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

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Date:   Wed, 14 Nov 2018 03:35:25 -0700
From:   William Kucharski <william.kucharski@...cle.com>
To:     "Isaac J. Manjarres" <isaacm@...eaurora.org>
Cc:     Kees Cook <keescook@...omium.org>, crecklin@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        psodagud@...eaurora.org, tsoni@...eaurora.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm/usercopy: Use memory range to be accessed for
 wraparound check



> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <isaacm@...eaurora.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