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26fbe9772b
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> |
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| .. | ||
| buffer.c | ||
| config.c | ||
| devices.c | ||
| devio.c | ||
| driver.c | ||
| endpoint.c | ||
| file.c | ||
| generic.c | ||
| hcd-pci.c | ||
| hcd.c | ||
| hub.c | ||
| hub.h | ||
| Kconfig | ||
| ledtrig-usbport.c | ||
| Makefile | ||
| message.c | ||
| notify.c | ||
| of.c | ||
| otg_productlist.h | ||
| phy.c | ||
| phy.h | ||
| port.c | ||
| quirks.c | ||
| sysfs.c | ||
| urb.c | ||
| usb-acpi.c | ||
| usb.c | ||
| usb.h | ||