| .. 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. |
