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authorJason Yan <yanaijie@huawei.com>2019-09-20 17:45:46 +0800
committerMichael Ellerman <mpe@ellerman.id.au>2019-11-13 19:27:58 +1100
commitc2d1a13520eee7f0ac64ffb94f8756006320e4b8 (patch)
tree3d0c6ab3c5d5005a821bfdec40a02ad32e87b593 /Documentation/powerpc
parent74277f00b23263066772fd9e9106acb6a280f84f (diff)
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>
Diffstat (limited to 'Documentation/powerpc')
-rw-r--r--Documentation/powerpc/index.rst1
-rw-r--r--Documentation/powerpc/kaslr-booke32.rst42
2 files changed, 43 insertions, 0 deletions
diff --git a/Documentation/powerpc/index.rst b/Documentation/powerpc/index.rst
index db7b6a880f52..ba5edb3211c0 100644
--- a/Documentation/powerpc/index.rst
+++ b/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
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.