Subject: Re: chmod 111
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Nick Warne <nick@linicks.net>,
       Felipe Alfaro Solana <felipe.alfaro@gmail.com>,
       linux-kernel@vger.kernel.org
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	 <Pine.LNX.4.64.0603171036240.3618@g5.osdl.org>
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Date: Fri, 17 Mar 2006 13:55:28 -0500
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On Fri, 2006-03-17 at 10:43 -0800, Linus Torvalds wrote:
> 
> On Fri, 17 Mar 2006, Steven Rostedt wrote:
> > 
> > So I guess if you need to debug a system binary, you need it readable.
> > But I guess that can also be a security problem, and having system
> > binaries not readable, might make you system a little more secure.
> 
> NOTE! The kernel does not guarantee that you can't read execute-only 
> binaries.
> 
> In particular, it's fairly easy to create a shared library that replaces a 
> system library (LD_LIBRARY_PATH) and then just dumps out the binary image.
> 
> So anybody who thinks that 0111 permissions are somehow "more secure" than 
> 0755 is just setting himself up for disappointment.  You're much better 
> off just having all binaries be 0755 and getting the security through 
> other means.
> 
> Basically, you should think of the "executable" bit as a way to say "this 
> file is appropriate for execve(), and btw, that does imply that we'll need 
> to read it into memory too". You should _not_ depend on it for security, 
> although dropping the readability bits will mean that certain -trivial- 
> programs won't be able to read it.
> 
> For example, making a binary unreadable is a perfectly good way to stop a 
> web browser or other interface from exporting it outside the machine: but 
> it's not so much about security as about _accidental_ leaking. 
> 
> So from a security standpoint, you're much better off thinking "executable 
> means readable", than lulling yourself into some false sense of security. 

Yep, I agree whole heartily.  I should have stressed the "little" part
in the above quote. "might make your system a __little__ more secure.".
I didn't want to get into the details that you described, but you are
entirely right.  I just wanted to note that it does stop someone at some
level. But it's more of an annoyance to someone with a little more
experience in hacking into a computer.

But since it does stop some amateur's does give it some weight to
actually do it.  But it is far from actually protecting yourself from a
more devious cracker.

But thanks for the critical disclaimer.

-- Steve


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