Add document to explain how we implement KASLR for fsl_booke32. Signed-off-by: Jason Yan <yanai...@huawei.com> Cc: Diana Craciun <diana.crac...@nxp.com> Cc: Michael Ellerman <m...@ellerman.id.au> Cc: Christophe Leroy <christophe.le...@c-s.fr> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> Cc: Paul Mackerras <pau...@samba.org> Cc: Nicholas Piggin <npig...@gmail.com> Cc: Kees Cook <keesc...@chromium.org> --- Documentation/powerpc/kaslr-booke32.rst | 42 +++++++++++++++++++++++++ 1 file changed, 42 insertions(+) create mode 100644 Documentation/powerpc/kaslr-booke32.rst
diff --git a/Documentation/powerpc/kaslr-booke32.rst b/Documentation/powerpc/kaslr-booke32.rst new file mode 100644 index 000000000000..8b259fdfdf03 --- /dev/null +++ b/Documentation/powerpc/kaslr-booke32.rst @@ -0,0 +1,42 @@ +.. 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 enable and you +want to disable it at runtime, add "nokaslr" to the kernel cmdline. -- 2.17.2