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randomize_kstack: Improve docs on requirements/rationale
There were some recent questions about where and why to use the random_kstack routines when applying them to new architectures[1]. Update the header comments to reflect the design choices for the routines. [1] https://lore.kernel.org/lkml/1652173338.7bltwybi0c.astroid@bobo.none Cc: Nicholas Piggin <npiggin@gmail.com> Cc: Xiu Jianfeng <xiujianfeng@huawei.com> Signed-off-by: Kees Cook <keescook@chromium.org>
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1 changed files with 24 additions and 3 deletions
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@ -40,10 +40,14 @@ DECLARE_PER_CPU(u32, kstack_offset);
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*/
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#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
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/*
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* These macros must be used during syscall entry when interrupts and
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/**
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* add_random_kstack_offset - Increase stack utilization by previously
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* chosen random offset
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*
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* This should be used in the syscall entry path when interrupts and
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* preempt are disabled, and after user registers have been stored to
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* the stack.
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* the stack. For testing the resulting entropy, please see:
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* tools/testing/selftests/lkdtm/stack-entropy.sh
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*/
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#define add_random_kstack_offset() do { \
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if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
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@ -55,6 +59,23 @@ DECLARE_PER_CPU(u32, kstack_offset);
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} \
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} while (0)
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/**
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* choose_random_kstack_offset - Choose the random offset for the next
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* add_random_kstack_offset()
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*
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* This should only be used during syscall exit when interrupts and
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* preempt are disabled. This position in the syscall flow is done to
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* frustrate attacks from userspace attempting to learn the next offset:
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* - Maximize the timing uncertainty visible from userspace: if the
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* offset is chosen at syscall entry, userspace has much more control
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* over the timing between choosing offsets. "How long will we be in
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* kernel mode?" tends to be more difficult to predict than "how long
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* will we be in user mode?"
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* - Reduce the lifetime of the new offset sitting in memory during
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* kernel mode execution. Exposure of "thread-local" memory content
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* (e.g. current, percpu, etc) tends to be easier than arbitrary
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* location memory exposure.
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*/
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#define choose_random_kstack_offset(rand) do { \
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if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
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&randomize_kstack_offset)) { \
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