Messages in this thread |  | | Date | Tue, 24 May 2011 23:02:33 +0200 | | From | Ingo Molnar <> | | Subject | Re: [RFC][PATCH] Randomize kernel base address on boot |
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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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