[PATCH] usb: gadget: udc-xilinx: Fix wild pointer dereference on empty EP0 queue

[Li Ximo <lixicaicai@gmail.com>]

From: Wang Haoran <haoranwangsec@gmail.com>

list_first_entry() does not return NULL when the list is empty; it
returns the result of container_of() on the list head, i.e. a non-NULL
wild pointer pointing into the enclosing struct xusb_ep rather than to
a valid request. This was found by code audit.

xudc_ep0_out() and xudc_ep0_in() are reached from the endpoint 0
interrupt handler xudc_ctrl_ep_handler() purely based on the hardware
FIFO status bits, without checking whether ep0->queue actually holds a
request. The queue can be empty at that point: the gadget driver's
setup() callback may not queue a request for every control transfer
stage, and xudc_done() removes the request once it completes, after
which bus timing can raise another FIFO ready/free interrupt. The
unconditional list_first_entry() then yields a wild pointer, and the
subsequent accesses to req->usb_req.actual, req->usb_req.buf and
req->usb_req.length read or write unrelated kernel memory, leading to
memory corruption or a crash. On real hardware this is triggerable by
an external USB host controlling the token timing. Note that the
enqueue side __xudc_ep0_queue() already guards the same queue with
list_empty(), which shows the queue can legitimately be empty; the
dequeue side was simply missing the symmetric check.

Switch both functions to list_first_entry_or_null() and return early
when no request is queued, mirroring the existing list_empty() guard
on the enqueue side.

Reported-by: Wang Haoran <haoranwangsec@gmail.com>
Reported-by: Li Ximo <lixicaicai@gmail.com>
Signed-off-by: Wang Haoran <haoranwangsec@gmail.com>
---
 drivers/usb/gadget/udc/udc-xilinx.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/udc/udc-xilinx.c b/drivers/usb/gadget/udc/udc-xilinx.c
index bef06fe75..032d9f04f 100644
--- a/drivers/usb/gadget/udc/udc-xilinx.c
+++ b/drivers/usb/gadget/udc/udc-xilinx.c
@@ -1822,7 +1822,9 @@ static void xudc_ep0_out(struct xusb_udc *udc)
 	unsigned int bytes_to_rx;
 	void *buffer;
 
-	req = list_first_entry(&ep0->queue, struct xusb_req, queue);
+	req = list_first_entry_or_null(&ep0->queue, struct xusb_req, queue);
+	if (!req)
+		return;
 
 	switch (udc->setupseqrx) {
 	case STATUS_PHASE:
@@ -1875,7 +1877,10 @@ static void xudc_ep0_in(struct xusb_udc *udc)
 	u8 *ep0rambase;
 	u8 test_mode = le16_to_cpu(udc->setup.wIndex) >> 8;
 
-	req = list_first_entry(&ep0->queue, struct xusb_req, queue);
+	req = list_first_entry_or_null(&ep0->queue, struct xusb_req, queue);
+	if (!req)
+		return;
+
 	bytes_to_tx = req->usb_req.length - req->usb_req.actual;
 
 	switch (udc->setupseqtx) {
-- 
2.30.2