Date: Tue, 24 May 2011 23:02:33 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Dan Rosenberg <drosenberg@vsecurity.com>
Cc: Tony Luck <tony.luck@gmail.com>, linux-kernel@vger.kernel.org,
        davej@redhat.com, kees.cook@canonical.com, davem@davemloft.net,
        eranian@google.com, torvalds@linux-foundation.org, adobriyan@gmail.com,
        penberg@kernel.org, hpa@zytor.com,
        Arjan van de Ven <arjan@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, Valdis.Kletnieks@vt.edu,
        pageexec@freemail.hu
Subject: Re: [RFC][PATCH] Randomize kernel base address on boot
Message-ID: <20110524210233.GI27634@elte.hu>
References: <1306269105.21443.20.camel@dan>
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* Dan Rosenberg <drosenberg@vsecurity.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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