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SubjectRe: [PATCH 1/1] kasan: fix object remain in offline per-cpu quarantine
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On Fri, 2020-11-13 at 08:03 +0100, Dmitry Vyukov wrote:
> On Fri, Nov 13, 2020 at 3:32 AM Kuan-Ying Lee
> <Kuan-Ying.Lee@mediatek.com> wrote:
> >
> > On Thu, 2020-11-12 at 09:39 +0100, Dmitry Vyukov wrote:
> > > On Thu, Nov 12, 2020 at 7:25 AM Kuan-Ying Lee
> > > <Kuan-Ying.Lee@mediatek.com> wrote:
> > > >
> > > > We hit this issue in our internal test.
> > > > When enabling generic kasan, a kfree()'d object is put into per-cpu
> > > > quarantine first. If the cpu goes offline, object still remains in
> > > > the per-cpu quarantine. If we call kmem_cache_destroy() now, slub
> > > > will report "Objects remaining" error.
> > > >
> > > > [ 74.982625] =============================================================================
> > > > [ 74.983380] BUG test_module_slab (Not tainted): Objects remaining in test_module_slab on __kmem_cache_shutdown()
> > > > [ 74.984145] -----------------------------------------------------------------------------
> > > > [ 74.984145]
> > > > [ 74.984883] Disabling lock debugging due to kernel taint
> > > > [ 74.985561] INFO: Slab 0x(____ptrval____) objects=34 used=1 fp=0x(____ptrval____) flags=0x2ffff00000010200
> > > > [ 74.986638] CPU: 3 PID: 176 Comm: cat Tainted: G B 5.10.0-rc1-00007-g4525c8781ec0-dirty #10
> > > > [ 74.987262] Hardware name: linux,dummy-virt (DT)
> > > > [ 74.987606] Call trace:
> > > > [ 74.987924] dump_backtrace+0x0/0x2b0
> > > > [ 74.988296] show_stack+0x18/0x68
> > > > [ 74.988698] dump_stack+0xfc/0x168
> > > > [ 74.989030] slab_err+0xac/0xd4
> > > > [ 74.989346] __kmem_cache_shutdown+0x1e4/0x3c8
> > > > [ 74.989779] kmem_cache_destroy+0x68/0x130
> > > > [ 74.990176] test_version_show+0x84/0xf0
> > > > [ 74.990679] module_attr_show+0x40/0x60
> > > > [ 74.991218] sysfs_kf_seq_show+0x128/0x1c0
> > > > [ 74.991656] kernfs_seq_show+0xa0/0xb8
> > > > [ 74.992059] seq_read+0x1f0/0x7e8
> > > > [ 74.992415] kernfs_fop_read+0x70/0x338
> > > > [ 74.993051] vfs_read+0xe4/0x250
> > > > [ 74.993498] ksys_read+0xc8/0x180
> > > > [ 74.993825] __arm64_sys_read+0x44/0x58
> > > > [ 74.994203] el0_svc_common.constprop.0+0xac/0x228
> > > > [ 74.994708] do_el0_svc+0x38/0xa0
> > > > [ 74.995088] el0_sync_handler+0x170/0x178
> > > > [ 74.995497] el0_sync+0x174/0x180
> > > > [ 74.996050] INFO: Object 0x(____ptrval____) @offset=15848
> > > > [ 74.996752] INFO: Allocated in test_version_show+0x98/0xf0 age=8188 cpu=6 pid=172
> > > > [ 75.000802] stack_trace_save+0x9c/0xd0
> > > > [ 75.002420] set_track+0x64/0xf0
> > > > [ 75.002770] alloc_debug_processing+0x104/0x1a0
> > > > [ 75.003171] ___slab_alloc+0x628/0x648
> > > > [ 75.004213] __slab_alloc.isra.0+0x2c/0x58
> > > > [ 75.004757] kmem_cache_alloc+0x560/0x588
> > > > [ 75.005376] test_version_show+0x98/0xf0
> > > > [ 75.005756] module_attr_show+0x40/0x60
> > > > [ 75.007035] sysfs_kf_seq_show+0x128/0x1c0
> > > > [ 75.007433] kernfs_seq_show+0xa0/0xb8
> > > > [ 75.007800] seq_read+0x1f0/0x7e8
> > > > [ 75.008128] kernfs_fop_read+0x70/0x338
> > > > [ 75.008507] vfs_read+0xe4/0x250
> > > > [ 75.008990] ksys_read+0xc8/0x180
> > > > [ 75.009462] __arm64_sys_read+0x44/0x58
> > > > [ 75.010085] el0_svc_common.constprop.0+0xac/0x228
> > > > [ 75.011006] kmem_cache_destroy test_module_slab: Slab cache still has objects
> > > >
> > > > Register a cpu hotplug function to remove all objects in the offline
> > > > per-cpu quarantine when cpu is going offline. Set a per-cpu variable
> > > > to indicate this cpu is offline.
> > > >
> > > > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> > > > ---
> > > > mm/kasan/quarantine.c | 59 +++++++++++++++++++++++++++++++++++++++++--
> > > > 1 file changed, 57 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
> > > > index 4c5375810449..67fb91ae2bd0 100644
> > > > --- a/mm/kasan/quarantine.c
> > > > +++ b/mm/kasan/quarantine.c
> > > > @@ -29,6 +29,7 @@
> > > > #include <linux/srcu.h>
> > > > #include <linux/string.h>
> > > > #include <linux/types.h>
> > > > +#include <linux/cpuhotplug.h>
> > > >
> > > > #include "../slab.h"
> > > > #include "kasan.h"
> > > > @@ -97,6 +98,7 @@ static void qlist_move_all(struct qlist_head *from, struct qlist_head *to)
> > > > * guarded by quarantine_lock.
> > > > */
> > >
> > > Hi Kuan-Ying,
> > >
> > > Thanks for fixing this.
> > >
> > > > static DEFINE_PER_CPU(struct qlist_head, cpu_quarantine);
> > > > +static DEFINE_PER_CPU(int, cpu_quarantine_offline);
> > >
> > > I think cpu_quarantine_offline is better be part of cpu_quarantine
> > > because it logically is and we already obtain a pointer to
> > > cpu_quarantine in quarantine_put, so it will also make the code a bit
> > > shorter.
> > >
> >
> > Ok. Got it.
> >
> > >
> > > > /* Round-robin FIFO array of batches. */
> > > > static struct qlist_head global_quarantine[QUARANTINE_BATCHES];
> > > > @@ -176,6 +178,8 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache)
> > > > unsigned long flags;
> > > > struct qlist_head *q;
> > > > struct qlist_head temp = QLIST_INIT;
> > > > + int *offline;
> > > > + struct qlist_head q_offline = QLIST_INIT;
> > > >
> > > > /*
> > > > * Note: irq must be disabled until after we move the batch to the
> > > > @@ -187,8 +191,16 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache)
> > > > */
> > > > local_irq_save(flags);
> > > >
> > > > - q = this_cpu_ptr(&cpu_quarantine);
> > > > - qlist_put(q, &info->quarantine_link, cache->size);
> > > > + offline = this_cpu_ptr(&cpu_quarantine_offline);
> > > > + if (*offline == 0) {
> > > > + q = this_cpu_ptr(&cpu_quarantine);
> > > > + qlist_put(q, &info->quarantine_link, cache->size);
> > > > + } else {
> > > > + qlist_put(&q_offline, &info->quarantine_link, cache->size);
> > > > + qlist_free_all(&q_offline, cache);
> > >
> > > This looks like a convoluted way to call qlink_free. I think it will
> > > be better to call qlink_free directly here.
> > >
> > > And why do we need this? Because CPU shutdown code can still free some
> > > objects afterwards?
> > >
> >
> > Yes, it is because IRQ can happen during CPU shutdown and free some
> > objects into offline CPU quarantine.
> >
> > > > + local_irq_restore(flags);
> > > > + return;
> > >
> > > You add both if/else and early return, this looks like unnecessary
> > > code complication. It would be simpler with:
> > >
> > > if (*offline) {
> > > qlink_free(...);
> > > return;
> > > }
> > > ... all current per-cpu local ...
> > >
> > >
> >
> > Thank you for reminder. v2 Will do it.
> >
> > > > + }
> > > > if (unlikely(q->bytes > QUARANTINE_PERCPU_SIZE)) {
> > > > qlist_move_all(q, &temp);
> > > >
> > > > @@ -328,3 +340,46 @@ void quarantine_remove_cache(struct kmem_cache *cache)
> > > >
> > > > synchronize_srcu(&remove_cache_srcu);
> > > > }
> > > > +
> > > > +static int kasan_cpu_online(unsigned int cpu)
> > > > +{
> > > > + int *offline;
> > > > + unsigned long flags;
> > > > +
> > > > + local_irq_save(flags);
> > >
> > > I assume this local_irq_save/restore is to prevent some warnings from
> > > this_cpu_ptr.
> > > But CPU online/offline callbacks should run without preemption already
> > > (preempting/rescheduling on other CPUs does not make sense for them,
> > > right?), so I would assume that is already at least preemption
> > > disabled or something. Is there this_cpu_ptr variant that won't
> > > produce warnings on its own in cpu online/offline callbacks?
> > > This whole function could be a 1-liner:
> > > this_cpu_ptr(&cpu_quarantine)->offline = true;
> > > So I am trying to understand if we could avoid all this unnecessary danse.
> > >
> >
> > Yes, it's unnecessary. v2 will fix it.
> >
> > >
> > > > + offline = this_cpu_ptr(&cpu_quarantine_offline);
> > > > + *offline = 0;
> > > > + local_irq_restore(flags);
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static int kasan_cpu_offline(unsigned int cpu)
> > > > +{
> > > > + struct kmem_cache *s;
> > > > + int *offline;
> > > > + unsigned long flags;
> > > > +
> > > > + local_irq_save(flags);
> > > > + offline = this_cpu_ptr(&cpu_quarantine_offline);
> > > > + *offline = 1;
> > > > + local_irq_restore(flags);
> > > > +
> > > > + mutex_lock(&slab_mutex);
> > > > + list_for_each_entry(s, &slab_caches, list) {
> > > > + per_cpu_remove_cache(s);
> > > > + }
> > > > + mutex_unlock(&slab_mutex);
> > >
> > > We just want to drop the whole per-cpu cache at once, right? I would
> > > assume there should be a simpler way to do this all at once, rather
> > > than doing this per-slab.
> > >
> >
> > Yes.
> > Is removing objects in per-cpu quarantine directly better?
>
> Yes, single qlist_free_all call looks much better than iteration over
> all slabs and removing in parts under the mutex.
>

Hi Dmitry,

Got it.
Thanks for your review and reminder.
I will fix them in v2.

Kuan-Ying


> > struct qlist_head *q;
> > q = this_cpu_ptr(&cpu_quaratine);
> > q->offline = true;
> > qlist_free_all(q, NULL);
> >
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static int __init kasan_cpu_offline_quarantine_init(void)
> > > > +{
> > > > + int ret = 0;
> > > > +
> > > > + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/kasan:online",
> > > > + kasan_cpu_online, kasan_cpu_offline);
> > > > + if (ret)
> > > > + pr_err("kasan offline cpu quarantine register failed [%d]\n", ret);
> > > > + return ret;
> > > > +}
> > > > +late_initcall(kasan_cpu_offline_quarantine_init);
> > > > --
> > > > 2.18.0
> > > >
> > > > --
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> >
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