Re: [PATCH] media: uvc: Avoid cyclic entity chains due to malformed USB descriptors
From: Laurent Pinchart
Date: Wed Oct 02 2019 - 09:09:30 EST
Hi Will,
Thank you for the patch.
On Wed, Oct 02, 2019 at 12:27:53PM +0100, Will Deacon wrote:
> Way back in 2017, fuzzing the 4.14-rc2 USB stack with syzkaller kicked
> up the following WARNING from the UVC chain scanning code:
>
> | list_add double add: new=ffff880069084010, prev=ffff880069084010,
> | next=ffff880067d22298.
> | ------------[ cut here ]------------
> | WARNING: CPU: 1 PID: 1846 at lib/list_debug.c:31 __list_add_valid+0xbd/0xf0
> | Modules linked in:
> | CPU: 1 PID: 1846 Comm: kworker/1:2 Not tainted
> | 4.14.0-rc2-42613-g1488251d1a98 #238
> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> | Workqueue: usb_hub_wq hub_event
> | task: ffff88006b01ca40 task.stack: ffff880064358000
> | RIP: 0010:__list_add_valid+0xbd/0xf0 lib/list_debug.c:29
> | RSP: 0018:ffff88006435ddd0 EFLAGS: 00010286
> | RAX: 0000000000000058 RBX: ffff880067d22298 RCX: 0000000000000000
> | RDX: 0000000000000058 RSI: ffffffff85a58800 RDI: ffffed000c86bbac
> | RBP: ffff88006435dde8 R08: 1ffff1000c86ba52 R09: 0000000000000000
> | R10: 0000000000000002 R11: 0000000000000000 R12: ffff880069084010
> | R13: ffff880067d22298 R14: ffff880069084010 R15: ffff880067d222a0
> | FS: 0000000000000000(0000) GS:ffff88006c900000(0000) knlGS:0000000000000000
> | CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> | CR2: 0000000020004ff2 CR3: 000000006b447000 CR4: 00000000000006e0
> | Call Trace:
> | __list_add ./include/linux/list.h:59
> | list_add_tail+0x8c/0x1b0 ./include/linux/list.h:92
> | uvc_scan_chain_forward.isra.8+0x373/0x416
> | drivers/media/usb/uvc/uvc_driver.c:1471
> | uvc_scan_chain drivers/media/usb/uvc/uvc_driver.c:1585
> | uvc_scan_device drivers/media/usb/uvc/uvc_driver.c:1769
> | uvc_probe+0x77f2/0x8f00 drivers/media/usb/uvc/uvc_driver.c:2104
>
> Looking into the output from usbmon, the interesting part is the
> following data packet:
>
> ffff880069c63e00 30710169 C Ci:1:002:0 0 143 = 09028f00 01030080
> 00090403 00000e01 00000924 03000103 7c003328 010204db
>
> If we drop the lead configuration and interface descriptors, we're left
> with an output terminal descriptor describing a generic display:
>
> /* Output terminal descriptor */
> buf[0] 09
> buf[1] 24
> buf[2] 03 /* UVC_VC_OUTPUT_TERMINAL */
> buf[3] 00 /* ID */
> buf[4] 01 /* type == 0x0301 (UVC_OTT_DISPLAY) */
> buf[5] 03
> buf[6] 7c
> buf[7] 00 /* source ID refers to self! */
> buf[8] 33
>
> The problem with this descriptor is that it is self-referential: the
> source ID of 0 matches itself! This causes the 'struct uvc_entity'
> representing the display to be added to its chain list twice during
> 'uvc_scan_chain()': once via 'uvc_scan_chain_entity()' when it is
> processed directly from the 'dev->entities' list and then again
> immediately afterwards when trying to follow the source ID in
> 'uvc_scan_chain_forward()'
>
> Add a check before adding an entity to a chain list to ensure that the
> entity is not already part of a chain.
>
> Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: c0efd232929c ("V4L/DVB (8145a): USB Video Class driver")
> Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-media/CAAeHK+z+Si69jUR+N-SjN9q4O+o5KFiNManqEa-PjUta7EOb7A@xxxxxxxxxxxxxx/
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>
> I don't have a way to reproduce the original issue, so this change is
> based purely on inspection. Considering I'm not familiar with USB nor
> UVC, I may well have missed something!
I may also be missing something, I haven't touched this code for a long
time :-)
uvc_scan_chain_entity(), at the end of the function, adds the entity to
the list of entities in the chain with
list_add_tail(&entity->chain, &chain->entities);
uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and
iterates over all entities connected to the entity being scanned.
while (1) {
forward = uvc_entity_by_reference(chain->dev, entity->id,
forward);
At this point forward may be equal to entity, if entity references
itself.
if (forward == NULL)
break;
if (forward == prev)
continue;
if (forward->chain.next || forward->chain.prev) {
uvc_trace(UVC_TRACE_DESCR, "Found reference to "
"entity %d already in chain.\n", forward->id);
return -EINVAL;
}
But then this check should trigger, as forward == entity and entity is
in the chain's list of entities.
> drivers/media/usb/uvc/uvc_driver.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 66ee168ddc7e..e24420b1750a 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -1493,6 +1493,11 @@ static int uvc_scan_chain_forward(struct uvc_video_chain *chain,
> break;
> if (forward == prev)
> continue;
> + if (forward->chain.next || forward->chain.prev) {
> + uvc_trace(UVC_TRACE_DESCR, "Found reference to "
> + "entity %d already in chain.\n", forward->id);
> + return -EINVAL;
> + }
>
> switch (UVC_ENTITY_TYPE(forward)) {
> case UVC_VC_EXTENSION_UNIT:
> @@ -1574,6 +1579,13 @@ static int uvc_scan_chain_backward(struct uvc_video_chain *chain,
> return -1;
> }
>
> + if (term->chain.next || term->chain.prev) {
> + uvc_trace(UVC_TRACE_DESCR, "Found reference to "
> + "entity %d already in chain.\n",
> + term->id);
> + return -EINVAL;
> + }
> +
> if (uvc_trace_param & UVC_TRACE_PROBE)
> printk(KERN_CONT " %d", term->id);
>
--
Regards,
Laurent Pinchart