Re: [PATCH 1/1] kasan: fix object remain in offline per-cpu quarantine

[Kuan-Ying Lee]

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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > ---
> >  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?

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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