Re: [PATCH v2 1/1] kasan: fix object remain in offline per-cpu quarantine

From: Kuan-Ying Lee
Date: Tue Nov 17 2020 - 01:46:39 EST

On Mon, 2020-11-16 at 10:26 +0100, Dmitry Vyukov wrote:
> On Mon, Nov 16, 2020 at 7:30 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>
> > Suggested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Reported-by: Guangye Yang <guangye.yang@xxxxxxxxxxxx>
> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> > ---
> > mm/kasan/quarantine.c | 35 +++++++++++++++++++++++++++++++++++
> > 1 file changed, 35 insertions(+)
> >
> > diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
> > index 4c5375810449..16e618ea805e 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"
> > @@ -43,6 +44,7 @@ struct qlist_head {
> > struct qlist_node *head;
> > struct qlist_node *tail;
> > size_t bytes;
> > + bool offline;
> > };
> >
> > #define QLIST_INIT { NULL, NULL, 0 }
> > @@ -188,6 +190,11 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache)
> > local_irq_save(flags);
> >
> > q = this_cpu_ptr(&cpu_quarantine);
> > + if (q->offline) {
> > + qlink_free(&info->quarantine_link, cache);
> > + local_irq_restore(flags);
> > + return;
> > + }

I think we need to make sure objects will not be put in per-cpu
quarantine which is offline.

> > qlist_put(q, &info->quarantine_link, cache->size);
> > if (unlikely(q->bytes > QUARANTINE_PERCPU_SIZE)) {
> > qlist_move_all(q, &temp);
> > @@ -328,3 +335,31 @@ void quarantine_remove_cache(struct kmem_cache *cache)
> >
> > synchronize_srcu(&remove_cache_srcu);
> > }
> > +
> > +static int kasan_cpu_online(unsigned int cpu)
> > +{
> > + this_cpu_ptr(&cpu_quarantine)->offline = false;
> > + return 0;
> > +}
> > +
> > +static int kasan_cpu_offline(unsigned int cpu)
> > +{
> > + struct qlist_head *q;
> > +
> > + q = this_cpu_ptr(&cpu_quarantine);
> > + q->offline = true;
> > + qlist_free_all(q, NULL);
>
> Looks much nicer now!
>
> What is the story with interrupts in these callbacks?
> In the previous patch you mentioned that this CPU can still receive
> interrupts for a brief period of time. If these interrupts also free
> something, can't we corrupt the per-cpu quarantine? In quarantine_put
> we protect it by disabling interrupts I think.
>

Here is a situation.
After we freed all objects from the per-cpu quarantine which is going
offline, the interrupts happened. These interrupts free something and
put objects into this per-cpu quarantine. If we call
kmem_cache_destroy() now, slub still detect objects remain in
the per-cpu quarantine and report "Object remaining" error.

Thus, we need to check q->offline in quarantine_put and make sure
the offline per-cpu quarantine is not corrupted.

>
> > + 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 < 0)
> > + pr_err("kasan offline cpu quarantine register failed [%d]\n", ret);
> > + return ret;
> > +}
> > +late_initcall(kasan_cpu_offline_quarantine_init);
> > --
> > 2.18.0