[PATCH 4.19 16/86] USB: core: Fix hang in usb_kill_urb by adding memory barriers

Greg Kroah-Hartman
2022-02-07T11:23:15Z

From: Alan Stern <stern@rowland.harvard.edu>

commit 26fbe9772b8c459687930511444ce443011f86bf upstream.

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.

CC: <stable@vger.kernel.org>
Reported-and-tested-by: syzbot+76629376e06e2c2ad626@syzkaller.appspotmail.com
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/Ye8K0QYee0Q0Nna2@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/usb/core/hcd.c | 14 ++++++++++++++
drivers/usb/core/urb.c | 12 ++++++++++++
2 files changed, 26 insertions(+)

--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1670,6 +1670,13 @@ 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);
@@ -1779,6 +1786,13 @@ 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);
--- a/drivers/usb/core/urb.c
+++ b/drivers/usb/core/urb.c
@@ -692,6 +692,12 @@ 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);
@@ -733,6 +739,12 @@ 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;