Re: [PATCH] kasan: infer the requested size by scanning shadow memory

[Dmitry Vyukov]

On Fri, 13 Jan 2023 at 08:59, 'Kuan-Ying Lee (李冠穎)' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2023-01-09 at 07:51 +0100, Dmitry Vyukov wrote:
> > On Tue, 3 Jan 2023 at 08:56, 'Kuan-Ying Lee' via kasan-dev
> > <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > We scan the shadow memory to infer the requested size instead of
> > > printing cache->object_size directly.
> > >
> > > This patch will fix the confusing generic kasan report like below.
> > > [1]
> > > Report shows "cache kmalloc-192 of size 192", but user
> > > actually kmalloc(184).
> > >
> > > ==================================================================
> > > BUG: KASAN: slab-out-of-bounds in _find_next_bit+0x143/0x160
> > > lib/find_bit.c:109
> > > Read of size 8 at addr ffff8880175766b8 by task kworker/1:1/26
> > > ...
> > > The buggy address belongs to the object at ffff888017576600
> > >  which belongs to the cache kmalloc-192 of size 192
> > > The buggy address is located 184 bytes inside of
> > >  192-byte region [ffff888017576600, ffff8880175766c0)
> > > ...
> > > Memory state around the buggy address:
> > >  ffff888017576580: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> > >  ffff888017576600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > > ffff888017576680: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
> > >
> > >                                         ^
> > >  ffff888017576700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > >  ffff888017576780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > > ==================================================================
> > >
> > > After this patch, report will show "cache kmalloc-192 of size 184".
> > >
> > > Link:
> > > https://urldefense.com/v3/__https://bugzilla.kernel.org/show_bug.cgi?id=216457__;!!CTRNKA9wMg0ARbw!mLNcuZ83c39d0Xkut-WMY3CcvZcAYDuLCmv4mu7IAldw4_n4i6XvX8GORBfjOadWxOa6d-ODQdx6ZCSvB2g13Q$
> > > $   [1]
> > >
> > > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@xxxxxxxxxxxx>
> > > ---
> > >  mm/kasan/kasan.h          |  5 +++++
> > >  mm/kasan/report.c         |  3 ++-
> > >  mm/kasan/report_generic.c | 18 ++++++++++++++++++
> > >  3 files changed, 25 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > > index 32413f22aa82..7bb627d21580 100644
> > > --- a/mm/kasan/kasan.h
> > > +++ b/mm/kasan/kasan.h
> > > @@ -340,8 +340,13 @@ static inline void
> > > kasan_print_address_stack_frame(const void *addr) { }
> > >
> > >  #ifdef CONFIG_KASAN_GENERIC
> > >  void kasan_print_aux_stacks(struct kmem_cache *cache, const void
> > > *object);
> > > +int kasan_get_alloc_size(void *object_addr, struct kmem_cache
> > > *cache);
> > >  #else
> > >  static inline void kasan_print_aux_stacks(struct kmem_cache
> > > *cache, const void *object) { }
> > > +static inline int kasan_get_alloc_size(void *object_addr, struct
> > > kmem_cache *cache)
> > > +{
> > > +       return cache->object_size;
> > > +}
> > >  #endif
> > >
> > >  bool kasan_report(unsigned long addr, size_t size,
> > > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > > index 1d02757e90a3..6de454bb2cad 100644
> > > --- a/mm/kasan/report.c
> > > +++ b/mm/kasan/report.c
> > > @@ -236,12 +236,13 @@ static void describe_object_addr(const void
> > > *addr, struct kmem_cache *cache,
> > >  {
> > >         unsigned long access_addr = (unsigned long)addr;
> > >         unsigned long object_addr = (unsigned long)object;
> > > +       int real_size = kasan_get_alloc_size((void *)object_addr,
> > > cache);
> > >         const char *rel_type;
> > >         int rel_bytes;
> > >
> > >         pr_err("The buggy address belongs to the object at %px\n"
> > >                " which belongs to the cache %s of size %d\n",
> > > -               object, cache->name, cache->object_size);
> > > +               object, cache->name, real_size);
> > >
> > >         if (access_addr < object_addr) {
> > >                 rel_type = "to the left";
> > > diff --git a/mm/kasan/report_generic.c b/mm/kasan/report_generic.c
> > > index 043c94b04605..01b38e459352 100644
> > > --- a/mm/kasan/report_generic.c
> > > +++ b/mm/kasan/report_generic.c
> > > @@ -43,6 +43,24 @@ void *kasan_find_first_bad_addr(void *addr,
> > > size_t size)
> > >         return p;
> > >  }
> > >
> > > +int kasan_get_alloc_size(void *addr, struct kmem_cache *cache)
> > > +{
> > > +       int size = 0;
> > > +       u8 *shadow = (u8 *)kasan_mem_to_shadow(addr);
> > > +
> > > +       while (size < cache->object_size) {
> > > +               if (*shadow == 0)
> > > +                       size += KASAN_GRANULE_SIZE;
> > > +               else if (*shadow >= 1 && *shadow <=
> > > KASAN_GRANULE_SIZE - 1)
> > > +                       size += *shadow;
> > > +               else
> > > +                       return size;
> > > +               shadow++;
> >
> > This only works for out-of-bounds reports, but I don't see any checks
> > for report type. Won't this break reporting for all other report
> > types?
> >
>
> I think it won't break reporting for other report types.
> This function is only called by slab OOB and UAF.

I meant specifically UAF reports.
During UAF there are no 0s in the object shadow.

> > I would also print the cache name anyway. Sometimes reports are
> > perplexing and/or this logic may return a wrong result for some
> > reason. The total object size may be useful to understand harder
> > cases.
> >
>
> Ok. I will keep the cache name and the total object_size.
>
> > > +       }
> > > +
> > > +       return cache->object_size;
> > > +}
> > > +
> > >  static const char *get_shadow_bug_type(struct kasan_report_info
> > > *info)
> > >  {
> > >         const char *bug_type = "unknown-crash";
>
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