From c86bc06338e96e74dbe91f422c2e2655e5aee404 Mon Sep 17 00:00:00 2001 From: Andrey Ryabinin Date: Sat, 24 Aug 2019 17:55:09 -0700 Subject: [PATCH] mm/kasan: fix false positive invalid-free reports with CONFIG_KASAN_SW_TAGS=y The code like this: ptr = kmalloc(size, GFP_KERNEL); page = virt_to_page(ptr); offset = offset_in_page(ptr); kfree(page_address(page) + offset); may produce false-positive invalid-free reports on the kernel with CONFIG_KASAN_SW_TAGS=y. In the example above we lose the original tag assigned to 'ptr', so kfree() gets the pointer with 0xFF tag. In kfree() we check that 0xFF tag is different from the tag in shadow hence print false report. Instead of just comparing tags, do the following: 1) Check that shadow doesn't contain KASAN_TAG_INVALID. Otherwise it's double-free and it doesn't matter what tag the pointer have. 2) If pointer tag is different from 0xFF, make sure that tag in the shadow is the same as in the pointer. Link: http://lkml.kernel.org/r/20190819172540.19581-1-aryabinin@virtuozzo.com Fixes: 4f384717fb9c ("kasan: add hooks implementation for tag-based mode") Signed-off-by: Andrey Ryabinin Reported-by: Walter Wu Reported-by: Mark Rutland Reviewed-by: Andrey Konovalov Cc: Alexander Potapenko Cc: Dmitry Vyukov Cc: Catalin Marinas Cc: Will Deacon Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/kasan/common.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/mm/kasan/common.c b/mm/kasan/common.c index 2277b82902d83..95d16a42db6bc 100644 --- a/mm/kasan/common.c +++ b/mm/kasan/common.c @@ -407,8 +407,14 @@ static inline bool shadow_invalid(u8 tag, s8 shadow_byte) if (IS_ENABLED(CONFIG_KASAN_GENERIC)) return shadow_byte < 0 || shadow_byte >= KASAN_SHADOW_SCALE_SIZE; - else - return tag != (u8)shadow_byte; + + /* else CONFIG_KASAN_SW_TAGS: */ + if ((u8)shadow_byte == KASAN_TAG_INVALID) + return true; + if ((tag != KASAN_TAG_KERNEL) && (tag != (u8)shadow_byte)) + return true; + + return false; } static bool __kasan_slab_free(struct kmem_cache *cache, void *object, -- 2.39.5