.On Tue, 2 Aug 2022 at 11:43, Vlastimil Babka <vbabka@xxxxxxx> wrote: > > On 8/2/22 09:06, Dmitry Vyukov wrote: > > On Tue, 2 Aug 2022 at 08:55, 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 > >> + > >> 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? kasan_free_meta is placed in the object data after freeing, so it can be sizeof(kasan_free_meta) > > 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. > > >> 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 > >> > > > >> > > >> > >> -- > >> You received this message because you are subscribed to the Google Groups "kasan-dev" group. > >> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@xxxxxxxxxxxxxxxx. > >> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/YujKCxu2lJJFm73P%40feng-skl. >
