On Tue, Aug 02, 2022 at 03:59:00PM +0800, Dmitry Vyukov wrote: > 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 Thanks for the catch! will change. > And KASAN_HW_TAGS has chances of being enabled with DEBUG_SLUB in > real-world uses (with Arm MTE). I only have device to test the kasan-generic mode, and not SW/HW tag. But if there is conflict, we may have to apply the similar solution :) > > > > > + > > > > 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? Christoph has explained in one earlier mail that CONFIG_SLUB_DEBUG only compile in the debug support but not activate it. Option CONFIG_SLUB_DEBUG_ON will enable it, and each slub debug flag bits can also be enabled by changing kernel cmdline for some or all slabs. > And KASAN enables CONFIG_SLUB_DEBUG, this means that this is stored > always when KASAN is enabled? Looks wrong. Thanks, Feng
