===================================================================
@@ -1563,6 +1563,13 @@ int usb_hcd_submit_urb (struct urb *urb,
urb->hcpriv = NULL;
INIT_LIST_HEAD(&urb->urb_list);
atomic_dec(&urb->use_count);
+ /*
+ * Order the write of urb->use_count above before the read
+ * of urb->reject below. Pairs with the memory barriers in
+ * usb_kill_urb() and usb_poison_urb().
+ */
+ smp_mb__after_atomic();
+
atomic_dec(&urb->dev->urbnum);
if (atomic_read(&urb->reject))
wake_up(&usb_kill_urb_queue);
@@ -1665,6 +1672,13 @@ static void __usb_hcd_giveback_urb(struc
usb_anchor_resume_wakeups(anchor);
atomic_dec(&urb->use_count);
+ /*
+ * Order the write of urb->use_count above before the read
+ * of urb->reject below. Pairs with the memory barriers in
+ * usb_kill_urb() and usb_poison_urb().
+ */
+ smp_mb__after_atomic();
+
if (unlikely(atomic_read(&urb->reject)))
wake_up(&usb_kill_urb_queue);
usb_put_urb(urb);
===================================================================
@@ -715,6 +715,12 @@ void usb_kill_urb(struct urb *urb)
if (!(urb && urb->dev && urb->ep))
return;
atomic_inc(&urb->reject);
+ /*
+ * Order the write of urb->reject above before the read
+ * of urb->use_count below. Pairs with the barriers in
+ * __usb_hcd_giveback_urb() and usb_hcd_submit_urb().
+ */
+ smp_mb__after_atomic();
usb_hcd_unlink_urb(urb, -ENOENT);
wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0);
@@ -756,6 +762,12 @@ void usb_poison_urb(struct urb *urb)
if (!urb)
return;
atomic_inc(&urb->reject);
+ /*
+ * Order the write of urb->reject above before the read
+ * of urb->use_count below. Pairs with the barriers in
+ * __usb_hcd_giveback_urb() and usb_hcd_submit_urb().
+ */
+ smp_mb__after_atomic();
if (!urb->dev || !urb->ep)
return;
The syzbot fuzzer has identified a bug in which processes hang waiting for usb_kill_urb() to return. It turns out the issue is not unlinking the URB; that works just fine. Rather, the problem arises when the wakeup notification that the URB has completed is not received. The reason is memory-access ordering on SMP systems. In outline form, usb_kill_urb() and __usb_hcd_giveback_urb() operating concurrently on different CPUs perform the following actions: CPU 0 CPU 1 ---------------------------- --------------------------------- usb_kill_urb(): __usb_hcd_giveback_urb(): ... ... atomic_inc(&urb->reject); atomic_dec(&urb->use_count); ... ... wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0); if (atomic_read(&urb->reject)) wake_up(&usb_kill_urb_queue); Confining your attention to urb->reject and urb->use_count, you can see that the overall pattern of accesses on CPU 0 is: write urb->reject, then read urb->use_count; whereas the overall pattern of accesses on CPU 1 is: write urb->use_count, then read urb->reject. This pattern is referred to in memory-model circles as SB (for "Store Buffering"), and it is well known that without suitable enforcement of the desired order of accesses -- in the form of memory barriers -- it is entirely possible for one or both CPUs to execute their reads ahead of their writes. The end result will be that sometimes CPU 0 sees the old un-decremented value of urb->use_count while CPU 1 sees the old un-incremented value of urb->reject. Consequently CPU 0 ends up on the wait queue and never gets woken up, leading to the observed hang in usb_kill_urb(). The same pattern of accesses occurs in usb_poison_urb() and the failure pathway of usb_hcd_submit_urb(). The problem is fixed by adding suitable memory barriers. To provide proper memory-access ordering in the SB pattern, a full barrier is required on both CPUs. The atomic_inc() and atomic_dec() accesses themselves don't provide any memory ordering, but since they are present, we can use the optimized smp_mb__after_atomic() memory barrier in the various routines to obtain the desired effect. This patch adds the necessary memory barriers. Reported-and-tested-by: syzbot+76629376e06e2c2ad626@syzkaller.appspotmail.com Signed-off-by: Alan Stern <stern@rowland.harvard.edu> CC: <stable@vger.kernel.org> --- [as1969] drivers/usb/core/hcd.c | 14 ++++++++++++++ drivers/usb/core/urb.c | 12 ++++++++++++ 2 files changed, 26 insertions(+)