From a5989d4ed40cef0cadede2393c714a1ff9179f65 Mon Sep 17 00:00:00 2001 From: Juntong Deng Date: Mon, 20 Nov 2023 04:46:29 +0800 Subject: [PATCH] kasan: improve free meta storage in Generic KASAN Currently free meta can only be stored in object if the object is not smaller than free meta. After the improvement, when the object is smaller than free meta and SLUB DEBUG is not enabled, it is possible to store part of the free meta in the object, reducing the increased size of the red zone. Example: free meta size: 16 bytes alloc meta size: 16 bytes object size: 8 bytes optimal redzone size (object_size <= 64): 16 bytes Before improvement: actual redzone size = alloc meta size + free meta size = 32 bytes After improvement: actual redzone size = alloc meta size + (free meta size - object size) = 24 bytes [juntong.deng@outlook.com: make kasan_metadata_size() adapt to the improved free meta storage] Link: https://lkml.kernel.org/r/VI1P193MB0752675D6E0A2D16CE656F8299BAA@VI1P193MB0752.EURP193.PROD.OUTLOOK.COM Link: https://lkml.kernel.org/r/VI1P193MB0752DE2CCD9046B5FED0AA8E99B5A@VI1P193MB0752.EURP193.PROD.OUTLOOK.COM Signed-off-by: Juntong Deng Suggested-by: Dmitry Vyukov Cc: Alexander Potapenko Cc: Andrey Konovalov Cc: Andrey Ryabinin Cc: Dmitry Vyukov Cc: Vincenzo Frascino Signed-off-by: Andrew Morton --- mm/kasan/generic.c | 39 +++++++++++++++++++++++++++++---------- 1 file changed, 29 insertions(+), 10 deletions(-) diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c index 50cc519e23f4..54e20b2bc3e1 100644 --- a/mm/kasan/generic.c +++ b/mm/kasan/generic.c @@ -362,6 +362,8 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, { unsigned int ok_size; unsigned int optimal_size; + unsigned int rem_free_meta_size; + unsigned int orig_alloc_meta_offset; if (!kasan_requires_meta()) return; @@ -395,6 +397,9 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, /* Continue, since free meta might still fit. */ } + ok_size = *size; + orig_alloc_meta_offset = cache->kasan_info.alloc_meta_offset; + /* * Add free meta into redzone when it's not possible to store * it in the object. This is the case when: @@ -402,23 +407,37 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, * be touched after it was freed, or * 2. Object has a constructor, which means it's expected to * retain its content until the next allocation, or - * 3. Object is too small. + * 3. Object is too small and SLUB DEBUG is enabled. Avoid + * free meta that exceeds the object size corrupts the + * SLUB DEBUG metadata. * Otherwise cache->kasan_info.free_meta_offset = 0 is implied. + * If the object is smaller than the free meta and SLUB DEBUG + * is not enabled, it is still possible to store part of the + * free meta in the object. */ - if ((cache->flags & SLAB_TYPESAFE_BY_RCU) || cache->ctor || - cache->object_size < sizeof(struct kasan_free_meta)) { - ok_size = *size; - + if ((cache->flags & SLAB_TYPESAFE_BY_RCU) || cache->ctor) { cache->kasan_info.free_meta_offset = *size; *size += sizeof(struct kasan_free_meta); - - /* If free meta doesn't fit, don't add it. */ - if (*size > KMALLOC_MAX_SIZE) { - cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META; - *size = ok_size; + } else if (cache->object_size < sizeof(struct kasan_free_meta)) { + if (__slub_debug_enabled()) { + cache->kasan_info.free_meta_offset = *size; + *size += sizeof(struct kasan_free_meta); + } else { + rem_free_meta_size = sizeof(struct kasan_free_meta) - + cache->object_size; + *size += rem_free_meta_size; + if (cache->kasan_info.alloc_meta_offset != 0) + cache->kasan_info.alloc_meta_offset += rem_free_meta_size; } } + /* If free meta doesn't fit, don't add it. */ + if (*size > KMALLOC_MAX_SIZE) { + cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META; + cache->kasan_info.alloc_meta_offset = orig_alloc_meta_offset; + *size = ok_size; + } + /* Calculate size with optimal redzone. */ optimal_size = cache->object_size + optimal_redzone(cache->object_size); /* Limit it with KMALLOC_MAX_SIZE (relevant for SLAB only). */