On Tue, Dec 20, 2016 at 10:11:01AM -0800, Greg Thelen wrote: > Per memcg slab accounting and kasan have a problem with kmem_cache > destruction. > - kmem_cache_create() allocates a kmem_cache, which is used for > allocations from processes running in root (top) memcg. > - Processes running in non root memcg and allocating with either > __GFP_ACCOUNT or from a SLAB_ACCOUNT cache use a per memcg kmem_cache. > - Kasan catches use-after-free by having kfree() and kmem_cache_free() > defer freeing of objects. Objects are placed in a quarantine. > - kmem_cache_destroy() destroys root and non root kmem_caches. It takes > care to drain the quarantine of objects from the root memcg's > kmem_cache, but ignores objects associated with non root memcg. This > causes leaks because quarantined per memcg objects refer to per memcg > kmem cache being destroyed. > > To see the problem: > 1) create a slab cache with kmem_cache_create(,,,SLAB_ACCOUNT,) > 2) from non root memcg, allocate and free a few objects from cache > 3) dispose of the cache with kmem_cache_destroy() > kmem_cache_destroy() will trigger a "Slab cache still has objects" > warning indicating that the per memcg kmem_cache structure was leaked. > > Fix the leak by draining kasan quarantined objects allocated from non > root memcg. > > Racing memcg deletion is tricky, but handled. kmem_cache_destroy() => > shutdown_memcg_caches() => __shutdown_memcg_cache() => shutdown_cache() > flushes per memcg quarantined objects, even if that memcg has been > rmdir'd and gone through memcg_deactivate_kmem_caches(). > > This leak only affects destroyed SLAB_ACCOUNT kmem caches when kasan is > enabled. So I don't think it's worth patching stable kernels. > > Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx> Reviewed-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>