Re: [PATCH] usb: gadget: Stall OS descriptor request for unsupported functions

[Jack Pham]

Hi Wesley,

On Mon, Mar 22, 2021 at 06:50:17PM -0700, Wesley Cheng wrote:
> From: Chandana Kishori Chiluveru <cchiluve@xxxxxxxxxxxxxx>
> 
> Hosts which request "OS descriptors" from gadgets do so during
> the enumeration phase and before the configuration is set with
> SET_CONFIGURATION. Composite driver supports OS descriptor
> handling in composite_setup function. This requires to pass
> signature field, vendor code, compatibleID and subCompatibleID
> from user space.
> 
> For USB compositions that contain functions which don't implement os
> descriptors, Windows is sending vendor specific requests for os
> descriptors and composite driver handling this request with invalid
> data. With this invalid info host resetting the bus and never
> selecting the configuration and leading enumeration issue.
> 
> Fix this by bailing out from the OS descriptor setup request
> handling if the functions does not have OS descriptors compatibleID.
> 
> Signed-off-by: Chandana Kishori Chiluveru <cchiluve@xxxxxxxxxxxxxx>
> Signed-off-by: Wesley Cheng <wcheng@xxxxxxxxxxxxxx>
> ---
>  drivers/usb/gadget/composite.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> index 72a9797..473edda6 100644
> --- a/drivers/usb/gadget/composite.c
> +++ b/drivers/usb/gadget/composite.c
> @@ -1945,6 +1945,12 @@ composite_setup(struct usb_gadget *gadget, const struct usb_ctrlrequest *ctrl)
>  				buf[6] = w_index;
>  				/* Number of ext compat interfaces */
>  				count = count_ext_compat(os_desc_cfg);
> +				/*
> +				 * Bailout if device does not
> +				 * have ext_compat interfaces.
> +				 */
> +				if (count == 0)
> +					break;
>  				buf[8] = count;
>  				count *= 24; /* 24 B/ext compat desc */
>  				count += 16; /* header */

Do we still need this fix? IIRC we had this change in our downstream
kernel to fix the case when dynamically re-configuring ConfigFS, i.e.
changing the composition of functions wherein none of the interfaces
support OS Descriptors, so this causes count_ext_compat() to return
0 and results in the issue described in $SUBJECT.

But I think this is more of a problem of an improperly configured
ConfigFS gadget. If userspace instead removes the config from the
gadget's os_desc subdirectory that should cause cdev->os_desc_config to
be set to NULL and hence composite_setup() should never enter this
handling at all, right?

Jack
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