Re: [PATCH] media: Fix invalid link creation when source entity has 0 pads
From: Laurent Pinchart
Date: Wed Apr 09 2025 - 16:55:06 EST
Hi Gabriel,
On Tue, Apr 08, 2025 at 01:35:00AM -0400, Gabriel Shahrouzi wrote:
> On Tue, Apr 1, 2025 at 8:30 PM Laurent Pinchart wrote:
> > On Sat, Mar 29, 2025 at 01:50:00PM -0400, Gabriel wrote:
> > > Hi Laurent,
> > >
> > > I’ve analyzed the bug report, and the root cause of the
> > > "WARNING-media_create_pad_link" issue is a mismatch in terminal
> > > references in the USB descriptor.
> > >
> > > The format type descriptor references terminal ID 6, while the audio
> > > streaming interface descriptor points to terminal ID 5. This
> > > discrepancy triggers the warning: "No streaming interface found for
> > > terminal 6", followed by the media pad link warning.
> >
> > Can you share the USB descriptors.
>
> The USB descriptors via the Syzkaller reproducer:
> "\x12\x01\x00\x00\xfb\x5d\x7d\x08\x6d\x04\xc3\x08\x16\x6b\x01\x02\x03"
> "\x01\x09\x02\x50\x00\x01\x00\x00\x00\x00\x09\x04\x1f\x00\x00\xff\x01"
> "\x00\x00\x0a\x24\x02\x00\x00\x05\x02\x01\x02\x07\x24\x07\x05\x00\x00"
> "\x18\xc2\x24\x08\x05\x04\x00\x04\x96\x0d\x24\x06\x01\x01\x03\x02\x00"
> "\x01\x00\x06\x00\x06\x09\x24\x03\x05\x05\x03\x06\x05\x81\x09\x24\x03"
> "\x06\x01\x01\x04\x05\x05\x07\x24\x04\x05\x01\x00\x9c\xbd\x89"
If I haven't made any mistake in the manual decode process (is there any
Linux tool that can decode a binary descriptors dump the same way lsusb
decodes descriptors from a device ?), the relevant UVC descriptors there
are
0x0a bLength
0x24 bDescriptorType USB_DT_CS_INTERFACE
0x02 bDescriptorSubtype VC_INPUT_TERMINAL
0x00 bTerminalID 0 (invalid)
0x00, 0x05 bTerminalType 0x0500 (invalid)
0x02 bAssocTerminal 2 (invalid)
0x01 iTerminal 1
0x02, 0x07
0x09 bLength
0x24 bDescriptorType USB_DT_CS_INTERFACE
0x03 bDescriptorSubtype VC_OUTPUT_TERMINAL
0x06 bTerminalID 6
0x01, 0x01 bTerminalType TT_STREAMING
0x04 bAssocTerminal 4 (invalid)
0x05 bSourceID 5
0x05 iTerminal 5
0x07 bLength
0x24 bDescriptorType USB_DT_CS_INTERFACE
0x04 bDescriptorSubtype VC_SELECTOR_UNIT
0x05 bUnitID 5
0x01 bNrInPins 1
0x00 baSourceID(1) 0
0x9c iSelector 156
Ignoring a few invalid values (bTerminalID shouldb't be 0, bTerminalType
0x0500 is defined by the specification, and the two bAssocTerminal ids
are also invalid), this creates the following chain:
VC_INPUT_TERMINAL (0) -> VC_SELECTOR_UNIT (5) -> VC_OUTPUT_TERMINAL (6)
Looking at uvc_mc_init_entity() where the media_entity->num_pads field
gets assigned by calling media_entity_pads_init(), a media entity is
only initialized when the entity type is not TT_STREAMING (so it's a
subdev), or when the entity has an associated video device. I think
that what is happening here is that the second entity in the above list
(VC_OUTPUT_TERMINAL, id 6) fails to initialize properly in
uvc_register_terms() is there is no corresponding streaming interface in
the device. This is confirmed by the
usb 1-1: No streaming interface found for terminal 6.
message in the syzbot kernel log. No video device is created for the
terminal, and no media_entity is initialized. Trying to later link the
entity in uvc_mc_create_links() then fails.
I don't want to address this in uvc_mc_create_links() as the invalid
terminal in the chain means we could have other issues elsewhere. One
option is to fail turn the missing streaming interface check in a hard
failure, at least for the chain being registered. The driver could still
proceed to registering other chains.
There's a small risk of regression for buggy devices. If that's a
problem, we could instead remove invalid terminals from the device
entities list before we proceed to scanning chains.
> > > I confirmed this by changing the terminal ID in the format descriptor
> > > from 6 to 5, which eliminates both warnings. This shows the warning is
> > > correctly identifying an invalid descriptor configuration, not a
> > > kernel bug.
> >
> > There's still something not quite right. uvc_entity->num_pads should
> > always be equal to the corresponding media_entity->num_pads. That's not
> > the case here, and I think it indicates a bug.
>
> Ah ok - the mismatch itself shouldn't happen regardless of the descriptor
>
> > > Since the USB descriptor is invalid, I believe the warning is
> > > necessary and should remain. The code should stay as is.
> >
> > There should be a warning, but I think it needs to be caught in a
> > different place, earlier.
>
> Got it.
>
> > > On Tue, Mar 25, 2025 at 8:13 PM Laurent Pinchart wrote:
> > > > On Tue, Mar 25, 2025 at 06:05:00PM -0400, Gabriel wrote:
> > > > > Hi Ricardo,
> > > > >
> > > > > > I cannot reach that URL
> > > > > I was unable to access the URL from my email client when I initially
> > > > > sent the email, but a couple of hours later, I was able to. Initially,
> > > > > copying and pasting the URL into the browser provided a workaround.
> > > > >
> > > > > > Shouldn't it be?:
> > > > > > Fixes: 4ffc2d89f38a ("[media] uvcvideo: Register subdevices for each entity")
> > > > > You're right, I incorrectly referenced the wrong commit. However, I’m
> > > > > not certain if it should reference a96aa5342d57 (Fixes: a96aa5342d57 -
> > > > > '[media] uvcvideo: Ignore entities for terminals with no supported
> > > > > format') as it's the latest commit affecting the line I'm changing or
> > > > > the one you mentioned.
> > > > >
> > > > > > Shouldn't source->num_pads be the same as remote->num_pads?
> > > > > The fuzzer (Syzkaller) that triggered the warning appears to have
> > > > > encountered a case where source->num_pads and remote->num_pads were
> > > > > different. When analyzing the case in GDB, remote->num_pads was 1,
> > > > > while source->num_pads was 0.
> > > >
> > > > This seems like the real bug that should be fixed.
> > > >
> > > > > > Are you sure that your kernel does not contain?
> > > > > > https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/media/usb/uvc/uvc_entity.c?id=41ddb251c68ac75c101d3a50a68c4629c9055e4c
> > > > > Yes, it should be included since I am running the upstream kernel.
--
Regards,
Laurent Pinchart