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randomize_kstack: Improve docs on requirements/rationale

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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>
This commit is contained in:
Kees Cook 2022-05-10 10:29:36 -07:00
parent 8c6a490e40
commit 1ff297584f
1 changed files with 24 additions and 3 deletions
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27
include/linux/randomize_kstack.h
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@ -40,10 +40,14 @@ DECLARE_PER_CPU(u32, kstack_offset);
*/
#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
/*
* These macros must be used during syscall entry when interrupts and
/**
* add_random_kstack_offset - Increase stack utilization by previously
* chosen random offset
*
* This should be used in the syscall entry path when interrupts and
* preempt are disabled, and after user registers have been stored to
* the stack.
* the stack. For testing the resulting entropy, please see:
* tools/testing/selftests/lkdtm/stack-entropy.sh
*/
#define add_random_kstack_offset() do { \
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
@ -55,6 +59,23 @@ DECLARE_PER_CPU(u32, kstack_offset);
} \
} while (0)
/**
* choose_random_kstack_offset - Choose the random offset for the next
* add_random_kstack_offset()
*
* This should only be used during syscall exit when interrupts and
* preempt are disabled. This position in the syscall flow is done to
* frustrate attacks from userspace attempting to learn the next offset:
* - Maximize the timing uncertainty visible from userspace: if the
* offset is chosen at syscall entry, userspace has much more control
* over the timing between choosing offsets. "How long will we be in
* kernel mode?" tends to be more difficult to predict than "how long
* will we be in user mode?"
* - Reduce the lifetime of the new offset sitting in memory during
* kernel mode execution. Exposure of "thread-local" memory content
* (e.g. current, percpu, etc) tends to be easier than arbitrary
* location memory exposure.
*/
#define choose_random_kstack_offset(rand) do { \
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \