[PATCH] usb: r8a66597-hcd: fix potential divide-by-zero in prepare_packet_read()

[PATCH] usb: r8a66597-hcd: fix potential divide-by-zero in prepare_packet_read()

[pip-izony]

From: Seungjin Bae <eeodqql09@gmail.com>

The `prepare_packet_read()` function calculates the number of packets
required for a transfer by dividing the transfer buffer length by the
maximum packet size (`td->maxpacket`). The `td->maxpacket` is
initialized in `r8a66597_make_td()` function based on the endpoint
descriptor. However, it does not validate whether `td->maxpacket` is
zero before using it in the `DIV_ROUND_UP` macro.

If a malicious USB device sends a descriptor with `wMaxPacketSize` set to
0, it triggers a divide-by-zero exception (kernel panic).

Fix this by ensuring `td->maxpacket` is greater than 0 before performing
the division.

Fixes: 5d3043586db4 ("USB: r8a66597-hcd: host controller driver for R8A66597")
Signed-off-by: Seungjin Bae <eeodqql09@gmail.com>
---
 drivers/usb/host/r8a66597-hcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/r8a66597-hcd.c b/drivers/usb/host/r8a66597-hcd.c
index d21a03cf5c17..445551b4b05f 100644
--- a/drivers/usb/host/r8a66597-hcd.c
+++ b/drivers/usb/host/r8a66597-hcd.c
@@ -1097,7 +1097,7 @@ static void prepare_packet_read(struct r8a66597 *r8a66597,
 		pipe_start(r8a66597, td->pipe);
 		pipe_irq_enable(r8a66597, urb, td->pipenum);
 	} else {
-		if (urb->actual_length == 0) {
+		if (urb->actual_length == 0 && td->maxpacket > 0) {
 			pipe_irq_disable(r8a66597, td->pipenum);
 			pipe_setting(r8a66597, td);
 			pipe_stop(r8a66597, td->pipe);
-- 
2.43.0

Re: [PATCH] usb: r8a66597-hcd: fix potential divide-by-zero in prepare_packet_read()

[Greg Kroah-Hartman]

On Sat, Jan 10, 2026 at 02:22:33PM -0500, pip-izony wrote:
> From: Seungjin Bae <eeodqql09@gmail.com>
> 
> The `prepare_packet_read()` function calculates the number of packets
> required for a transfer by dividing the transfer buffer length by the
> maximum packet size (`td->maxpacket`). The `td->maxpacket` is
> initialized in `r8a66597_make_td()` function based on the endpoint
> descriptor. However, it does not validate whether `td->maxpacket` is
> zero before using it in the `DIV_ROUND_UP` macro.
> 
> If a malicious USB device sends a descriptor with `wMaxPacketSize` set to
> 0, it triggers a divide-by-zero exception (kernel panic).

Same here, when can this happen, before a device is bound to the device
or afterward?

thanks,

greg k-h

Re: [PATCH] usb: r8a66597-hcd: fix potential divide-by-zero in prepare_packet_read()

[Greg Kroah-Hartman]

On Sun, Jan 11, 2026 at 02:25:54AM -0500, Seungjin Bae wrote:
> 2026년 1월 10일 (토) PM 3:02, Greg Kroah-Hartman <gregkh@linuxfoundation.org>님이
> 작성:
> 
> > On Sat, Jan 10, 2026 at 02:22:33PM -0500, pip-izony wrote:
> > > From: Seungjin Bae <eeodqql09@gmail.com>
> > >
> > > The `prepare_packet_read()` function calculates the number of packets
> > > required for a transfer by dividing the transfer buffer length by the
> > > maximum packet size (`td->maxpacket`). The `td->maxpacket` is
> > > initialized in `r8a66597_make_td()` function based on the endpoint
> > > descriptor. However, it does not validate whether `td->maxpacket` is
> > > zero before using it in the `DIV_ROUND_UP` macro.
> > >
> > > If a malicious USB device sends a descriptor with `wMaxPacketSize` set to
> > > 0, it triggers a divide-by-zero exception (kernel panic).
> >
> > Same here, when can this happen, before a device is bound to the device
> > or afterward?
> >
> > thanks,
> >
> > greg k-h
> >
> 
> This logic triggers after the driver is bound, specifically during the
> packet preparation phase.
> 
> I checked `usb_parse_endpoint()` and confirmed that if `wMaxPacketSize`
> is 0, it only prints a warning and continues the binding process.
> It does not reject the device at that stage.
> 
> However, I missed that `usb_submit_urb()` explicitly checks if
> `wMaxPacketSize` is 0 before calling the driver's enqueue function.
> Since the core rejects such requests with -EMSGSIZE, this code path
> is unreachable in practice.

That's great to verify, thanks for checking.

also, when writing a patch that says "this will fix a crash", please
verify that the tool that found this issue is actually correct and not
making things up.  We have a lot of bad AI generated bug reports these
days, so be careful as we are getting much more sensitive to this.

thanks,

greg k-h