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Subject: Re: [PATCH] mm/usercopy: Use memory range to be accessed for
 wraparound check
From:   William Kucharski <william.kucharski@oracle.com>
In-Reply-To: <50baa4900e55b523f18eea2759f8efae@codeaurora.org>
Date:   Wed, 14 Nov 2018 15:50:05 -0700
Cc:     David Laight <David.Laight@aculab.com>,
        Kees Cook <keescook@chromium.org>, crecklin@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        psodagud@codeaurora.org, tsoni@codeaurora.org,
        stable@vger.kernel.org
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To:     "Isaac J. Manjarres" <isaacm@codeaurora.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 14, 2018, at 10:32 AM, isaacm@codeaurora.org wrote:
>=20
> Thank you and David for your feedback. The check_bogus_address() =
routine is only invoked from one place in the kernel, which is =
__check_object_size(). Before invoking check_bogus_address, =
__check_object_size ensures that n is non-zero, so it is not possible to =
call this routine with n being 0. Therefore, we shouldn't run into the =
scenario you described. Also, in the case where we are copying a page's =
contents into a kernel space buffer and will not have that buffer =
interacting with userspace at all, this change to that check should =
still be valid, correct?

Having fixed more than one bug resulting from a "only called in one =
place" routine later being called elsewhere,
I am wary, but ultimately it's likely not worth the performance hit of a =
check or BUG_ON().

It's a generic math check for overflow, so it should work with any =
address.

Reviewed-by: William Kucharski <william.kucharski@oracle.com>=