43 lines
1.7 KiB
ReStructuredText
43 lines
1.7 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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===========================
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KASLR for Freescale BookE32
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===========================
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The word KASLR stands for Kernel Address Space Layout Randomization.
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This document tries to explain the implementation of the KASLR for
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Freescale BookE32. KASLR is a security feature that deters exploit
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attempts relying on knowledge of the location of kernel internals.
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Since CONFIG_RELOCATABLE has already supported, what we need to do is
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map or copy kernel to a proper place and relocate. Freescale Book-E
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parts expect lowmem to be mapped by fixed TLB entries(TLB1). The TLB1
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entries are not suitable to map the kernel directly in a randomized
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region, so we chose to copy the kernel to a proper place and restart to
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relocate.
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Entropy is derived from the banner and timer base, which will change every
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build and boot. This not so much safe so additionally the bootloader may
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pass entropy via the /chosen/kaslr-seed node in device tree.
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We will use the first 512M of the low memory to randomize the kernel
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image. The memory will be split in 64M zones. We will use the lower 8
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bit of the entropy to decide the index of the 64M zone. Then we chose a
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16K aligned offset inside the 64M zone to put the kernel in::
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KERNELBASE
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|--> 64M <--|
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+---------------+ +----------------+---------------+
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| |....| |kernel| | |
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+---------------+ +----------------+---------------+
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|-----> offset <-----|
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kernstart_virt_addr
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To enable KASLR, set CONFIG_RANDOMIZE_BASE = y. If KASLR is enabled and you
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want to disable it at runtime, add "nokaslr" to the kernel cmdline.
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