linux-kernel - [PATCH 5.14 092/172] mac80211-hwsim: fix late beacon hrtimer handling

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Date:   Mon,  4 Oct 2021 14:52:22 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+0e964fad69a9c462bc1e@...kaller.appspotmail.com,
        Johannes Berg <johannes.berg@...el.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.14 092/172] mac80211-hwsim: fix late beacon hrtimer handling

From: Johannes Berg <johannes.berg@...el.com>

[ Upstream commit 313bbd1990b6ddfdaa7da098d0c56b098a833572 ]

Thomas explained in https://lore.kernel.org/r/87mtoeb4hb.ffs@tglx
that our handling of the hrtimer here is wrong: If the timer fires
late (e.g. due to vCPU scheduling, as reported by Dmitry/syzbot)
then it tries to actually rearm the timer at the next deadline,
which might be in the past already:

 1          2          3          N          N+1
 |          |          |   ...    |          |

 ^ intended to fire here (1)
            ^ next deadline here (2)
                                      ^ actually fired here

The next time it fires, it's later, but will still try to schedule
for the next deadline (now 3), etc. until it catches up with N,
but that might take a long time, causing stalls etc.

Now, all of this is simulation, so we just have to fix it, but
note that the behaviour is wrong even per spec, since there's no
value then in sending all those beacons unaligned - they should be
aligned to the TBTT (1, 2, 3, ... in the picture), and if we're a
bit (or a lot) late, then just resume at that point.

Therefore, change the code to use hrtimer_forward_now() which will
ensure that the next firing of the timer would be at N+1 (in the
picture), i.e. the next interval point after the current time.

Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Reported-by: syzbot+0e964fad69a9c462bc1e@...kaller.appspotmail.com
Fixes: 01e59e467ecf ("mac80211_hwsim: hrtimer beacon")
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lore.kernel.org/r/20210915112936.544f383472eb.I3f9712009027aa09244b65399bf18bf482a8c4f1@changeid
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/net/wireless/mac80211_hwsim.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/mac80211_hwsim.c b/drivers/net/wireless/mac80211_hwsim.c
index ffa894f7312a..0adae76eb8df 100644
--- a/drivers/net/wireless/mac80211_hwsim.c
+++ b/drivers/net/wireless/mac80211_hwsim.c
@@ -1867,8 +1867,8 @@ mac80211_hwsim_beacon(struct hrtimer *timer)
 		bcn_int -= data->bcn_delta;
 		data->bcn_delta = 0;
 	}
-	hrtimer_forward(&data->beacon_timer, hrtimer_get_expires(timer),
-			ns_to_ktime(bcn_int * NSEC_PER_USEC));
+	hrtimer_forward_now(&data->beacon_timer,
+			    ns_to_ktime(bcn_int * NSEC_PER_USEC));
 	return HRTIMER_RESTART;
 }

-- 
2.33.0