linux-kernel - Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 17 Dec 2007 17:48:06 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	John Reiser <jreiser@...Wagon.com>,
	Andrew Morton <akpm@...ux-foundation.org>, security@...nel.org,
	tytso@....edu,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	mpm@...enic.com, linux-sparse@...r.kernel.org
Subject: Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)

On Mon, Dec 17, 2007 at 09:28:57AM -0800, Linus Torvalds wrote:
> 
> 
> On Sat, 15 Dec 2007, Herbert Xu wrote:
> > 
> > There ought to be a warning about this sort of thing.
> 
> We could add it to sparse. The appended (untested) patch seems to say 
> there's a lot of those signed divides-by-power-of-twos.

I'm not sure that you are warning about the right things.  If you want
a real nightmare scenario in that area, consider this:

	int x[20];

	int *p = x + n;
	int *q = x + m;

	p - q
	((char *)p - (char *)q)/4
	((char *)p - (char *)q)/sizeof(int)

The first two are equivalent on all targets we care about.  However, an
attempt to make the second one "more portable" silently creates the
code that'll do something entirely different as soon as we get m > n...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/