Re: [PATCH] isdnloop: NUL-terminate strings from userspace
From: Hannes Frederic Sowa
Date: Tue Apr 01 2014 - 07:03:48 EST
On Tue, Apr 01, 2014 at 12:46:37PM +0200, Vegard Nossum wrote:
> On 04/01/2014 12:30 PM, Hannes Frederic Sowa wrote:
> >Looking down the problem, it seems the problem is that the strlen in
> >could read beyond the input buffer?
> >To prevent this problem in other parts of the kernel wouldn't it be better
> >replace the strlen with strnlen in strlcpy?
> Sorry, I should have included the link to the previous thread:
> I only resent (adding netdev to Cc) to get this into David Miller's
> patch queue.
Ah ok, sorry I don't follow lkml as closely as netdev@.
> As you can see from the previous discussion, we _could_ change the Linux
> kernel's definition of strlcpy(), but I wouldn't recommend it for the
> following reasons:
> 1. Both BSD man page and BSD implementation _require_ the source string
> to be 0-terminated. Changing the semantics of strlcpy() in the Linux
> kernel would probably be a bad idea and cause even more confusion that
> what we already have.
Sure, we shouldn't change the documented semantics. If at all it would
be an additional safety net. Your patch would still be needed.
> 2. Even if we changed strlcpy() to use strnlen(), it would still be
> unsafe if the source string is not 0-terminated and the source buffer is
> shorter than the destination buffer. That's because the size passed to
> strlcpy() is conceptually the length of the _destination_ buffer, not
> the source string.
> I'm not against changing strlcpy() per se (changing to strnlen() might
> be a performance improvement), but we shouldn't use that as an excuse to
> use the interface incorrectly.
I am totally with you there.
Actually in some cases it could hinder finding such bugs as we're more
unlikely to hit a RED_ZONE which should crash the kernel (I actually
think crashes to find such bugs are good). But I guess the propability
is pretty high to hit another NUL byte before that and if at that point a
RED_ZONE is mapped.
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