forked from mirrors/linux
1ff297584f
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>
92 lines
3.7 KiB
C
92 lines
3.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only */
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#ifndef _LINUX_RANDOMIZE_KSTACK_H
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#define _LINUX_RANDOMIZE_KSTACK_H
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#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
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#include <linux/kernel.h>
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#include <linux/jump_label.h>
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#include <linux/percpu-defs.h>
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DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
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randomize_kstack_offset);
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DECLARE_PER_CPU(u32, kstack_offset);
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/*
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* Do not use this anywhere else in the kernel. This is used here because
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* it provides an arch-agnostic way to grow the stack with correct
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* alignment. Also, since this use is being explicitly masked to a max of
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* 10 bits, stack-clash style attacks are unlikely. For more details see
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* "VLAs" in Documentation/process/deprecated.rst
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*
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* The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
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* only with Clang and not GCC). Initializing the unused area on each syscall
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* entry is expensive, and generating an implicit call to memset() may also be
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* problematic (such as in noinstr functions). Therefore, if the compiler
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* supports it (which it should if it initializes allocas), always use the
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* "uninitialized" variant of the builtin.
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*/
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#if __has_builtin(__builtin_alloca_uninitialized)
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#define __kstack_alloca __builtin_alloca_uninitialized
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#else
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#define __kstack_alloca __builtin_alloca
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#endif
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/*
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* Use, at most, 10 bits of entropy. We explicitly cap this to keep the
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* "VLA" from being unbounded (see above). 10 bits leaves enough room for
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* per-arch offset masks to reduce entropy (by removing higher bits, since
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* high entropy may overly constrain usable stack space), and for
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* compiler/arch-specific stack alignment to remove the lower bits.
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*/
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#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
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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. 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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&randomize_kstack_offset)) { \
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u32 offset = raw_cpu_read(kstack_offset); \
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u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
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/* Keep allocation even after "ptr" loses scope. */ \
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asm volatile("" :: "r"(ptr) : "memory"); \
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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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u32 offset = raw_cpu_read(kstack_offset); \
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offset ^= (rand); \
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raw_cpu_write(kstack_offset, offset); \
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} \
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} while (0)
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#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#define add_random_kstack_offset() do { } while (0)
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#define choose_random_kstack_offset(rand) do { } while (0)
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#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#endif
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