Re: [PATCH] usb: gadget: f_printer: prevent OOB write in GET_DEVICE_ID

On Fri, Aug 21, 2026 at 04:34:28PM +0800, Haofeng Li wrote:
> printer_func_setup() services the Printer Class GET_DEVICE_ID request
> by echoing the PnP string previously stored in the gadget's configfs
> pnp_string attribute:
> 
> 	value = strlen(*dev->pnp_string);
> 	buf[0] = (value >> 8) & 0xFF;
> 	buf[1] = value & 0xFF;
> 	memcpy(buf + 2, *dev->pnp_string, value);
> 
> The EP0 response buffer is exactly USB_COMP_EP0_BUFSIZ (4096) bytes,
> allocated once by composite_dev_prepare():
> 
> 	cdev->req->buf = kzalloc(USB_COMP_EP0_BUFSIZ, GFP_KERNEL);
> 
> The two-byte length prefix plus the string body must therefore fit
> into 4096 bytes.  pnp_string is stored via kstrndup(page, len,
> GFP_KERNEL) in f_printer_opts_pnp_string_store(); configfs passes at
> most PAGE_SIZE - 1 (4095) bytes down to the store callback, so a
> 4095-byte string combined with the 2-byte length field makes the
> memcpy() write buf[2..4096], one byte past the end of the allocation.
> 
> Attack chain (USB Printer gadget on the victim device):
> 
>   1. pnp_string is set to a 4095-byte value through the gadget's
>      configfs attribute
>      (~/config/usb_gadget/<gadget>/functions/printer.usb0/pnp_string);
>      configfs accepts up to PAGE_SIZE - 1 bytes (fs/configfs/file.c).
>   2. The printer function is enabled and the gadget is bound to its
>      UDC.  An attacker in control of the connecting USB host sends a
>      Printer Class GET_DEVICE_ID request (bmRequestType=0xA1,
>      bRequest=0x00, wIndex pointing at the printer interface); the
>      usblp host driver also issues this request on enumeration.
>   3. composite_setup() -> printer_func_setup() -> memcpy(buf + 2,
>      pnp_string, 4095) performs a 4097-byte write into the 4096-byte
>      EP0 response buffer, overflowing the heap object by one byte and
>      potentially corrupting adjacent slab objects or allocator
>      metadata (CWE-787).
> 
> With KASAN enabled the overflow is reliably reported (this is
> reproducible end to end with a configfs gadget + dummy_hcd):
> 
> 	BUG: KASAN: slab-out-of-bounds in printer_func_setup+0x2ec/0x3c0
> 	Write of size 4095 at addr ffff88818e461002
> 
> Fix it at both ends:
> 
>   - clamp the string length to USB_COMP_EP0_BUFSIZ - 2 in
>     printer_func_setup() so the copy can never exceed the EP0 buffer,
>     and
>   - reject pnp_string values longer than USB_COMP_EP0_BUFSIZ - 2 in
>     f_printer_opts_pnp_string_store() so an oversized string is never
>     stored in the first place.
> 
> Signed-off-by: Haofeng Li <[email protected]>
> ---
>  drivers/usb/gadget/function/f_printer.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)

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