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um: time-travel: fix time corruption

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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>
This commit is contained in:
Johannes Berg 2023-10-25 22:45:05 +02:00 committed by Richard Weinberger
parent 7d748f60a4
commit abe4eaa861
1 changed files with 27 additions and 5 deletions
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32
arch/um/kernel/time.c
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@ -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;
}