[PATCH AUTOSEL 6.7 06/13] um: time-travel: fix time corruption

Sasha Levin
2024-01-24T14:28:39Z

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

[ Upstream commit abe4eaa8618bb36c2b33e9cdde0499296a23448c ]

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>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 fddd1dec27e6..3e270da6b6f6 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -432,9 +432,29 @@ 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);

@@ -568,7 +588,11 @@ 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)
{
}

@@ -720,9 +744,7 @@ 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;
}

--
2.43.0