linux-kernel - Re: [PATCH] isdnloop: NUL-terminate strings from userspace

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Message-ID: <20140307115253.GL4774@mwanda>
Date:	Fri, 7 Mar 2014 14:52:54 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] isdnloop: NUL-terminate strings from userspace

On Fri, Mar 07, 2014 at 12:42:12PM +0100, Vegard Nossum wrote:
> On 03/07/2014 12:26 PM, Dan Carpenter wrote:
> >On Fri, Mar 07, 2014 at 11:56:04AM +0100, Vegard Nossum wrote:
> >>Both the in-kernel and BSD strlcpy() require that the source string is
> >>NUL terminated.
> >
> >No.  You're obviously wrong.  What on earth?
> 
> Well, from lib/string.c:
> 
> size_t strlcpy(char *dest, const char *src, size_t size)
> {
>         size_t ret = strlen(src);
>

Ah...  So you mean that we could read far beyond the end of the string
and it would be a DoS because there would be 4 gigs of memory before we
hit a NUL character.  That won't happen in this case because the user
only controls a small buffer.  Normal memory is full of NUL chars.

I don't know the speed impact of changing the strlen() there to
strnlen().

> The BSD man page:
> 
> "Also note that strlcpy() and strlcat() only operate on true ``C''
> strings.  This means that for strlcpy() src must be NUL-terminated
> and for strlcat() both src and dst must be NUL-terminated."

It's talking about the kind of strings.  If it's a string which includes
NUL characters the strlcpy() won't work for that.  Or if it is not
*supposed* to end in a NUL character then it won't work for that.

We are using C strings here.

regards,
dan carpenter

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