ff ff ff 48 83 c4 30 5b 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc 0f 0b eb e9 0f 0b eb 0a 0f 0b eb 06 <0f> 0b eb 02 0f 0b b8 ea ff ff ff eb d4 66 2e 0f 1f 84 00 00 00 00
[   21.046801] RSP: 0018:ffffc9000004b318 EFLAGS: 00010246
[   21.047227] RAX: ffff888004e5d458 RBX: 0000000000000000 RCX: ffffffff818fccf1
[   21.047719] RDX: 000000000000007b RSI: 0000000000000000 RDI: ffff888004313290
[   21.048241] RBP: ffff888004313290 R08: 0001ffffffffffff R09: 0000000000000000
[   21.048701] R10: 0000000000000013 R11: 0001888004313290 R12: 0000000000000003
[   21.049138] R13: ffff888004313080 R14: ffff888004313080 R15: 0000000000000000
[   21.049648] FS:  0000000000000000(0000) GS:ffff88803ec00000(0000) knlGS:0000000000000000
[   21.050271] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   21.050688] CR2: 0000592cc27635b0 CR3: 000000000431c000 CR4: 0000000000750ef0
[   21.051136] PKRU: 55555554
[   21.051331] Call Trace:
[   21.051480]  <TASK>
[   21.051611]  ? __warn+0xc4/0x210
[   21.051861]  ? media_create_pad_link+0x2c4/0x2e0
[   21.052252]  ? report_bug+0x11b/0x1a0
[   21.052540]  ? trace_hardirqs_on+0x31/0x40
[   21.052901]  ? handle_bug+0x3d/0x70
[   21.053197]  ? exc_invalid_op+0x1a/0x50
[   21.053511]  ? asm_exc_invalid_op+0x1a/0x20
[   21.053924]  ? media_create_pad_link+0x91/0x2e0
[   21.054364]  ? media_create_pad_link+0x2c4/0x2e0
[   21.054834]  ? media_create_pad_link+0x91/0x2e0
[   21.055131]  ? _raw_spin_unlock+0x1e/0x40
[   21.055441]  ? __v4l2_device_register_subdev+0x202/0x210
[   21.055837]  uvc_mc_register_entities+0x358/0x400
[   21.056144]  uvc_register_chains+0x1
---truncated---
References: https://git.kernel.org/stable/c/000b2a6bed7f30e0aadfb19bce9af6458d879304, https://git.kernel.org/stable/c/0e2ee70291e64a30fe36960c85294726d34a103e, https://git.kernel.org/stable/c/0f140cede24334b3ee55e3e1127071266cbb8287