Re: [mm/slub] 3616799128: BUG_kmalloc-#(Not_tainted):kmalloc_Redzone_overwritten

Dmitry Vyukov
2022-08-02T10:32:03Z

On Tue, 2 Aug 2022 at 11:43, Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 8/2/22 09:06, Dmitry Vyukov wrote:
> > On Tue, 2 Aug 2022 at 08:55, Feng Tang <feng.tang@intel.com> wrote:
> >>
> >> On Mon, Aug 01, 2022 at 10:23:23PM +0800, Vlastimil Babka wrote:
> >> > On 8/1/22 08:21, Feng Tang wrote:
> >> [snip]
> >> > > Cc kansan mail list.
> >> > >
> >> > > This is really related with KASAN debug, that in free path, some
> >> > > kmalloc redzone ([orig_size+1, object_size]) area is written by
> >> > > kasan to save free meta info.
> >> > >
> >> > > The callstack is:
> >> > >
> >> > > kfree
> >> > > slab_free
> >> > > slab_free_freelist_hook
> >> > > slab_free_hook
> >> > > __kasan_slab_free
> >> > > ____kasan_slab_free
> >> > > kasan_set_free_info
> >> > > kasan_set_track
> >> > >
> >> > > And this issue only happens with "kmalloc-16" slab. Kasan has 2
> >> > > tracks: alloc_track and free_track, for x86_64 test platform, most
> >> > > of the slabs will reserve space for alloc_track, and reuse the
> >> > > 'object' area for free_track. The kasan free_track is 16 bytes
> >> > > large, that it will occupy the whole 'kmalloc-16's object area,
> >> > > so when kmalloc-redzone is enabled by this patch, the 'overwritten'
> >> > > error is triggered.
> >> > >
> >> > > But it won't hurt other kmalloc slabs, as kasan's free meta won't
> >> > > conflict with kmalloc-redzone which stay in the latter part of
> >> > > kmalloc area.
> >> > >
> >> > > So the solution I can think of is:
> >> > > * skip the kmalloc-redzone for kmalloc-16 only, or
> >> > > * skip kmalloc-redzone if kasan is enabled, or
> >> > > * let kasan reserve the free meta (16 bytes) outside of object
> >> > > just like for alloc meta
> >> >
> >> > Maybe we could add some hack that if both kasan and SLAB_STORE_USER is
> >> > enabled, we bump the stored orig_size from <16 to 16? Similar to what
> >> > __ksize() does.
> >>
> >> How about the following patch:
> >>
> >> ---
> >> diff --git a/mm/slub.c b/mm/slub.c
> >> index added2653bb0..33bbac2afaef 100644
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -830,6 +830,16 @@ static inline void set_orig_size(struct kmem_cache *s,
> >> if (!slub_debug_orig_size(s))
> >> return;
> >>
> >> +#ifdef CONFIG_KASAN
> >> + /*
> >> + * When kasan is enabled, it could save its free meta data in the
> >> + * start part of object area, so skip the kmalloc redzone check
> >> + * for small kmalloc slabs to avoid the data conflict.
> >> + */
> >> + if (s->object_size <= 32)
> >> + orig_size = s->object_size;
> >> +#endif
> >> +
> >> p += get_info_end(s);
> >> p += sizeof(struct track) * 2;
> >>
> >> I extend the size to 32 for potential's kasan meta data size increase.
> >> This is tested locally, if people are OK with it, I can ask for 0Day's
> >> help to verify this.
>
> Is there maybe some KASAN macro we can use instead of hardcoding 32?
>
> >
> > Where is set_orig_size() function defined? Don't see it upstream nor
> > in linux-next.
> > This looks fine but my only concern is that this should not increase
> > memory consumption when slub debug tracking is not enabled, which
> > should be the main operation mode when KASAN is enabled. But I can't
> > figure this out w/o context.
>
> It won't increase memory consumption even if slub_debug tracking is enabled.
> It just fakes a bit the size that was passed to kmalloc() and which we newly
> store (thanks to Feng's patches) for statistics and debugging purposes.

Then it looks good to me. Thanks for double checking.

> >> Thanks,
> >> Feng
> >>
> >> >
> >> > > I don't have way to test kasan's SW/HW tag configuration, which
> >> > > is only enabled on arm64 now. And I don't know if there will
> >> > > also be some conflict.
> >> > >
> >> > > Thanks,
> >> > > Feng
> >> > >
> >> >
> >>
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>