linux-kernel - Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of attacking

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Date:	Wed, 10 Nov 2010 09:53:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Marcus Meissner <meissner@...e.de>,
	linux-kernel@...r.kernel.org, jason.wessel@...driver.com,
	fweisbec@...il.com, tj@...nel.org, mort@....com, akpm@...l.org,
	security@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of
 attacking


* H. Peter Anvin <hpa@...or.com> wrote:

> We already do virtual relocation on 32 bits, and replicating that on 64 bits 
> wouldn't be hard.  However, the linkage script strongly assumes congruency mod 2/4 
> MiB, and that is probably nontrivial to change.  However, that still gives about 9 
> bits of entrophy to play with.  The question is if that is enough, or if we'd have 
> to do more clever hacks.

Even 1 bit of entropy would bring a visible improvement: a failed exploit attempt to 
the wrong address can crash the kernel with a 50% chance. 9 bits would be very nice.

If an exploit can be brute-forced without crashing the kernel then only some 
significantly large bitness would help. So while 9 bits would be rather low for a 
user-space ASLR scheme [many user-space bugs can be brute-forced without crashing 
the system and raising alarms], it's very attractive for kernel ASLR.

	Ingo
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