85 lines
2.9 KiB
C
85 lines
2.9 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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#include <linux/prandom.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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/*
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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, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is
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* to keep the "VLA" from being unbounded (see above). Additionally clear
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* the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack
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* alignment will always be at least word size. This makes the compiler
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* code gen better when it is applying the actual per-arch alignment to
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* the final offset. The resulting randomness is reasonable without overly
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* constraining usable stack space.
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*/
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#ifdef CONFIG_64BIT
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#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000)
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#else
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#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100)
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#endif
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DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
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static __always_inline u32 get_kstack_offset(void)
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{
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struct rnd_state *state;
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u32 rnd;
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state = &get_cpu_var(kstack_rnd_state);
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rnd = prandom_u32_state(state);
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put_cpu_var(kstack_rnd_state);
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return rnd;
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}
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/**
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* add_random_kstack_offset - Increase stack utilization by a random offset.
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*
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* This should be used in the syscall entry path after user registers have been
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* stored to the stack. Preemption may be enabled. For testing the resulting
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* entropy, please see: 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 = get_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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#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#define add_random_kstack_offset() do { } while (0)
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#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#endif
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