On 03/23/2018 09:05 PM, Andrey Konovalov wrote:
> This commit adds KHWASAN hooks implementation.
>
> 1. When a new slab cache is created, KHWASAN rounds up the size of the
> objects in this cache to KASAN_SHADOW_SCALE_SIZE (== 16).
>
> 2. On each kmalloc KHWASAN generates a random tag, sets the shadow memory,
> that corresponds to this object to this tag, and embeds this tag value
> into the top byte of the returned pointer.
>
> 3. On each kfree KHWASAN poisons the shadow memory with a random tag to
> allow detection of use-after-free bugs.
>
> The rest of the logic of the hook implementation is very much similar to
> the one provided by KASAN. KHWASAN saves allocation and free stack metadata
> to the slab object the same was KASAN does this.
>
> Signed-off-by: Andrey Konovalov <[email protected]>
> ---
> mm/kasan/khwasan.c | 200 ++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 197 insertions(+), 3 deletions(-)
>
> diff --git a/mm/kasan/khwasan.c b/mm/kasan/khwasan.c
> index da4b17997c71..e8bed5a078c7 100644
> --- a/mm/kasan/khwasan.c
> +++ b/mm/kasan/khwasan.c
> @@ -90,69 +90,260 @@ void *khwasan_reset_tag(const void *addr)
> return reset_tag(addr);
> }
>
> +void kasan_poison_shadow(const void *address, size_t size, u8 value)
> +{
> + void *shadow_start, *shadow_end;
> +
> + /* Perform shadow offset calculation based on untagged address */
> + address = reset_tag(address);
> +
> + shadow_start = kasan_mem_to_shadow(address);
> + shadow_end = kasan_mem_to_shadow(address + size);
> +
> + memset(shadow_start, value, shadow_end - shadow_start);
> +}
> +
> void kasan_unpoison_shadow(const void *address, size_t size)
> {
> + /* KHWASAN only allows 16-byte granularity */
> + size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
> + kasan_poison_shadow(address, size, get_tag(address));
> }
>
This is way too much of copy-paste/code duplication. Ideally, you should have
only
check_memory_region() stuff separated, the rest (poisoning/unpoisoning, slabs
management) should be
in common.c code.
So it should be something like this:
in kasan.h
...
#ifdef CONFIG_KASAN_CLASSIC
#define KASAN_FREE_PAGE 0xFF /* page was freed */
#define KASAN_PAGE_REDZONE 0xFE /* redzone for kmalloc_large allocations
*/
#define KASAN_KMALLOC_REDZONE 0xFC /* redzone inside slub object */
#define KASAN_KMALLOC_FREE 0xFB /* object was freed
(kmem_cache_free/kfree) */
#else
#define KASAN_FREE_PAGE 0xFE
#define KASAN_PAGE_REDZONE 0xFE
#define KASAN_KMALLOC_REDZONE 0xFE
#define KASAN_KMALLOC_FREE 0xFE
#endif
...
#ifdef CONFIG_KASAN_CLASSIC
static inline void *reset_tag(const void *addr)
{
return (void *)addr;
}
static inline u8 get_tag(const void *addr)
{
return 0;
}
#else
static inline u8 get_tag(const void *addr)
{
return (u8)((u64)addr >> KHWASAN_TAG_SHIFT);
}
static inline void *reset_tag(const void *addr)
{
return set_tag(addr, KHWASAN_TAG_KERNEL);
}
#endif
in kasan/common.c:
void kasan_poison_shadow(const void *address, size_t size, u8 value)
{
void *shadow_start, *shadow_end;
address = reset_tag(address);
shadow_start = kasan_mem_to_shadow(address);
shadow_end = kasan_mem_to_shadow(address + size);
memset(shadow_start, value, shadow_end - shadow_start);
}
void kasan_unpoison_shadow(const void *address, size_t size)
{
kasan_poison_shadow(address, size, get_tag(address));
if (size & KASAN_SHADOW_MASK) {
u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
if (IS_ENABLED(CONFIG_KASAN_TAGS)
*shadow = get_tag(address);
else
*shadow = size & KASAN_SHADOW_MASK;
}
}
void kasan_free_pages(struct page *page, unsigned int order)
{
if (likely(!PageHighMem(page)))
kasan_poison_shadow(page_address(page),
PAGE_SIZE << order,
KASAN_FREE_PAGE);
}
etc.
> void check_memory_region(unsigned long addr, size_t size, bool write,
> unsigned long ret_ip)
> {
> + u8 tag;
> + u8 *shadow_first, *shadow_last, *shadow;
> + void *untagged_addr;
> +
> + tag = get_tag((const void *)addr);
> +
> + /* Ignore accesses for pointers tagged with 0xff (native kernel
> + * pointer tag) to suppress false positives caused by kmap.
> + *
> + * Some kernel code was written to account for archs that don't keep
> + * high memory mapped all the time, but rather map and unmap particular
> + * pages when needed. Instead of storing a pointer to the kernel memory,
> + * this code saves the address of the page structure and offset within
> + * that page for later use. Those pages are then mapped and unmapped
> + * with kmap/kunmap when necessary and virt_to_page is used to get the
> + * virtual address of the page. For arm64 (that keeps the high memory
> + * mapped all the time), kmap is turned into a page_address call.
> +
> + * The issue is that with use of the page_address + virt_to_page
> + * sequence the top byte value of the original pointer gets lost (gets
> + * set to 0xff.
> + */
> + if (tag == 0xff)
> + return;
You can save tag somewhere in page struct and make page_address() return tagged
address.
I'm not sure it might be even possible to squeeze the tag into page->flags on
some configurations,
see include/linux/page-flags-layout.h
> void *kasan_slab_alloc(struct kmem_cache *cache, void *object, gfp_t flags)
> {
> + if (!READ_ONCE(khwasan_enabled))
> + return object;
...
> void *kasan_kmalloc(struct kmem_cache *cache, const void *object,
> size_t size, gfp_t flags)
> {
> + if (!READ_ONCE(khwasan_enabled))
> + return (void *)object;
> +
...
> void *kasan_kmalloc_large(const void *ptr, size_t size, gfp_t flags)
> {
...
> +
> + if (!READ_ONCE(khwasan_enabled))
> + return (void *)ptr;
> +
I don't see any possible way of khwasan_enabled being 0 here.
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