Re: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Wed, Nov 14, 2018 at 4:35 AM, William Kucharski
<william.kucharski@xxxxxxxxxx> wrote:
>
>
>> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <isaacm@xxxxxxxxxxxxxx> wrote:
>>
>> diff --git a/mm/usercopy.c b/mm/usercopy.c
>> index 852eb4e..0293645 100644
>> --- a/mm/usercopy.c
>> +++ b/mm/usercopy.c
>> @@ -151,7 +151,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
>>                                      bool to_user)
>> {
>>       /* Reject if object wraps past end of memory. */
>> -     if (ptr + n < ptr)
>> +     if (ptr + (n - 1) < ptr)
>>               usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
>
> I'm being paranoid, but is it possible this routine could ever be passed "n" set to zero?

It's a single-use inline, and zero is tested just before getting called:

        /* Skip all tests if size is zero. */
        if (!n)
                return;

        /* Check for invalid addresses. */
        check_bogus_address((const unsigned long)ptr, n, to_user);


>
> If so, it will erroneously abort indicating a wrapped address as (n - 1) wraps to ULONG_MAX.
>
> Easily fixed via:
>
>         if ((n != 0) && (ptr + (n - 1) < ptr))

Agreed. Thanks for noticing this!

-Kees

-- 
Kees Cook