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

Dan Rosenberg
2011-05-24T22:56:10Z

On Tue, 2011-05-24 at 23:02 +0200, Ingo Molnar wrote:
> * 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.
>

Alright, I'll start working on this.

> > 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.
>

Good idea, will do.

-Dan