From: Juergen Gross jgross@suse.com
mainline inclusion from mainline-v5.10 commit f01337197419b7e8a492e83089552b77d3b5fb90 category: bugfix bugzilla: NA CVE: CVE-2020-27673
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Unmasking a fifo event channel can result in unmasking it twice, once directly in the kernel and once via a hypercall in case the event was pending.
Fix that by doing the local unmask only if the event is not pending.
This is part of XSA-332.
Cc: stable@vger.kernel.org Signed-off-by: Juergen Gross jgross@suse.com Reviewed-by: Jan Beulich jbeulich@suse.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com Reviewed-by: Jason Yan yanaijie@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- drivers/xen/events/events_fifo.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/drivers/xen/events/events_fifo.c b/drivers/xen/events/events_fifo.c index 9e460ccf38d2..33462521bfd0 100644 --- a/drivers/xen/events/events_fifo.c +++ b/drivers/xen/events/events_fifo.c @@ -227,19 +227,25 @@ static bool evtchn_fifo_is_masked(unsigned port) return sync_test_bit(EVTCHN_FIFO_BIT(MASKED, word), BM(word)); } /* - * Clear MASKED, spinning if BUSY is set. + * Clear MASKED if not PENDING, spinning if BUSY is set. + * Return true if mask was cleared. */ -static void clear_masked(volatile event_word_t *word) +static bool clear_masked_cond(volatile event_word_t *word) { event_word_t new, old, w;
w = *word;
do { + if (w & (1 << EVTCHN_FIFO_PENDING)) + return false; + old = w & ~(1 << EVTCHN_FIFO_BUSY); new = old & ~(1 << EVTCHN_FIFO_MASKED); w = sync_cmpxchg(word, old, new); } while (w != old); + + return true; }
static void evtchn_fifo_unmask(unsigned port) @@ -248,8 +254,7 @@ static void evtchn_fifo_unmask(unsigned port)
BUG_ON(!irqs_disabled());
- clear_masked(word); - if (evtchn_fifo_is_pending(port)) { + if (!clear_masked_cond(word)) { struct evtchn_unmask unmask = { .port = port }; (void)HYPERVISOR_event_channel_op(EVTCHNOP_unmask, &unmask); }