Re: [PATCH v2] xen/events: Fix race in set_evtchn_to_irq

[Boris Ostrovsky]

On 8/12/21 9:09 AM, Maximilian Heyne wrote:
> There is a TOCTOU issue in set_evtchn_to_irq. Rows in the evtchn_to_irq
> mapping are lazily allocated in this function. The check whether the row
> is already present and the row initialization is not synchronized. Two
> threads can at the same time allocate a new row for evtchn_to_irq and
> add the irq mapping to the their newly allocated row. One thread will
> overwrite what the other has set for evtchn_to_irq[row] and therefore
> the irq mapping is lost. This will trigger a BUG_ON later in
> bind_evtchn_to_cpu:
>
>   INFO: pci 0000:1a:15.4: [1d0f:8061] type 00 class 0x010802
>   INFO: nvme 0000:1a:12.1: enabling device (0000 -> 0002)
>   INFO: nvme nvme77: 1/0/0 default/read/poll queues
>   CRIT: kernel BUG at drivers/xen/events/events_base.c:427!
>   WARN: invalid opcode: 0000 [#1] SMP NOPTI
>   WARN: Workqueue: nvme-reset-wq nvme_reset_work [nvme]
>   WARN: RIP: e030:bind_evtchn_to_cpu+0xc2/0xd0
>   WARN: Call Trace:
>   WARN:  set_affinity_irq+0x121/0x150
>   WARN:  irq_do_set_affinity+0x37/0xe0
>   WARN:  irq_setup_affinity+0xf6/0x170
>   WARN:  irq_startup+0x64/0xe0
>   WARN:  __setup_irq+0x69e/0x740
>   WARN:  ? request_threaded_irq+0xad/0x160
>   WARN:  request_threaded_irq+0xf5/0x160
>   WARN:  ? nvme_timeout+0x2f0/0x2f0 [nvme]
>   WARN:  pci_request_irq+0xa9/0xf0
>   WARN:  ? pci_alloc_irq_vectors_affinity+0xbb/0x130
>   WARN:  queue_request_irq+0x4c/0x70 [nvme]
>   WARN:  nvme_reset_work+0x82d/0x1550 [nvme]
>   WARN:  ? check_preempt_wakeup+0x14f/0x230
>   WARN:  ? check_preempt_curr+0x29/0x80
>   WARN:  ? nvme_irq_check+0x30/0x30 [nvme]
>   WARN:  process_one_work+0x18e/0x3c0
>   WARN:  worker_thread+0x30/0x3a0
>   WARN:  ? process_one_work+0x3c0/0x3c0
>   WARN:  kthread+0x113/0x130
>   WARN:  ? kthread_park+0x90/0x90
>   WARN:  ret_from_fork+0x3a/0x50
>
> This patch sets evtchn_to_irq rows via a cmpxchg operation so that they
> will be set only once. The row is now cleared before writing it to
> evtchn_to_irq in order to not create a race once the row is visible for
> other threads.
>
> While at it, do not require the page to be zeroed, because it will be
> overwritten with -1's in clear_evtchn_to_irq_row anyway.
>
> Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
> Fixes: d0b075ffeede ("xen/events: Refactor evtchn_to_irq array to be dynamically allocated")


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>