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

From: Dan Carpenter
Date: Tue Apr 01 2014 - 08:36:01 EST

On Tue, Apr 01, 2014 at 01:02:55PM +0200, Hannes Frederic Sowa wrote:
> 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
> > >strlcpy
> > >could read beyond the input buffer?
> > >
> > >To prevent this problem in other parts of the kernel wouldn't it be better
> > >to
> > >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.

Guys, really? How would the patch "still be needed"? I feel like if
someone said we had to rub a chicken head on this code we do it in the
name of security...

I don't understand what you think the point of strlcpy() is, if it's not
to deal with source strings which aren't NUL terminated.

I still maintain that the since the stack is full of NUL characters the
current implimentation of strlcpy() is ok for this isdn_loop function
and the patch is not needed at all without the strnlen() change.

However for other heap allocated variables then I could imagine that
the strlen() might be a problem. I have two theories why we have never
seen problems with this in running code. 1) The string would have to be
at the end of a struct allocated at the end of a page. You have to be
very unlucky to hit this requirement. 2) Most people pass valid data.

dan carpenter

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