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. 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 > > >
