On Tue, 2 Aug 2022 at 09:47, Feng Tang <feng.tang@xxxxxxxxx> 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 I think this can be done only when CONFIG_KASAN_GENERIC. Only CONFIG_KASAN_GENERIC stores free meta info in objects: https://elixir.bootlin.com/linux/latest/source/mm/kasan/common.c#L176 And KASAN_HW_TAGS has chances of being enabled with DEBUG_SLUB in real-world uses (with Arm MTE). > > > + > > > 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. > > > > 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. > > Yes, the patchset was only posted on LKML, and not in any tree now. > The link to the original patches is: > > https://lore.kernel.org/lkml/20220727071042.8796-1-feng.tang@xxxxxxxxx/t/ Lots of code... This SLAB_STORE_USER seems to be set on all kmalloc slabs by default when CONFIG_SLUB_DEBUG is enabled, right? And KASAN enables CONFIG_SLUB_DEBUG, this means that this is stored always when KASAN is enabled? Looks wrong.
