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

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Message-ID: <a4676065-571a-4dda-bd3a-798a9b2d7edf@email.android.com>
Date:	Sun, 07 Nov 2010 10:32:33 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>,
	Marcus Meissner <meissner@...e.de>
CC:	Ingo Molnar <mingo@...e.hu>, 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

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.



"Andi Kleen" <andi@...stfloor.org> wrote:

>Marcus Meissner <meissner@...e.de> writes:
>>
>> I also briefly thought about kernel ASLR, but my knowledge of the
>kernel
>> loading is too limited whether this is even possible or at all
>useful.
>
>Kernel ASLR sounds like a good idea, although there are some traps.
>
>On 32bit the available range is not too great, only a few hundred MB
>max. Probably less on a larger systems, there will conflicts with a
>large mem_map.  On 64bit x86 it's nearly 2GB and somewhat easier
>(although a large mem_map may still be a problem)
>
>You still want to not stray too much from a 2MB alignment
>to make sure most of the main kernel is handled by a single 2MB TLB
>entry. 
>
>It would not be too hard to do today using kexec and loading the kernel
>twice.  Right now the kexec command doesn't allow specifying
>the address, but the kernel interface supports it, so it could
>be just implemented in the user tool.
>
>Doing it with a single boot sequence would be a bit more work.
>Right now the relocation entries are not put into the bzImage
>and that would be needed.
>
>That would not cover modules, but it shouldn't be too difficult
>to do it for those either.
>
>-Andi
>
>-- 
>ak@...ux.intel.com -- Speaking for myself only.

-- 
Sent from my mobile phone.  Please pardon any lack of formatting.
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