On Fri, Sep 27, 2019 at 5:43 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
>
> memmove() and memcpy() have missing underflow issues.
> When -7 <= size < 0, then KASAN will miss to catch the underflow issue.
> It looks like shadow start address and shadow end address is the same,
> so it does not actually check anything.
>
> The following test is indeed not caught by KASAN:
>
> char *p = kmalloc(64, GFP_KERNEL);
> memset((char *)p, 0, 64);
> memmove((char *)p, (char *)p + 4, -2);
> kfree((char*)p);
>
> It should be checked here:
>
> void *memmove(void *dest, const void *src, size_t len)
> {
> check_memory_region((unsigned long)src, len, false, _RET_IP_);
> check_memory_region((unsigned long)dest, len, true, _RET_IP_);
>
> return __memmove(dest, src, len);
> }
>
> We fix the shadow end address which is calculated, then generic KASAN
> get the right shadow end address and detect this underflow issue.
>
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=199341
>
> Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---
> lib/test_kasan.c | 36 ++++++++++++++++++++++++++++++++++++
> mm/kasan/generic.c | 8 ++++++--
> 2 files changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index b63b367a94e8..8bd014852556 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -280,6 +280,40 @@ static noinline void __init kmalloc_oob_in_memset(void)
> kfree(ptr);
> }
>
> +static noinline void __init kmalloc_oob_in_memmove_underflow(void)
> +{
> + char *ptr;
> + size_t size = 64;
> +
> + pr_info("underflow out-of-bounds in memmove\n");
> + ptr = kmalloc(size, GFP_KERNEL);
> + if (!ptr) {
> + pr_err("Allocation failed\n");
> + return;
> + }
> +
> + memset((char *)ptr, 0, 64);
> + memmove((char *)ptr, (char *)ptr + 4, -2);
> + kfree(ptr);
> +}
> +
> +static noinline void __init kmalloc_oob_in_memmove_overflow(void)
> +{
> + char *ptr;
> + size_t size = 64;
> +
> + pr_info("overflow out-of-bounds in memmove\n");
> + ptr = kmalloc(size, GFP_KERNEL);
> + if (!ptr) {
> + pr_err("Allocation failed\n");
> + return;
> + }
> +
> + memset((char *)ptr, 0, 64);
> + memmove((char *)ptr + size, (char *)ptr, 2);
> + kfree(ptr);
> +}
> +
> static noinline void __init kmalloc_uaf(void)
> {
> char *ptr;
> @@ -734,6 +768,8 @@ static int __init kmalloc_tests_init(void)
> kmalloc_oob_memset_4();
> kmalloc_oob_memset_8();
> kmalloc_oob_memset_16();
> + kmalloc_oob_in_memmove_underflow();
> + kmalloc_oob_in_memmove_overflow();
> kmalloc_uaf();
> kmalloc_uaf_memset();
> kmalloc_uaf2();
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 616f9dd82d12..34ca23d59e67 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -131,9 +131,13 @@ static __always_inline bool memory_is_poisoned_n(unsigned long addr,
> size_t size)
> {
> unsigned long ret;
> + void *shadow_start = kasan_mem_to_shadow((void *)addr);
> + void *shadow_end = kasan_mem_to_shadow((void *)addr + size - 1) + 1;
>
> - ret = memory_is_nonzero(kasan_mem_to_shadow((void *)addr),
> - kasan_mem_to_shadow((void *)addr + size - 1) + 1);
> + if ((long)size < 0)
> + shadow_end = kasan_mem_to_shadow((void *)addr + size);
Hi Walter,
Thanks for working on this.
If size<0, does it make sense to continue at all? We will still check
1PB of shadow memory? What happens when we pass such huge range to
memory_is_nonzero?
Perhaps it's better to produce an error and bail out immediately if size<0?
Also, what's the failure mode of the tests? Didn't they badly corrupt
memory? We tried to keep tests such that they produce the KASAN
reports, but don't badly corrupt memory b/c/ we need to run all of
them.
> + ret = memory_is_nonzero(shadow_start, shadow_end);
>
> if (unlikely(ret)) {
> unsigned long last_byte = addr + size - 1;
> --
> 2.18.0
>
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