linux-kernel - [PATCH v6 12/12] powerpc/fsl_booke/32: Document KASLR implementation

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Message-ID: <20190809100800.5426-13-yanaijie@huawei.com>
Date:   Fri, 9 Aug 2019 18:08:00 +0800
From:   Jason Yan <yanaijie@...wei.com>
To:     <mpe@...erman.id.au>, <linuxppc-dev@...ts.ozlabs.org>,
        <diana.craciun@....com>, <christophe.leroy@....fr>,
        <benh@...nel.crashing.org>, <paulus@...ba.org>,
        <npiggin@...il.com>, <keescook@...omium.org>,
        <kernel-hardening@...ts.openwall.com>
CC:     <linux-kernel@...r.kernel.org>, <wangkefeng.wang@...wei.com>,
        <yebin10@...wei.com>, <thunder.leizhen@...wei.com>,
        <jingxiangfeng@...wei.com>, <fanchengyang@...wei.com>,
        <zhaohongjiang@...wei.com>, Jason Yan <yanaijie@...wei.com>
Subject: [PATCH v6 12/12] powerpc/fsl_booke/32: Document KASLR implementation

Add document to explain how we implement KASLR for fsl_booke32.

Signed-off-by: Jason Yan <yanaijie@...wei.com>
Cc: Diana Craciun <diana.craciun@....com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Christophe Leroy <christophe.leroy@....fr>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Kees Cook <keescook@...omium.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