Re: [PATCH] kasan: fix the missing underflow in memmove and memcpy with CONFIG_KASAN_GENERIC=y

[Dmitry Vyukov]

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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