Re: [PATCH v4 2/4] kasan: record and print the free track

From: Dmitry Vyukov
Date: Wed May 20 2020 - 07:46:39 EST

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On Wed, May 20, 2020 at 1:42 PM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> > > > > > > > > > > On Tue, May 19, 2020 at 4:25 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > Move free track from slub alloc meta-data to slub free meta-data in
> > > > > > > > > > > > order to make struct kasan_free_meta size is 16 bytes. It is a good
> > > > > > > > > > > > size because it is the minimal redzone size and a good number of
> > > > > > > > > > > > alignment.
> > > > > > > > > > > >
> > > > > > > > > > > > For free track in generic KASAN, we do the modification in struct
> > > > > > > > > > > > kasan_alloc_meta and kasan_free_meta:
> > > > > > > > > > > > - remove free track from kasan_alloc_meta.
> > > > > > > > > > > > - add free track into kasan_free_meta.
> > > > > > > > > > > >
> > > > > > > > > > > > [1]https://bugzilla.kernel.org/show_bug.cgi?id=198437
> > > > > > > > > > > >
> > > > > > > > > > > > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > > > > > > > > > > > Suggested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > > > > > > > > > > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > > > > > > > > > > > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > > > > > > > > > > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > > > > > > > > > > > ---
> > > > > > > > > > > > mm/kasan/common.c | 22 ++--------------------
> > > > > > > > > > > > mm/kasan/generic.c | 18 ++++++++++++++++++
> > > > > > > > > > > > mm/kasan/kasan.h | 7 +++++++
> > > > > > > > > > > > mm/kasan/report.c | 20 --------------------
> > > > > > > > > > > > mm/kasan/tags.c | 37 +++++++++++++++++++++++++++++++++++++
> > > > > > > > > > > > 5 files changed, 64 insertions(+), 40 deletions(-)
> > > > > > > > > > > >
> > > > > > > > > > > > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > > > > > > > > > > > index 8bc618289bb1..47b53912f322 100644
> > > > > > > > > > > > --- a/mm/kasan/common.c
> > > > > > > > > > > > +++ b/mm/kasan/common.c
> > > > > > > > > > > > @@ -51,7 +51,7 @@ depot_stack_handle_t kasan_save_stack(gfp_t flags)
> > > > > > > > > > > > return stack_depot_save(entries, nr_entries, flags);
> > > > > > > > > > > > }
> > > > > > > > > > > >
> > > > > > > > > > > > -static inline void set_track(struct kasan_track *track, gfp_t flags)
> > > > > > > > > > > > +void kasan_set_track(struct kasan_track *track, gfp_t flags)
> > > > > > > > > > > > {
> > > > > > > > > > > > track->pid = current->pid;
> > > > > > > > > > > > track->stack = kasan_save_stack(flags);
> > > > > > > > > > > > @@ -299,24 +299,6 @@ struct kasan_free_meta *get_free_info(struct kmem_cache *cache,
> > > > > > > > > > > > return (void *)object + cache->kasan_info.free_meta_offset;
> > > > > > > > > > > > }
> > > > > > > > > > > >
> > > > > > > > > > > > -
> > > > > > > > > > > > -static void kasan_set_free_info(struct kmem_cache *cache,
> > > > > > > > > > > > - void *object, u8 tag)
> > > > > > > > > > > > -{
> > > > > > > > > > > > - struct kasan_alloc_meta *alloc_meta;
> > > > > > > > > > > > - u8 idx = 0;
> > > > > > > > > > > > -
> > > > > > > > > > > > - alloc_meta = get_alloc_info(cache, object);
> > > > > > > > > > > > -
> > > > > > > > > > > > -#ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> > > > > > > > > > > > - idx = alloc_meta->free_track_idx;
> > > > > > > > > > > > - alloc_meta->free_pointer_tag[idx] = tag;
> > > > > > > > > > > > - alloc_meta->free_track_idx = (idx + 1) % KASAN_NR_FREE_STACKS;
> > > > > > > > > > > > -#endif
> > > > > > > > > > > > -
> > > > > > > > > > > > - set_track(&alloc_meta->free_track[idx], GFP_NOWAIT);
> > > > > > > > > > > > -}
> > > > > > > > > > > > -
> > > > > > > > > > > > void kasan_poison_slab(struct page *page)
> > > > > > > > > > > > {
> > > > > > > > > > > > unsigned long i;
> > > > > > > > > > > > @@ -492,7 +474,7 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
> > > > > > > > > > > > KASAN_KMALLOC_REDZONE);
> > > > > > > > > > > >
> > > > > > > > > > > > if (cache->flags & SLAB_KASAN)
> > > > > > > > > > > > - set_track(&get_alloc_info(cache, object)->alloc_track, flags);
> > > > > > > > > > > > + kasan_set_track(&get_alloc_info(cache, object)->alloc_track, flags);
> > > > > > > > > > > >
> > > > > > > > > > > > return set_tag(object, tag);
> > > > > > > > > > > > }
> > > > > > > > > > > > diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> > > > > > > > > > > > index 3372bdcaf92a..763d8a13e0ac 100644
> > > > > > > > > > > > --- a/mm/kasan/generic.c
> > > > > > > > > > > > +++ b/mm/kasan/generic.c
> > > > > > > > > > > > @@ -344,3 +344,21 @@ void kasan_record_aux_stack(void *addr)
> > > > > > > > > > > > alloc_info->aux_stack[1] = alloc_info->aux_stack[0];
> > > > > > > > > > > > alloc_info->aux_stack[0] = kasan_save_stack(GFP_NOWAIT);
> > > > > > > > > > > > }
> > > > > > > > > > > > +
> > > > > > > > > > > > +void kasan_set_free_info(struct kmem_cache *cache,
> > > > > > > > > > > > + void *object, u8 tag)
> > > > > > > > > > > > +{
> > > > > > > > > > > > + struct kasan_free_meta *free_meta;
> > > > > > > > > > > > +
> > > > > > > > > > > > + free_meta = get_free_info(cache, object);
> > > > > > > > > > > > + kasan_set_track(&free_meta->free_track, GFP_NOWAIT);
> > > > > > > > > > > > +}
> > > > > > > > > > > > +
> > > > > > > > > > > > +struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > > > > > > > > > > > + void *object, u8 tag)
> > > > > > > > > > > > +{
> > > > > > > > > > > > + struct kasan_free_meta *free_meta;
> > > > > > > > > > > > +
> > > > > > > > > > > > + free_meta = get_free_info(cache, object);
> > > > > > > > > > > > + return &free_meta->free_track;
> > > > > > > > > > > > +}
> > > > > > > > > > > > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > > > > > > > > > > > index a7391bc83070..ad897ec36545 100644
> > > > > > > > > > > > --- a/mm/kasan/kasan.h
> > > > > > > > > > > > +++ b/mm/kasan/kasan.h
> > > > > > > > > > > > @@ -127,6 +127,9 @@ struct kasan_free_meta {
> > > > > > > > > > > > * Otherwise it might be used for the allocator freelist.
> > > > > > > > > > > > */
> > > > > > > > > > > > struct qlist_node quarantine_link;
> > > > > > > > > > > > +#ifdef CONFIG_KASAN_GENERIC
> > > > > > > > > > > > + struct kasan_track free_track;
> > > > > > > > > > > > +#endif
> > > > > > > > > > > > };
> > > > > > > > > > > >
> > > > > > > > > > > > struct kasan_alloc_meta *get_alloc_info(struct kmem_cache *cache,
> > > > > > > > > > > > @@ -168,6 +171,10 @@ void kasan_report_invalid_free(void *object, unsigned long ip);
> > > > > > > > > > > > struct page *kasan_addr_to_page(const void *addr);
> > > > > > > > > > > >
> > > > > > > > > > > > depot_stack_handle_t kasan_save_stack(gfp_t flags);
> > > > > > > > > > > > +void kasan_set_track(struct kasan_track *track, gfp_t flags);
> > > > > > > > > > > > +void kasan_set_free_info(struct kmem_cache *cache, void *object, u8 tag);
> > > > > > > > > > > > +struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > > > > > > > > > > > + void *object, u8 tag);
> > > > > > > > > > > >
> > > > > > > > > > > > #if defined(CONFIG_KASAN_GENERIC) && \
> > > > > > > > > > > > (defined(CONFIG_SLAB) || defined(CONFIG_SLUB))
> > > > > > > > > > > > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > > > > > > > > > > > index 6f8f2bf8f53b..96d2657fe70f 100644
> > > > > > > > > > > > --- a/mm/kasan/report.c
> > > > > > > > > > > > +++ b/mm/kasan/report.c
> > > > > > > > > > > > @@ -159,26 +159,6 @@ static void describe_object_addr(struct kmem_cache *cache, void *object,
> > > > > > > > > > > > (void *)(object_addr + cache->object_size));
> > > > > > > > > > > > }
> > > > > > > > > > > >
> > > > > > > > > > > > -static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > > > > > > > > > > > - void *object, u8 tag)
> > > > > > > > > > > > -{
> > > > > > > > > > > > - struct kasan_alloc_meta *alloc_meta;
> > > > > > > > > > > > - int i = 0;
> > > > > > > > > > > > -
> > > > > > > > > > > > - alloc_meta = get_alloc_info(cache, object);
> > > > > > > > > > > > -
> > > > > > > > > > > > -#ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> > > > > > > > > > > > - for (i = 0; i < KASAN_NR_FREE_STACKS; i++) {
> > > > > > > > > > > > - if (alloc_meta->free_pointer_tag[i] == tag)
> > > > > > > > > > > > - break;
> > > > > > > > > > > > - }
> > > > > > > > > > > > - if (i == KASAN_NR_FREE_STACKS)
> > > > > > > > > > > > - i = alloc_meta->free_track_idx;
> > > > > > > > > > > > -#endif
> > > > > > > > > > > > -
> > > > > > > > > > > > - return &alloc_meta->free_track[i];
> > > > > > > > > > > > -}
> > > > > > > > > > > > -
> > > > > > > > > > > > #ifdef CONFIG_KASAN_GENERIC
> > > > > > > > > > > > static void print_stack(depot_stack_handle_t stack)
> > > > > > > > > > > > {
> > > > > > > > > > > > diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
> > > > > > > > > > > > index 25b7734e7013..201dee5d6ae0 100644
> > > > > > > > > > > > --- a/mm/kasan/tags.c
> > > > > > > > > > > > +++ b/mm/kasan/tags.c
> > > > > > > > > > > > @@ -162,3 +162,40 @@ void __hwasan_tag_memory(unsigned long addr, u8 tag, unsigned long size)
> > > > > > > > > > > > kasan_poison_shadow((void *)addr, size, tag);
> > > > > > > > > > > > }
> > > > > > > > > > > > EXPORT_SYMBOL(__hwasan_tag_memory);
> > > > > > > > > > > > +
> > > > > > > > > > > > +void kasan_set_free_info(struct kmem_cache *cache,
> > > > > > > > > > > > + void *object, u8 tag)
> > > > > > > > > > > > +{
> > > > > > > > > > > > + struct kasan_alloc_meta *alloc_meta;
> > > > > > > > > > > > + u8 idx = 0;
> > > > > > > > > > > > +
> > > > > > > > > > > > + alloc_meta = get_alloc_info(cache, object);
> > > > > > > > > > > > +
> > > > > > > > > > > > +#ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> > > > > > > > > > > > + idx = alloc_meta->free_track_idx;
> > > > > > > > > > > > + alloc_meta->free_pointer_tag[idx] = tag;
> > > > > > > > > > > > + alloc_meta->free_track_idx = (idx + 1) % KASAN_NR_FREE_STACKS;
> > > > > > > > > > > > +#endif
> > > > > > > > > > > > +
> > > > > > > > > > > > + kasan_set_track(&alloc_meta->free_track[idx], GFP_NOWAIT);
> > > > > > > > > > > > +}
> > > > > > > > > > > > +
> > > > > > > > > > > > +struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > > > > > > > > > > > + void *object, u8 tag)
> > > > > > > > > > > > +{
> > > > > > > > > > > > + struct kasan_alloc_meta *alloc_meta;
> > > > > > > > > > > > + int i = 0;
> > > > > > > > > > > > +
> > > > > > > > > > > > + alloc_meta = get_alloc_info(cache, object);
> > > > > > > > > > > > +
> > > > > > > > > > > > +#ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> > > > > > > > > > > > + for (i = 0; i < KASAN_NR_FREE_STACKS; i++) {
> > > > > > > > > > > > + if (alloc_meta->free_pointer_tag[i] == tag)
> > > > > > > > > > > > + break;
> > > > > > > > > > > > + }
> > > > > > > > > > > > + if (i == KASAN_NR_FREE_STACKS)
> > > > > > > > > > > > + i = alloc_meta->free_track_idx;
> > > > > > > > > > > > +#endif
> > > > > > > > > > > > +
> > > > > > > > > > > > + return &alloc_meta->free_track[i];
> > > > > > > > > > > > +}
> > > > > > > > > > >
> > > > > > > > > > > Hi Walter,
> > > > > > > > > > >
> > > > > > > > > > > FTR I've uploaded this for review purposes here:
> > > > > > > > > > > https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/2458
> > > > > > > > > > >
> > > > > > > > > > > Diff from the previous version is available as:
> > > > > > > > > > > https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/2458/1..2
> > > > > > > > > > >
> > > > > > > > > > > I've tested this locally and with syzkaller. This is :
> > > > > > > > > > >
> > > > > > > > > > > [ 80.583021][ C3] Freed by task 0:
> > > > > > > > > > > [ 80.583480][ C3] kasan_save_stack+0x1b/0x40 mm/kasan/common.c:49
> > > > > > > > > > > [ 80.584056][ C3] kasan_set_track+0x1c/0x30 mm/kasan/common.c:57
> > > > > > > > > > > [ 80.584617][ C3] kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:354
> > > > > > > > > > > [ 80.585221][ C3] __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:438
> > > > > > > > > > > [ 80.585814][ C3] __cache_free mm/slab.c:3426 [inline]
> > > > > > > > > > > [ 80.585814][ C3] kfree+0x10b/0x2b0 mm/slab.c:3757
> > > > > > > > > > > [ 80.586291][ C3] kasan_rcu_reclaim+0x16/0x43 [test_kasan]
> > > > > > > > > > > [ 80.587009][ C3] rcu_do_batch kernel/rcu/tree.c:2207 [inline]
> > > > > > > > > > > [ 80.587009][ C3] rcu_core+0x59f/0x1370 kernel/rcu/tree.c:2434
> > > > > > > > > > > [ 80.587537][ C3] __do_softirq+0x26c/0x9fa kernel/softirq.c:292
> > > > > > > > > > > [ 80.588085][ C3]
> > > > > > > > > > > [ 80.588367][ C3] Last one call_rcu() call stack:
> > > > > > > > > > > [ 80.589052][ C3] kasan_save_stack+0x1b/0x40 mm/kasan/common.c:49
> > > > > > > > > > > [ 80.589622][ C3] kasan_record_aux_stack+0x82/0xb0 mm/kasan/generic.c:345
> > > > > > > > > > > [ 80.590254][ C3] __call_rcu kernel/rcu/tree.c:2672 [inline]
> > > > > > > > > > > [ 80.590254][ C3] call_rcu+0x14f/0x7f0 kernel/rcu/tree.c:2746
> > > > > > > > > > > [ 80.590782][ C3] kasan_rcu_uaf+0xe4/0xeb [test_kasan]
> > > > > > > > > > > [ 80.591697][ C3] kmalloc_tests_init+0xbc/0x1097 [test_kasan]
> > > > > > > > > > > [ 80.592900][ C3] do_one_initcall+0x10a/0x7d0 init/main.c:1196
> > > > > > > > > > > [ 80.593494][ C3] do_init_module+0x1e6/0x6d0 kernel/module.c:3539
> > > > > > > > > > > [ 80.594066][ C3] load_module+0x7464/0x9450 kernel/module.c:3890
> > > > > > > > > > > [ 80.594626][ C3] __do_sys_init_module+0x1e3/0x220 kernel/module.c:3953
> > > > > > > > > > > [ 80.595265][ C3] do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
> > > > > > > > > > > [ 80.595822][ C3] entry_SYSCALL_64_after_hwframe+0x49/0xb3
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > Overall this looks very good to me.
> > > > > > > > > > > But there is one aspect that bothers me. In the previous patch you had
> > > > > > > > > > > code that returned NULL from kasan_get_free_track() if the object is
> > > > > > > > > > > live (which means free meta is not available, it's occupied by object
> > > > > > > > > > > data). Now you dropped that code, but I think we still need it.
> > > > > > > > > > > Otherwise we cast user object data to free meta and print the free
> > > > > > > > > > > stack/pid from whatever garbage is there. This may lead to very
> > > > > > > > > > > confusing output and potentially to crashes in stackdepot.
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Yes, I totally agree with you. In the previous email I thought that
> > > > > > > > > > there is a problem with free track, but I did not point it out. Thank
> > > > > > > > > > you for pointing this problem. As you mentioned, we should fix it.
> > > > > > > > > >
> > > > > > > > > > > What do you think about this patch on top of your patches?
> > > > > > > > > > > https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/2478
> > > > > > > > > > > This way we very precisely mark the period of time when the object has
> > > > > > > > > > > free track live and set.
> > > > > > > > > > > If it looks good to you, feel free to incorporate it into your series.
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Thank you for providing good idea solution.
> > > > > > > > > >
> > > > > > > > > > I saw this patch, that is a great patch. I think it can fix the issue
> > > > > > > > > > which has garbage stack. it should work as described below.
> > > > > > > > > >
> > > > > > > > > > 1). When object is live, then don't print free stack.
> > > > > > > > > > 2). When object is NOT alive, after free object:
> > > > > > > > > > 2a). when object is in quarantine, then it can print free stack
> > > > > > > > > > 2b). when object is NOT in quarantine, then it can NOT print free stack.
> > > > > > > > > >
> > > > > > > > > > I have a question about 2), why we don't directly use
> > > > > > > > > > KASAN_KMALLOC_FREE? if we directly use it, then 2b) can print free
> > > > > > > > > > stack? 2b) may has use-after-free? so that it may need free stack.
> > > > > > > > >
> > > > > > >
> > > > > > > About 2b), I see another question. When do qlink_free(), it will be
> > > > > > > written KASAN_KMALLOC_FREE from KASAN_KMALLOC_FREETRACK? if we don't
> > > > > > > write shadow memory, it is still KASAN_KMALLOC_FREETRACK, then 2b) will
> > > > > > > have free stack? Because I see you add KASAN_KMALLOC_FREETRACK to get
> > > > > > > use-after-free in get_shadow_bug_type(). so should it not write
> > > > > > > KASAN_KMALLOC_FREE?
> > > > > >
> > > > > > It may or may not work.
> > > > > > The potential problem is that when qlink_free calls ___cache_free,
> > > > > > slab/slub may start using object memory for its own purposes, e.g.
> > > > > > store the next link. This next link may overwrite part of free meta.
> > > > > > It actually may work because the slab/slib next link is likely to
> > > > > > overlap with kasan_free_meta.quarantine_link only. And we may have
> > > > > > kasan_free_meta.free_track intact while KASAN_KMALLOC_FREE is set. But
> > > > > > this needs careful checking for both slab and slub and if they may use
> > > > > > more than 1 word in some configurations.
> > > > > >
> > > > >
> > > > > This problem looks like existing, even without this change? currently
> > > > > KASAN may get wrong free stack?
> > > >
> > > > No, we should not have this problem now. Currently free track is
> > > > stored in alloc meta. Alloc meta does not overlap with the object.
> > > > It's only free meta that overlaps with the object and slab metadata at
> > > > different periods of the block lifetime. Schematically what we have
> > > > is:
> > > >
> > > > struct block_t {
> > > > alloc_meta kasan_alloc_meta;
> > > > union {
> > > > user_data char[N];
> > > > slab_meta slab_meta;
> > > > free_meta kasan_free_meta;
> > > > };
> > > > }
> > > >
> > > > free_meta shared storage space with 2 other things.
> > > >
> > >
> > > Ah...I forget it is stored in alloc mata, Yes, it should not have this
> > > problem.
> > >
> > > Thanks for your detailed explanation
> > >
> > > > > Regardless of whether the shadow memory content is
> > > > > KASAN_KMALLOC_FREETRACK or KASAN_KMALLOC_FREE, it may have this problem?
> > > >
> > > > KASAN_KMALLOC_FREETRACK is set only when nobody else uses the storage.
> > > >
> > >
> > > Ok, I will use KASAN_KMALLOC_FREE. If you have any concerns, please tell me.
> > > Thanks.
> >
> > You mean KASAN_KMALLOC_FREETRACK?
> >
>
> Yes, sorry, I made you misunderstand what I meant. I mean that qlink_free()
> write KASAN_KMALLOC_FREE.

Good.

> > Or, you checked that using KASAN_KMALLOC_FREE is safe and will not
> > cause any bad overlap?
> >
>
> This item you refer to, maybe it can be done in the future.
>
> >
> > > > > But because of kasan_get_free_track() have conditions to get free track,
> > > > > so that if shadow memory content is KASAN_KMALLOC_FREE, then it will
> > > > > avoid this problem and always print right free stack.
> > > >
> > > >
> > > >
> > > > > > > > > We can't use KASAN_KMALLOC_FREE because of this part:
> > > > > > > > >
> > > > > > > > > static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
> > > > > > > > > unsigned long ip, bool quarantine)
> > > > > > > > > {
> > > > > > > > > ...
> > > > > > > > > kasan_poison_shadow(object, rounded_up_size, KASAN_KMALLOC_FREE);
> > > > > > > > >
> > > > > > > > > if ((IS_ENABLED(CONFIG_KASAN_GENERIC) && !quarantine) ||
> > > > > > > > > unlikely(!(cache->flags & SLAB_KASAN)))
> > > > > > > > > return false;
> > > > > > > > >
> > > > > > > > > kasan_set_free_info(cache, object, tag);
> > > > > > > > > ...
> > > > > > > > >
> > > > > > > >
> > > > > > > > Ok, I see. When return false, then the shadow memory content has
> > > > > > > > KASAN_KMALLOC_FREE, but it doesn't set free stack, so that we need to
> > > > > > > > avoid this situation. Thank for you reminder.
> > > > > > > >
> > > > > > > > >
> > > > > > > > > We may set KASAN_KMALLOC_FREE, but not set the track (or even have
> > > > > > > > > memory for the track!).
> > > > > > > > > The object may not have free meta allocated at all, e.g. very large
> > > > > > > > > object with ctor (no place to store meta), or it may be in a mempool:
> > > > > > > > > https://elixir.bootlin.com/linux/v5.7-rc6/source/mm/mempool.c#L109
> > > > > > > > > and mempool may be using the object memory itself (for its own next
> > > > > > > > > link or something).
> > > > > > > > >
> > > > > > > > > KASAN_KMALLOC_FREETRACK very explicitly tracks the exact condition we
> > > > > > > > > want: we have meta info live now and we have free track set.
> >
>
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