powerpc/fsl_booke/32: Document KASLR implementation

Add document to explain how we implement KASLR for fsl_booke32.
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Scott Wood <oss@buserror.net>
[mpe: Add it to the index as well]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/powerpc/index.rst

@@ -19,6 +19,7 @@ powerpc
     firmware-assisted-dump
     hvcs
     isa-versions
+    kaslr-booke32
     mpc52xx
     pci_iov_resource_on_powernv
     pmu-ebb

Documentation/powerpc/kaslr-booke32.rst

+.. 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.