Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: UBSAN: shift-out-of-bounds in load_balance | Date | Tue, 23 Feb 2021 15:51:29 +0000 |
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On 23/02/21 14:45, Vincent Guittot wrote: > On Tue, 23 Feb 2021 at 13:03, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> >> >> +Vincent >> >> On 22/02/21 09:12, syzbot wrote: >> > syzbot has found a reproducer for the following issue on: >> > >> > HEAD commit: 31caf8b2 Merge branch 'linus' of git://git.kernel.org/pub/.. >> > git tree: upstream >> > console output: https://syzkaller.appspot.com/x/log.txt?x=16ab2682d00000 >> > kernel config: https://syzkaller.appspot.com/x/.config?x=b81388f0b32761d4 >> > dashboard link: https://syzkaller.appspot.com/bug?extid=d7581744d5fd27c9fbe1 >> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1277457f500000 >> > >> > IMPORTANT: if you fix the issue, please add the following tag to the commit: >> > Reported-by: syzbot+d7581744d5fd27c9fbe1@syzkaller.appspotmail.com >> > >> > ================================================================================ >> > UBSAN: shift-out-of-bounds in kernel/sched/fair.c:7712:14 >> > shift exponent 149 is too large for 64-bit type 'long unsigned int' >> >> That 149 is surprising. > > Yes, surprising. But is it really a problem in itself ? shifting left > would be a problem because of the overflow but here we shift right to > divide and the result is correct >
I would tend to think so, but the UB seems to apply regardless of the shifting direction:
""" If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined. """
> Beside this, it seems that a significant number of previous attempts > to balance load has been done with another migration_type otherwise it > would have raised a problem earlier (at 65) if previous LB were also > migration_load. It would be good to understand why the 148 previous > ones failed > >> >> sd->cache_nice_tries is \in {1, 2}, and sd->nr_balanced_failed should be in >> the same ballpark. >> >> A successful load_balance() resets it to 0; a failed one increments >> it. Once it gets to sd->cache_nice_tries + 3, this should trigger an active >> balance, which will either set it to sd->cache_nice_tries+1 or reset it to >> 0. There is this one condition that could let it creep up uncontrollably: >> >> /* >> * Don't kick the active_load_balance_cpu_stop, >> * if the curr task on busiest CPU can't be >> * moved to this_cpu: >> */ >> if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) { >> raw_spin_unlock_irqrestore(&busiest->lock, >> flags); >> goto out_one_pinned; >> } >> >> So despite the resulting sd->balance_interval increase, repeatedly hitting >> this might yield the above. Would we then want something like this? >> >> --- >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 8a8bd7b13634..b65c24b5ae91 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -7422,6 +7422,11 @@ struct lb_env { >> struct list_head tasks; >> }; >> >> +static inline unsigned int sd_balance_failed_cap(struct sched_domain *sd) >> +{ >> + return sd->cache_nice_tries + 3; >> +} >> + >> /* >> * Is this task likely cache-hot: >> */ >> @@ -9493,7 +9498,7 @@ imbalanced_active_balance(struct lb_env *env) >> * threads on a system with spare capacity >> */ >> if ((env->migration_type == migrate_task) && >> - (sd->nr_balance_failed > sd->cache_nice_tries+2)) >> + (sd->nr_balance_failed >= sd_balance_failed_cap(sd))) >> return 1; >> >> return 0; >> @@ -9737,8 +9742,10 @@ static int load_balance(int this_cpu, struct rq *this_rq, >> * frequent, pollute the failure counter causing >> * excessive cache_hot migrations and active balances. >> */ >> - if (idle != CPU_NEWLY_IDLE) >> - sd->nr_balance_failed++; >> + if (idle != CPU_NEWLY_IDLE) { >> + sd->nr_balance_failed = min(sd->nr_balance_failed + 1, >> + sd_balance_failed_cap(sd)); > > nr_balance_failed is an interesting metric that we want to monitor > sometimes and we would like to be able to divide higher than > 2^(sd->cache_nice_tries + 3). > > If we really want to prevent out of bound shift, The below is more > appropriate IMO: > > index 636741fa27c9..4d0b3fa30849 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7707,7 +7707,7 @@ static int detach_tasks(struct lb_env *env) > * migrate. > */ > > - if ((load >> env->sd->nr_balance_failed) > > env->imbalance) > + if ((load >> min_t(int, > env->sd->nr_balance_failed, BITS_PER_LONG)) > env->imbalance) > goto next; >
From the UB definition above, sounds like we need to cap at
BITS_PER_TYPE(unsigned long) - 1
i.e. something like
#define shr_bound(val, shift) \ (val >> min_t(int, shift, BITS_PER_TYPE(val) - 1))
> env->imbalance -= load; > > >> + } >> >> if (need_active_balance(&env)) { >> unsigned long flags; >>
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