Re: [PATCH 2/3] kasan: record and print the free track

[Walter Wu]

On Wed, 2020-05-06 at 11:50 +0200, Dmitry Vyukov wrote:
> On Wed, May 6, 2020 at 7:22 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> >
> > We add new KASAN_RCU_STACK_RECORD configuration option. It will move
> > free track from slub meta-data (struct kasan_alloc_meta) into freed object.
> > Because we hope this options doesn't enlarge slub meta-data size.
> >
> > This option doesn't enlarge struct kasan_alloc_meta size.
> > - add two call_rcu() call stack into kasan_alloc_meta, size is 8 bytes.
> > - remove free track from kasan_alloc_meta, size is 8 bytes.
> >
> > This option is only suitable for generic KASAN, because we move free track
> > into the freed object, so free track is valid information only when it
> > exists in quarantine. If the object is in-use state, then the KASAN report
> > doesn't print call_rcu() free track information.
> >
> > [1]https://bugzilla.kernel.org/show_bug.cgi?id=198437
> >
> > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > ---
> >  mm/kasan/common.c | 10 +++++++++-
> >  mm/kasan/report.c | 24 +++++++++++++++++++++---
> >  2 files changed, 30 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 32d422bdf127..13ec03e225a7 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -321,8 +321,15 @@ void kasan_record_callrcu(void *addr)
> >                 /* record last call_rcu() call stack */
> >                 alloc_info->rcu_free_stack[1] = save_stack(GFP_NOWAIT);
> >  }
> > -#endif
> >
> > +static void kasan_set_free_info(struct kmem_cache *cache,
> > +               void *object, u8 tag)
> > +{
> > +       /* store free track into freed object */
> > +       set_track((struct kasan_track *)(object + BYTES_PER_WORD), GFP_NOWAIT);
> > +}
> > +
> > +#else
> >  static void kasan_set_free_info(struct kmem_cache *cache,
> >                 void *object, u8 tag)
> >  {
> > @@ -339,6 +346,7 @@ static void kasan_set_free_info(struct kmem_cache *cache,
> >
> >         set_track(&alloc_meta->free_track[idx], GFP_NOWAIT);
> >  }
> > +#endif
> >
> >  void kasan_poison_slab(struct page *page)
> >  {
> > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > index 7aaccc70b65b..f2b0c6b9dffa 100644
> > --- a/mm/kasan/report.c
> > +++ b/mm/kasan/report.c
> > @@ -175,8 +175,23 @@ static void kasan_print_rcu_free_stack(struct kasan_alloc_meta *alloc_info)
> >         print_track(&free_track, "Last call_rcu() call stack", true);
> >         pr_err("\n");
> >  }
> > -#endif
> >
> > +static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > +               void *object, u8 tag, const void *addr)
> > +{
> > +       u8 *shadow_addr = (u8 *)kasan_mem_to_shadow(addr);
> > +
> > +       /*
> > +        * Only the freed object can get free track,
> > +        * because free track information is stored to freed object.
> > +        */
> > +       if (*shadow_addr == KASAN_KMALLOC_FREE)
> > +               return (struct kasan_track *)(object + BYTES_PER_WORD);
> 
> Humm... the other patch defines BYTES_PER_WORD as 4... I would assume
> seeing 8 (or sizeof(long)) here. Why 4?
It should be a pointer size, maybe sizeof(long) makes more sense.

> Have you tested all 4 modes (RCU/no-RCU x SLAB/SLUB)? As far as I
> remember one of the allocators stored something in the object.
Good question, I only tested in RCU x SLUB, would you tell mew how do
no-RCU? I will test them in v2 pathset.

> 
> Also, does this work with objects with ctors and slabs destroyed by
> rcu? kasan_track may smash other things in these cases.
> Have you looked at the KASAN implementation when free_track was
> removed? That may have useful details :)
Set free_track before put into quarantine, free_track should not have to
be removed, it only have to overwirte itself.

> 
> 
> > +       else
> > +               return NULL;
> > +}
> > +
> > +#else
> >  static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> >                 void *object, u8 tag, const void *addr)
> >  {
> > @@ -196,6 +211,7 @@ static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> >
> >         return &alloc_meta->free_track[i];
> >  }
> > +#endif
> >
> >  static void describe_object(struct kmem_cache *cache, void *object,
> >                                 const void *addr, u8 tag)
> > @@ -208,8 +224,10 @@ static void describe_object(struct kmem_cache *cache, void *object,
> >                 print_track(&alloc_info->alloc_track, "Allocated", false);
> >                 pr_err("\n");
> >                 free_track = kasan_get_free_track(cache, object, tag, addr);
> > -               print_track(free_track, "Freed", false);
> > -               pr_err("\n");
> > +               if (free_track) {
> > +                       print_track(free_track, "Freed", false);
> > +                       pr_err("\n");
> > +               }
> >  #ifdef CONFIG_KASAN_RCU_STACK_RECORD
> >                 kasan_print_rcu_free_stack(alloc_info);
> >  #endif
> > --
> > 2.18.0
> >
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