linux-kernel - Re: [patch 2/2] lib/string.c: strlcpy() might read too far

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Date:	Mon, 14 Apr 2014 14:46:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Vegard Nossum <vegard.nossum@...il.com>
Subject: Re: [patch 2/2] lib/string.c: strlcpy() might read too far

On Wed, 2 Apr 2014 11:47:31 +0300 Dan Carpenter <dan.carpenter@...cle.com> wrote:

> Imagine you have a user controlled variable at the end of a struct which
> is allocated at the end of a page.  The strlen() could read beyond the
> mapped memory and cause an oops.

Well, it's hard to conceive of a situation where anything like this
could happen.  Code which copies a string from userspace should
immediately ensure that the kernel copy is null-terminated.  But...  I
guess it's defense in depth.

> Probably there are two reasons why we have never hit this condition in
> real life.  First you would have to be really unlucky for all the
> variables to line up so the oops can happen.  Second we don't do a lot
> of fuzzing with invalid strings.
> 
> The strnlen() call is obviously a little bit slower than strlen() but I
> have tested it and I think it's probably ok.

If we cared about speed we wouldn't take two passes across the input
string ;)


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