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

[William Kucharski <william.kucharski@xxxxxxxxxx>]

> 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?

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