[PATCH v2] um: time-travel: fix time corruption

From: Johannes Berg <johannes@sipsolutions.net>
To: linux-um@lists.infradead.org
Cc: Johannes Berg <johannes.berg@intel.com>,
	Vincent Whitchurch <Vincent.Whitchurch@axis.com>
Subject: [PATCH v2] um: time-travel: fix time corruption
Date: Wed, 25 Oct 2023 22:45:05 +0200	[thread overview]
Message-ID: <20231025224504.7be512cdfeb4.Ia20103ef9899e53311bd671b8cfbca4f057a4668@changeid> (raw)

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

In 'basic' time-travel mode (without =inf-cpu or =ext), we
still get timer interrupts. These can happen at arbitrary
points in time, i.e. while in timer_read(), which pushes
time forward just a little bit. Then, if we happen to get
the interrupt after calculating the new time to push to,
but before actually finishing that, the interrupt will set
the time to a value that's incompatible with the forward,
and we'll crash because time goes backwards when we do the
forwarding.

Fix this by reading the time_travel_time, calculating the
adjustment, and doing the adjustment all with interrupts
disabled.

Reported-by: Vincent Whitchurch <Vincent.Whitchurch@axis.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
v2: remove stray debug code
---
 arch/um/kernel/time.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index 8ff46bc86d09..0c01674e14d5 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -551,9 +551,29 @@ static void time_travel_update_time(unsigned long long next, bool idle)
 	time_travel_del_event(&ne);
 }
 
+static void time_travel_update_time_rel(unsigned long long offs)
+{
+	unsigned long flags;
+
+	/*
+	 * Disable interrupts before calculating the new time so
+	 * that a real timer interrupt (signal) can't happen at
+	 * a bad time e.g. after we read time_travel_time but
+	 * before we've completed updating the time.
+	 */
+	local_irq_save(flags);
+	time_travel_update_time(time_travel_time + offs, false);
+	local_irq_restore(flags);
+}
+
 void time_travel_ndelay(unsigned long nsec)
 {
-	time_travel_update_time(time_travel_time + nsec, false);
+	/*
+	 * Not strictly needed to use _rel() version since this is
+	 * only used in INFCPU/EXT modes, but it doesn't hurt and
+	 * is more readable too.
+	 */
+	time_travel_update_time_rel(nsec);
 }
 EXPORT_SYMBOL(time_travel_ndelay);
 
@@ -687,7 +707,11 @@ static void time_travel_set_start(void)
 #define time_travel_time 0
 #define time_travel_ext_waiting 0
 
-static inline void time_travel_update_time(unsigned long long ns, bool retearly)
+static inline void time_travel_update_time(unsigned long long ns, bool idle)
+{
+}
+
+static inline void time_travel_update_time_rel(unsigned long long offs)
 {
 }
 
@@ -839,9 +863,7 @@ static u64 timer_read(struct clocksource *cs)
 		 */
 		if (!irqs_disabled() && !in_interrupt() && !in_softirq() &&
 		    !time_travel_ext_waiting)
-			time_travel_update_time(time_travel_time +
-						TIMER_MULTIPLIER,
-						false);
+			time_travel_update_time_rel(TIMER_MULTIPLIER);
 		return time_travel_time / TIMER_MULTIPLIER;
 	}
 
-- 
2.41.0


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