include/linux/randomize_kstack.h

0001 /* SPDX-License-Identifier: GPL-2.0-only */
0002 #ifndef _LINUX_RANDOMIZE_KSTACK_H
0003 #define _LINUX_RANDOMIZE_KSTACK_H
0004
0005 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
0006 #include <linux/kernel.h>
0007 #include <linux/jump_label.h>
0008 #include <linux/percpu-defs.h>
0009
0010 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
0011              randomize_kstack_offset);
0012 DECLARE_PER_CPU(u32, kstack_offset);
0013
0014 /*
0015  * Do not use this anywhere else in the kernel. This is used here because
0016  * it provides an arch-agnostic way to grow the stack with correct
0017  * alignment. Also, since this use is being explicitly masked to a max of
0018  * 10 bits, stack-clash style attacks are unlikely. For more details see
0019  * "VLAs" in Documentation/process/deprecated.rst
0020  *
0021  * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
0022  * only with Clang and not GCC). Initializing the unused area on each syscall
0023  * entry is expensive, and generating an implicit call to memset() may also be
0024  * problematic (such as in noinstr functions). Therefore, if the compiler
0025  * supports it (which it should if it initializes allocas), always use the
0026  * "uninitialized" variant of the builtin.
0027  */
0028 #if __has_builtin(__builtin_alloca_uninitialized)
0029 #define __kstack_alloca __builtin_alloca_uninitialized
0030 #else
0031 #define __kstack_alloca __builtin_alloca
0032 #endif
0033
0034 /*
0035  * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
0036  * "VLA" from being unbounded (see above). 10 bits leaves enough room for
0037  * per-arch offset masks to reduce entropy (by removing higher bits, since
0038  * high entropy may overly constrain usable stack space), and for
0039  * compiler/arch-specific stack alignment to remove the lower bits.
0040  */
0041 #define KSTACK_OFFSET_MAX(x)    ((x) & 0x3FF)
0042
0043 /**
0044  * add_random_kstack_offset - Increase stack utilization by previously
0045  *                chosen random offset
0046  *
0047  * This should be used in the syscall entry path when interrupts and
0048  * preempt are disabled, and after user registers have been stored to
0049  * the stack. For testing the resulting entropy, please see:
0050  * tools/testing/selftests/lkdtm/stack-entropy.sh
0051  */
0052 #define add_random_kstack_offset() do {                 \
0053     if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
0054                 &randomize_kstack_offset)) {        \
0055         u32 offset = raw_cpu_read(kstack_offset);       \
0056         u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));   \
0057         /* Keep allocation even after "ptr" loses scope. */ \
0058         asm volatile("" :: "r"(ptr) : "memory");        \
0059     }                               \
0060 } while (0)
0061
0062 /**
0063  * choose_random_kstack_offset - Choose the random offset for the next
0064  *               add_random_kstack_offset()
0065  *
0066  * This should only be used during syscall exit when interrupts and
0067  * preempt are disabled. This position in the syscall flow is done to
0068  * frustrate attacks from userspace attempting to learn the next offset:
0069  * - Maximize the timing uncertainty visible from userspace: if the
0070  *   offset is chosen at syscall entry, userspace has much more control
0071  *   over the timing between choosing offsets. "How long will we be in
0072  *   kernel mode?" tends to be more difficult to predict than "how long
0073  *   will we be in user mode?"
0074  * - Reduce the lifetime of the new offset sitting in memory during
0075  *   kernel mode execution. Exposure of "thread-local" memory content
0076  *   (e.g. current, percpu, etc) tends to be easier than arbitrary
0077  *   location memory exposure.
0078  */
0079 #define choose_random_kstack_offset(rand) do {              \
0080     if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
0081                 &randomize_kstack_offset)) {        \
0082         u32 offset = raw_cpu_read(kstack_offset);       \
0083         offset ^= (rand);                   \
0084         raw_cpu_write(kstack_offset, offset);           \
0085     }                               \
0086 } while (0)
0087 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
0088 #define add_random_kstack_offset()      do { } while (0)
0089 #define choose_random_kstack_offset(rand)   do { } while (0)
0090 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
0091
0092 #endif