On Mon, Apr 18, 2016 at 05:09:48PM +0200, Ard Biesheuvel wrote:
> Currently, our KASLR implementation randomizes the placement of the core
> kernel at 2 MB granularity. This is based on the arm64 kernel boot
> protocol, which mandates that the kernel is loaded TEXT_OFFSET bytes above
> a 2 MB aligned base address. This requirement is a result of the fact that
> the block size used by the early mapping code may be 2 MB at the most (for
> a 4 KB granule kernel)
>
> But we can do better than that: since a KASLR kernel needs to be relocated
> in any case, we can tolerate a physical misalignment as long as the virtual
> misalignment relative to this 2 MB block size is equal in size, and code to
> deal with this is already in place.
>
> Since we align the kernel segments to 64 KB, let's randomize the physical
> offset at 64 KB granularity as well (unless CONFIG_DEBUG_ALIGN_RODATA is
> enabled). This way, the page table and TLB footprint is not affected.
>
> The higher granularity allows for 5 bits of additional entropy to be used.
>
> Signed-off-by: Ard Biesheuvel <[email protected]>
> ---
> drivers/firmware/efi/libstub/arm64-stub.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
Adding Matt to Cc, since this touches the stub and I'll need his ack
before I can merge it.
Will
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c
> b/drivers/firmware/efi/libstub/arm64-stub.c
> index a90f6459f5c6..eae693eb3e91 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -81,15 +81,24 @@ efi_status_t handle_kernel_image(efi_system_table_t
> *sys_table_arg,
>
> if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && phys_seed != 0) {
> /*
> + * If CONFIG_DEBUG_ALIGN_RODATA is not set, produce a
> + * displacement in the interval [0, MIN_KIMG_ALIGN) that
> + * is a multiple of the minimal segment alignment (SZ_64K)
> + */
> + u32 mask = (MIN_KIMG_ALIGN - 1) & ~(SZ_64K - 1);
> + u32 offset = !IS_ENABLED(CONFIG_DEBUG_ALIGN_RODATA) ?
> + (phys_seed >> 32) & mask : TEXT_OFFSET;
> +
> + /*
> * If KASLR is enabled, and we have some randomness available,
> * locate the kernel at a randomized offset in physical memory.
> */
> - *reserve_size = kernel_memsize + TEXT_OFFSET;
> + *reserve_size = kernel_memsize + offset;
> status = efi_random_alloc(sys_table_arg, *reserve_size,
> MIN_KIMG_ALIGN, reserve_addr,
> - phys_seed);
> + (u32)phys_seed);
>
> - *image_addr = *reserve_addr + TEXT_OFFSET;
> + *image_addr = *reserve_addr + offset;
> } else {
> /*
> * Else, try a straight allocation at the preferred offset.
> --
> 2.5.0
>
