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

    * 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.

    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.



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