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

[Dmitry Vyukov]

On Thu, Oct 3, 2019 at 3:51 PM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:>
> how about this?
>
> commit fd64691026e7ccb8d2946d0804b0621ac177df38
> Author: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> Date:   Fri Sep 27 09:54:18 2019 +0800
>
>     kasan: detect invalid size in memory operation function
>
>     It is an undefined behavior to pass a negative value to
> memset()/memcpy()/memmove()
>     , so need to be detected by KASAN.
>
>     KASAN report:
>
>      BUG: KASAN: invalid size 18446744073709551614 in
> kmalloc_memmove_invalid_size+0x70/0xa0
>
>      CPU: 1 PID: 91 Comm: cat Not tainted
> 5.3.0-rc1ajb-00001-g31943bbc21ce-dirty #7
>      Hardware name: linux,dummy-virt (DT)
>      Call trace:
>       dump_backtrace+0x0/0x278
>       show_stack+0x14/0x20
>       dump_stack+0x108/0x15c
>       print_address_description+0x64/0x368
>       __kasan_report+0x108/0x1a4
>       kasan_report+0xc/0x18
>       check_memory_region+0x15c/0x1b8
>       memmove+0x34/0x88
>       kmalloc_memmove_invalid_size+0x70/0xa0
>
>     [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>
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index b63b367a94e8..e4e517a51860 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -280,6 +280,23 @@ static noinline void __init
> kmalloc_oob_in_memset(void)
>         kfree(ptr);
>  }
>
> +static noinline void __init kmalloc_memmove_invalid_size(void)
> +{
> +       char *ptr;
> +       size_t size = 64;
> +
> +       pr_info("invalid size 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_uaf(void)
>  {
>         char *ptr;
> @@ -734,6 +751,7 @@ static int __init kmalloc_tests_init(void)
>         kmalloc_oob_memset_4();
>         kmalloc_oob_memset_8();
>         kmalloc_oob_memset_16();
> +       kmalloc_memmove_invalid_size;
>         kmalloc_uaf();
>         kmalloc_uaf_memset();
>         kmalloc_uaf2();
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 2277b82902d8..5fd377af7457 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -102,7 +102,8 @@ EXPORT_SYMBOL(__kasan_check_write);
>  #undef memset
>  void *memset(void *addr, int c, size_t len)
>  {
> -       check_memory_region((unsigned long)addr, len, true, _RET_IP_);
> +       if(!check_memory_region((unsigned long)addr, len, true, _RET_IP_))
> +               return NULL;

Overall approach looks good to me.
A good question is what we should return here. All bets are off after
a report, but we still try to "minimize damage". One may argue for
returning addr here and in other functions. But the more I think about
this, the more I think it does not matter.


>         return __memset(addr, c, len);
>  }
> @@ -110,7 +111,8 @@ void *memset(void *addr, int c, size_t len)
>  #undef memmove
>  void *memmove(void *dest, const void *src, size_t len)
>  {
> -       check_memory_region((unsigned long)src, len, false, _RET_IP_);
> +       if(!check_memory_region((unsigned long)src, len, false, _RET_IP_))
> +               return NULL;
>         check_memory_region((unsigned long)dest, len, true, _RET_IP_);
>
>         return __memmove(dest, src, len);
> @@ -119,7 +121,8 @@ void *memmove(void *dest, const void *src, size_t
> len)
>  #undef memcpy
>  void *memcpy(void *dest, const void *src, size_t len)
>  {
> -       check_memory_region((unsigned long)src, len, false, _RET_IP_);
> +       if(!check_memory_region((unsigned long)src, len, false, _RET_IP_))
> +               return NULL;
>         check_memory_region((unsigned long)dest, len, true, _RET_IP_);
>
>         return __memcpy(dest, src, len);
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 616f9dd82d12..02148a317d27 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -173,6 +173,11 @@ static __always_inline bool
> check_memory_region_inline(unsigned long addr,
>         if (unlikely(size == 0))
>                 return true;
>
> +       if (unlikely((long)size < 0)) {
> +               kasan_report(addr, size, write, ret_ip);
> +               return false;
> +       }
> +
>         if (unlikely((void *)addr <
>                 kasan_shadow_to_mem((void *)KASAN_SHADOW_START))) {
>                 kasan_report(addr, size, write, ret_ip);
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 0e5f965f1882..0cd317ef30f5 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -68,11 +68,16 @@ __setup("kasan_multi_shot", kasan_set_multi_shot);
>
>  static void print_error_description(struct kasan_access_info *info)
>  {
> -       pr_err("BUG: KASAN: %s in %pS\n",
> -               get_bug_type(info), (void *)info->ip);
> -       pr_err("%s of size %zu at addr %px by task %s/%d\n",
> -               info->is_write ? "Write" : "Read", info->access_size,
> -               info->access_addr, current->comm, task_pid_nr(current));
> +       if ((long)info->access_size < 0) {
> +               pr_err("BUG: KASAN: invalid size %zu in %pS\n",
> +                       info->access_size, (void *)info->ip);

I would not introduce a new bug type.
These are parsed and used by some systems, e.g. syzbot. If size is
user-controllable, then a new bug type for this will mean 2 bug
reports.
It also won't harm to print Read/Write, definitely the address, so no
reason to special case this out of a dozen of report formats.
This can qualify as out-of-bounds (definitely will cross some
bounds!), so I would change get_bug_type() to return
"slab-out-of-bounds" (as the most common OOB) in such case (with a
comment).


> +       } else {
> +               pr_err("BUG: KASAN: %s in %pS\n",
> +                       get_bug_type(info), (void *)info->ip);
> +               pr_err("%s of size %zu at addr %px by task %s/%d\n",
> +                       info->is_write ? "Write" : "Read", info->access_size,
> +                       info->access_addr, current->comm, task_pid_nr(current));
> +       }
>  }
>
>  static DEFINE_SPINLOCK(report_lock);
> diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
> index 0e987c9ca052..b829535a3ad7 100644
> --- a/mm/kasan/tags.c
> +++ b/mm/kasan/tags.c
> @@ -86,6 +86,11 @@ bool check_memory_region(unsigned long addr, size_t
> size, bool write,
>         if (unlikely(size == 0))
>                 return true;
>
> +       if (unlikely((long)size < 0)) {
> +               kasan_report(addr, size, write, ret_ip);
> +               return false;
> +       }
> +
>         tag = get_tag((const void *)addr);
>
>         /*
>
>
> --
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