From: Michael Eager <eager@mvista.com>
To: Florian Weimer <fw@deneb.enyo.de>
Cc: "Joseph S. Myers" <jsm28@cam.ac.uk>, gcc@gcc.gnu.org
Subject: Re: Buffer Overflow Attacks
Date: Thu, 18 Oct 2001 15:02:00 -0000 [thread overview]
Message-ID: <3BCF5170.12820483@mvista.com> (raw)
In-Reply-To: <87y9meypy7.fsf@deneb.enyo.de>
Florian Weimer wrote:
>
> "Joseph S. Myers" <jsm28@cam.ac.uk> writes:
>
> > On Sun, 14 Oct 2001, Florian Weimer wrote:
> >
> >> According to the language standard, buffer overflow detection for
> >> character pointer types is possible only for buffers which are not
> >> nested in other objects (in struct or union objects). Overflowing
> >> character buffers has a well-defined effect if the buffer is contained
> >> in an object (and other objects follow the buffer inside this object),
> >> so a C implementation is not free to detect such errors (which is only
> >> possible if the buffer overflow triggers undefined behavior). ;-)
> >
> > This is a gross over-simplification of the problem of exactly when an
> > object can be accessed through a given pointer. For details see Nick
> > Maclaren's "What is an object when it is at home?" paper, except that I
> > don't think he's published it beyond the UK and WG14 reflectors. For
> > examples where the committee has ruled that bounds of sub-objects can't be
> > exceeded, see DR#017 question 16, and DRs #051 and #178 relating to the
> > "struct hack".
>
> The struct hack for character arrays results in strictly conforming
> programs because arithmetic on pointers to a character type is not
> restricted in any way, as long the enclosing object is not left. This
> is a consequence of 6.3.2.3(7).
I think that this is stretching reading of this paragraph. Creating
a valid pointer address does not mean that deferencing the pointer is
defined.
If you have the following:
struct foo {
char c[31];
int i;
} f;
char *p1;
You can create a pointer which points beyond the end of the array f.c:
p1 = &f.c[32];
The effect of dereferencing this pointer is undefined. There are few
restrictions on the layout of structures. Commonly there would be
padding bytes placed between c and i. Referencing these padding bytes
is undefined.
Indeed, in a hypothetical processor with very fine grained memory
protection, any padding bytes placed between c and i in the struct may
be both unreadable and/or unwritable. (We won't consider the havoc
this might cause to memcopy.)
--
Michael Eager eager@mvista.com 408-328-8426
MontaVista Software, Inc. 1237 E. Arques Ave., Sunnyvale, CA 94085
next prev parent reply other threads:[~2001-10-18 15:02 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-10-14 8:01 dewar
2001-10-14 10:25 ` Florian Weimer
2001-10-14 11:08 ` Joseph S. Myers
2001-10-14 12:14 ` Florian Weimer
2001-10-14 12:29 ` Joseph S. Myers
2001-10-18 15:02 ` Michael Eager [this message]
-- strict thread matches above, loose matches on Subject: below --
2001-10-31 16:39 mike stump
2001-10-18 17:00 mike stump
2001-10-31 10:01 ` Florian Weimer
2001-10-14 12:25 dewar
2001-10-14 5:32 Frank Pilhofer
2001-10-14 7:33 ` Carlo Wood
2001-10-14 10:50 ` Florian Weimer
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