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

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Message-ID: <1306278296.1921.10.camel@dan>
Date:	Tue, 24 May 2011 19:04:56 -0400
From:	Dan Rosenberg <drosenberg@...curity.com>
To:	"H. Peter Anvin" <hpa@...or.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,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Valdis.Kletnieks@...edu, Ingo Molnar <mingo@...e.hu>,
	pageexec@...email.hu
Subject: Re: [RFC][PATCH] Randomize kernel base address on boot

On Tue, 2011-05-24 at 15:31 -0700, H. Peter Anvin wrote:
> On 05/24/2011 01:31 PM, Dan Rosenberg 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.
> > 
> > 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.
> 
> As written, I think this is unsafe, simply because the kernel has no
> idea what memory is actually safe to relocate into, and your code
> doesn't actually make any attempt at doing so.
> 
> The fact that you change CONFIG_PHYSICAL_ALIGN is particularly
> devastating, and will introduce boot failures on real systems.
> 
> For this to be acceptable, you need to at the very least:
> 
> 1. Verify the in the address map passed to the kernel where it is safe
>    to locate the kernel;

I'll do this, thanks.

> 2. Not introduce a performance regression (we avoid locating in the
>    bottom 16 MiB for performance reasons, except on very small systems);

I altered the boot code so that it uses CONFIG_PHYSICAL_START, which
defaults to 16 MiB, as a lower bound on location.  So nothing will ever
get loaded below there, and I still can take advantage of higher
alignment granularity.  Are there other problems I'm not anticipating?

> 3. Make sure not to break kdump.
> 

Ok, I'll be sure to add this to the list of things to test.

Thanks for the feedback.

-Dan


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