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authorJuergen Gross <jgross@suse.com>2020-10-20 07:52:55 +0300
committerJuergen Gross <jgross@suse.com>2020-10-20 11:21:58 +0300
commitf01337197419b7e8a492e83089552b77d3b5fb90 (patch)
treed49649615f31a412be79528e3c3deb7985a8eb87
parent4d3fe31bd993ef504350989786858aefdb877daa (diff)
downloadlinux-f01337197419b7e8a492e83089552b77d3b5fb90.tar.xz
xen/events: fix race in evtchn_fifo_unmask()
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>
-rw-r--r--drivers/xen/events/events_fifo.c13
1 files changed, 9 insertions, 4 deletions
diff --git a/drivers/xen/events/events_fifo.c b/drivers/xen/events/events_fifo.c
index c60ee0450173..7fd39c64d4b5 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(evtchn_port_t 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(evtchn_port_t port)
@@ -248,8 +254,7 @@ static void evtchn_fifo_unmask(evtchn_port_t 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);
}