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

[William Kucharski]

> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <isa...@codeaurora.org> 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?

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))

William Kucharski