linux-kernel - Re: [RFC][PATCH] Randomize kernel base address on boot

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Message-ID: <20110524210233.GI27634@elte.hu>
Date:	Tue, 24 May 2011 23:02:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	Tony Luck <tony.luck@...il.com>, linux-kernel@...r.kernel.org,
	davej@...hat.com, kees.cook@...onical.com, davem@...emloft.net,
	eranian@...gle.com, torvalds@...ux-foundation.org,
	adobriyan@...il.com, penberg@...nel.org, hpa@...or.com,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Valdis.Kletnieks@...edu, pageexec@...email.hu
Subject: Re: [RFC][PATCH] Randomize kernel base address on boot


* Dan Rosenberg <drosenberg@...curity.com> wrote:

> This introduces CONFIG_RANDOMIZE_BASE, which randomizes the address at
> which the kernel is decompressed at boot as a security feature that
> deters exploit attempts relying on knowledge of the location of kernel
> internals.  The default values of the kptr_restrict and dmesg_restrict
> sysctls are set to (1) when this is enabled, since hiding kernel
> pointers is necessary to preserve the secrecy of the randomized base
> address.

That was quick! :-)

> This feature also uses a fixed mapping to move the IDT (if not already
> done as a fix for the F00F bug), to avoid exposing the location of
> kernel internals relative to the original IDT.  This has the additional
> security benefit of marking the new virtual address of the IDT
> read-only.

Btw., as i suggested before the IDT should be made percpu, that way we could 
split out and evaluate the IDT change independently of any security 
considerations, as a potential scalability improvement. Makes the decision 
easier because right now moving the IDT to a 4K TLB increases the kernel's TLB 
footprint a tiny bit.

> Entropy is generated using the RDRAND instruction if it is supported. If not, 
> then RDTSC is used, if supported. If neither RDRAND nor RDTSC are supported, 
> then no randomness is introduced. Support for the CPUID instruction is 
> required to check for the availability of these two instructions.

Btw., i'd suggest to fall back not to zero but to something system specific 
like RAM size or a BIOS signature such as the contents of 0xf0000 or so. This, 
while clearly not random, will at least *somewhat* randomize the kernel against 
remote attackers who do not know the RAM size or the system type.

Thanks,

	Ingo
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