Re: NULL dereference on disconnection during usb_set_interface()

["Michał Pecio" <michal.pecio@gmail.com>]

Hi Greg,

> There are a number of known-race-conditions in the v4l interface that
> can happen when devices go away and userspace is still holding a
> reference on the character device node.

I wrote to linux-usb because I think this particular crash is a bug in
the USB subsystem - namely, usb_set_interface() appears to crash when
the device is disconnected during its execution.

Indeed, today I came up with an artificial way to reproduce this crash.
I added msleep(1000) right before the call to usb_hcd_alloc_bandwidth()
in usb_set_interface() and pulled the USB plug when it slept.

(BTW, previously the device was not physically disconnected, it looks
like the host controller dropped it due to I/O errors).

Anyway, here's my new crash log:

# this is what normal execution looks like, nothing special happens yet
[  210.644611] usb_set_interface called from uvc_video_start_transfer
[  210.644615] sleeping before usb_hcd_alloc_bandwidth
[  211.668754] usb_set_interface returned

# and now I will disconnect the device during the sleep
[  216.700611] usb_set_interface called from uvc_video_start_transfer
[  216.700616] sleeping before usb_hcd_alloc_bandwidth
[  217.144340] usb 12-1.3: USB disconnect, device number 3
[  217.746182] BUG: kernel NULL pointer dereference, address: 0000000000000000
[  217.746190] #PF: supervisor read access in kernel mode
[  217.746192] #PF: error_code(0x0000) - not-present page
[  217.746195] PGD 0 P4D 0 
[  217.746197] Oops: 0000 [#1] PREEMPT SMP
[  217.746200] CPU: 0 PID: 815 Comm: yavta Not tainted 6.7.0 #4
[  217.746204] Hardware name: System manufacturer System Product Name/M4A88TD-M EVO, BIOS 1801    08/09/2012
[  217.746206] RIP: 0010:usb_ifnum_to_if+0x38/0x50
[  217.746212] Code: d2 74 32 0f b6 4a 04 84 c9 74 2e ff c9 48 8d 82 98 00 00 00 48 8d bc ca a0 00 00 00 eb 09 48 83 c0 08 48 39 f8 74 12 48 8b 10 <48> 8b 0a 0f b6 49 02 39 f1 75 e9 48 89 d0 c3 31 d2 48 89 d0 c3 0f
[  217.746215] RSP: 0018:ffffc90000b07b90 EFLAGS: 00010206
[  217.746217] RAX: ffff8880031ac498 RBX: ffff888003144800 RCX: 0000000000000003
[  217.746219] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8880031ac4b8
[  217.746221] RBP: 0000000000000000 R08: 0000000000000400 R09: 0000000000000000
[  217.746223] R10: 0000000000000000 R11: 00000000000003ad R12: ffff8880031acde8
[  217.746224] R13: 0000000000000000 R14: ffff8880031acc08 R15: ffff888102ca4000
[  217.746226] FS:  00007f8455cf2740(0000) GS:ffff88811bc00000(0000) knlGS:0000000000000000
[  217.746228] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  217.746230] CR2: 0000000000000000 CR3: 000000011af26000 CR4: 00000000000006f0
[  217.746231] Call Trace:
[  217.746234]  <TASK>
[  217.746237]  ? __die+0x2d/0x80
[  217.746240]  ? page_fault_oops+0x15d/0x420
[  217.746244]  ? fixup_exception+0x36/0x280
[  217.746248]  ? exc_page_fault+0x74/0x150
[  217.746252]  ? asm_exc_page_fault+0x22/0x30
[  217.746256]  ? usb_ifnum_to_if+0x38/0x50
[  217.746258]  usb_hcd_alloc_bandwidth+0x208/0x310
[  217.746263]  ? trace_raw_output_tick_stop+0x80/0x80
[  217.746267]  usb_set_interface+0x112/0x430
[  217.746269]  ? _printk+0x48/0x50
[  217.746273]  uvc_video_start_transfer+0x1db/0x650 [uvcvideo]