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- .. SPDX-License-Identifier: GPL-2.0
- ===========================
- KASLR for Freescale BookE32
- ===========================
- The word KASLR stands for Kernel Address Space Layout Randomization.
- This document tries to explain the implementation of the KASLR for
- Freescale BookE32. KASLR is a security feature that deters exploit
- attempts relying on knowledge of the location of kernel internals.
- Since CONFIG_RELOCATABLE has already supported, what we need to do is
- map or copy kernel to a proper place and relocate. Freescale Book-E
- parts expect lowmem to be mapped by fixed TLB entries(TLB1). The TLB1
- entries are not suitable to map the kernel directly in a randomized
- region, so we chose to copy the kernel to a proper place and restart to
- relocate.
- Entropy is derived from the banner and timer base, which will change every
- build and boot. This not so much safe so additionally the bootloader may
- pass entropy via the /chosen/kaslr-seed node in device tree.
- We will use the first 512M of the low memory to randomize the kernel
- image. The memory will be split in 64M zones. We will use the lower 8
- bit of the entropy to decide the index of the 64M zone. Then we chose a
- 16K aligned offset inside the 64M zone to put the kernel in::
- KERNELBASE
- |--> 64M <--|
- | |
- +---------------+ +----------------+---------------+
- | |....| |kernel| | |
- +---------------+ +----------------+---------------+
- | |
- |-----> offset <-----|
- kernstart_virt_addr
- To enable KASLR, set CONFIG_RANDOMIZE_BASE = y. If KASLR is enabled and you
- want to disable it at runtime, add "nokaslr" to the kernel cmdline.
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