From d5e5cd3654d2b5359a12ea6586120f05b28634ee Mon Sep 17 00:00:00 2001 From: Jinchao Wang Date: Thu, 16 Jul 2026 06:42:17 -0400 Subject: [PATCH] usb: gadget: dummy_hcd: prevent fifo_req reuse during giveback dummy_hcd embeds a single shared usb_request (dum->fifo_req) that the "emulated single-request FIFO" fast-path in dummy_queue() reuses for small IN transfers: it copies the caller's request into it (req->req = *_req) and queues it, treating list_empty(&fifo_req.queue) as "the slot is free". The completion side (dummy_timer/transfer/nuke/dummy_dequeue) follows the standard pattern: list_del_init(&req->queue) unlinks the request, then the lock is dropped and usb_gadget_giveback_request() invokes req->complete(). But list_del_init() makes fifo_req.queue look empty *before* the completion callback returns, so a concurrent dummy_queue() on another CPU sees the slot as free, reuses fifo_req and runs req->req = *_req -- overwriting req->complete while dummy_timer is mid-calling it. The indirect call then jumps to a clobbered pointer, causing a general protection fault / page fault in dummy_timer (syzkaller extid faf3a6cf579fc65591ca). The clobbering write is an in-bounds memcpy on a live shared object, so KASAN cannot flag it. Add a fifo_req_busy bit covering the shared request's whole lifetime: set it in dummy_queue() when the FIFO fast-path takes fifo_req (making it the fast-path guard, replacing the list_empty(&fifo_req.queue) test), and clear it after the completion callback has returned, via a dummy_giveback() helper used at all four gadget-request giveback sites. The shared slot can no longer be reused until its completion callback has finished. Reported-by: syzbot+faf3a6cf579fc65591ca@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=faf3a6cf579fc65591ca Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable Signed-off-by: Jinchao Wang Reviewed-by: Alan Stern Link: https://patch.msgid.link/5db8bba5b3499a86cd2e776f9918126b68b2508b.1784198306.git.wangjinchao600@gmail.com Signed-off-by: Greg Kroah-Hartman --- drivers/usb/gadget/udc/dummy_hcd.c | 40 ++++++++++++++++++++---------- 1 file changed, 27 insertions(+), 13 deletions(-) diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c index f47903461ed5..c0e40fa6dde5 100644 --- a/drivers/usb/gadget/udc/dummy_hcd.c +++ b/drivers/usb/gadget/udc/dummy_hcd.c @@ -278,6 +278,7 @@ struct dummy { unsigned ints_enabled:1; unsigned udc_suspended:1; unsigned pullup:1; + unsigned fifo_req_busy:1; /* * HOST side support @@ -329,6 +330,26 @@ static inline struct dummy *gadget_dev_to_dummy(struct device *dev) /* DEVICE/GADGET SIDE UTILITY ROUTINES */ +/* + * Give back a gadget request with dum->lock dropped around the callback. + * If @req is the shared fifo_req, clear fifo_req_busy afterward: the flag + * was set in dummy_queue() when the shared request was taken and must stay + * set until its completion callback has returned; list_del_init() alone + * makes the request look idle while the callback is still running. + * Caller holds dum->lock and has already done list_del_init() + status. + */ +static void dummy_giveback(struct dummy *dum, struct usb_ep *_ep, + struct dummy_request *req) +{ + bool fifo = req == &dum->fifo_req; + + spin_unlock(&dum->lock); + usb_gadget_giveback_request(_ep, &req->req); + spin_lock(&dum->lock); + if (fifo) + dum->fifo_req_busy = 0; +} + /* called with spinlock held */ static void nuke(struct dummy *dum, struct dummy_ep *ep) { @@ -339,9 +360,7 @@ static void nuke(struct dummy *dum, struct dummy_ep *ep) list_del_init(&req->queue); req->req.status = -ESHUTDOWN; - spin_unlock(&dum->lock); - usb_gadget_giveback_request(&ep->ep, &req->req); - spin_lock(&dum->lock); + dummy_giveback(dum, &ep->ep, req); } } @@ -728,10 +747,11 @@ static int dummy_queue(struct usb_ep *_ep, struct usb_request *_req, /* implement an emulated single-request FIFO */ if (ep->desc && (ep->desc->bEndpointAddress & USB_DIR_IN) && - list_empty(&dum->fifo_req.queue) && + !dum->fifo_req_busy && list_empty(&ep->queue) && _req->length <= FIFO_SIZE) { req = &dum->fifo_req; + dum->fifo_req_busy = 1; req->req = *_req; req->req.buf = dum->fifo_buf; memcpy(dum->fifo_buf, _req->buf, _req->length); @@ -785,9 +805,7 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req) dev_dbg(udc_dev(dum), "dequeued req %p from %s, len %d buf %p\n", req, _ep->name, _req->length, _req->buf); - spin_unlock(&dum->lock); - usb_gadget_giveback_request(_ep, _req); - spin_lock(&dum->lock); + dummy_giveback(dum, _ep, req); } spin_unlock_irqrestore(&dum->lock, flags); return retval; @@ -1523,9 +1541,7 @@ static int transfer(struct dummy_hcd *dum_hcd, struct urb *urb, if (req->req.status != -EINPROGRESS) { list_del_init(&req->queue); - spin_unlock(&dum->lock); - usb_gadget_giveback_request(&ep->ep, &req->req); - spin_lock(&dum->lock); + dummy_giveback(dum, &ep->ep, req); /* requests might have been unlinked... */ rescan = 1; @@ -1910,9 +1926,7 @@ static enum hrtimer_restart dummy_timer(struct hrtimer *t) dev_dbg(udc_dev(dum), "stale req = %p\n", req); - spin_unlock(&dum->lock); - usb_gadget_giveback_request(&ep->ep, &req->req); - spin_lock(&dum->lock); + dummy_giveback(dum, &ep->ep, req); ep->already_seen = 0; goto restart; }