Re: [PATCH] usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling

[Greg Kroah-Hartman]

On Tue, May 12, 2026 at 09:12:14AM -0500, Sebastian EM wrote:
> The WebUSB GET_URL handler in composite_setup() narrows
> landing_page_length to fit the host-supplied wLength using
> 
> landing_page_length = w_length
> - WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset;
> 
> If wLength is smaller than WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH the
> unsigned subtraction wraps, and the subsequent
> 
> memcpy(url_descriptor->URL,
>       cdev->landing_page + landing_page_offset,
>       landing_page_length - landing_page_offset);
> 
> ends up copying close to UINT_MAX bytes from cdev->landing_page into
> cdev->req->buf.  KASAN reports a slab-out-of-bounds in composite_setup
> on the kmalloc-2k gadget_info allocation, and FORTIFY_SOURCE traps the
> memcpy as a 4294967293-byte field-spanning write into
> url_descriptor->URL (size 252).
> 
> A USB host can reach this from a single SETUP packet against any
> gadget that has webusb/use=1 and a landingPage configured.
> 
> Handle the small-wLength case before the math: when the host requested
> fewer bytes than the URL descriptor header, only the header is
> meaningful and no URL bytes need to be copied.  Setting
> landing_page_length to landing_page_offset makes the existing memcpy a
> no-op and leaves the descriptor returned to the host unchanged for all
> larger wLength values.
> 
> Fixes: 93c473948c58 ("usb: gadget: add WebUSB landing page support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jeremy Erazo <mendozayt13@xxxxxxxxx>
> ---
> v2:
> - Drop the self Reported-by tag.
> - Add Fixes tag.
> - Cc stable.
> 
>  drivers/usb/gadget/composite.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> index a902184bd..dc3664374 100644
> --- a/drivers/usb/gadget/composite.c
> +++ b/drivers/usb/gadget/composite.c
> @@ -2172,7 +2172,10 @@ composite_setup(struct usb_gadget *gadget, const
> struct usb_ctrlrequest *ctrl)
>   sizeof(url_descriptor->URL)
>   - WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset);
> 
> - if (w_length < WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_length)
> + if (w_length < WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH)
> + landing_page_length = landing_page_offset;
> + else if (w_length <
> + WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_length)
>   landing_page_length = w_length
>   - WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset;
> 
> -- 
> 2.53.0
> 
> El mar, 12 may 2026 a las 8:29, Greg Kroah-Hartman (<
> gregkh@xxxxxxxxxxxxxxxxxxx>) escribió:
> 
> > On Tue, May 12, 2026 at 06:18:54AM -0500, Sebastian EM wrote:
> > > Hi Greg,
> > >
> > > Thanks for the review.
> > >
> > > You are right; the self Reported-by tag does not belong there, so I
> > dropped
> > > it in v2.
> > >
> > > The introducing commit is:
> > >
> > > 93c473948c58 ("usb: gadget: add WebUSB landing page support")
> > >
> > > I also added:
> > >
> > > Cc: stable@xxxxxxxxxxxxxxx
> > >
> > > since the issue was introduced with the WebUSB GET_URL handling path and
> > > the fix is a small bounds/underflow fix suitable for stable kernels.
> > >
> > > v2 is attached as a plain patch:
> > >
> > >
> > 0001-v2-usb-gadget-composite-fix-integer-underflow-in-WebUSB-GET_URL-handling.patch
> >
> > We can't take patches as attachments, just send this as a normal v2
> > patch.
> >
> > thanks,
> >
> > greg k-h
> >

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

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