1 /* SPDX-License-Identifier: GPL-2.0-only */
2 #ifndef _LINUX_RANDOMIZE_KSTACK_H
3 #define _LINUX_RANDOMIZE_KSTACK_H
4 
5 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
6 #include <linux/kernel.h>
7 #include <linux/jump_label.h>
8 #include <linux/percpu-defs.h>
9 
10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
11 			 randomize_kstack_offset);
12 DECLARE_PER_CPU(u32, kstack_offset);
13 
14 /*
15  * Do not use this anywhere else in the kernel. This is used here because
16  * it provides an arch-agnostic way to grow the stack with correct
17  * alignment. Also, since this use is being explicitly masked to a max of
18  * 10 bits, stack-clash style attacks are unlikely. For more details see
19  * "VLAs" in Documentation/process/deprecated.rst
20  *
21  * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
22  * only with Clang and not GCC). Initializing the unused area on each syscall
23  * entry is expensive, and generating an implicit call to memset() may also be
24  * problematic (such as in noinstr functions). Therefore, if the compiler
25  * supports it (which it should if it initializes allocas), always use the
26  * "uninitialized" variant of the builtin.
27  */
28 #if __has_builtin(__builtin_alloca_uninitialized)
29 #define __kstack_alloca __builtin_alloca_uninitialized
30 #else
31 #define __kstack_alloca __builtin_alloca
32 #endif
33 
34 /*
35  * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
36  * "VLA" from being unbounded (see above). 10 bits leaves enough room for
37  * per-arch offset masks to reduce entropy (by removing higher bits, since
38  * high entropy may overly constrain usable stack space), and for
39  * compiler/arch-specific stack alignment to remove the lower bits.
40  */
41 #define KSTACK_OFFSET_MAX(x)	((x) & 0x3FF)
42 
43 /**
44  * add_random_kstack_offset - Increase stack utilization by previously
45  *			      chosen random offset
46  *
47  * This should be used in the syscall entry path when interrupts and
48  * preempt are disabled, and after user registers have been stored to
49  * the stack. For testing the resulting entropy, please see:
50  * tools/testing/selftests/lkdtm/stack-entropy.sh
51  */
52 #define add_random_kstack_offset() do {					\
53 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
54 				&randomize_kstack_offset)) {		\
55 		u32 offset = raw_cpu_read(kstack_offset);		\
56 		u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));	\
57 		/* Keep allocation even after "ptr" loses scope. */	\
58 		asm volatile("" :: "r"(ptr) : "memory");		\
59 	}								\
60 } while (0)
61 
62 /**
63  * choose_random_kstack_offset - Choose the random offset for the next
64  *				 add_random_kstack_offset()
65  *
66  * This should only be used during syscall exit when interrupts and
67  * preempt are disabled. This position in the syscall flow is done to
68  * frustrate attacks from userspace attempting to learn the next offset:
69  * - Maximize the timing uncertainty visible from userspace: if the
70  *   offset is chosen at syscall entry, userspace has much more control
71  *   over the timing between choosing offsets. "How long will we be in
72  *   kernel mode?" tends to be more difficult to predict than "how long
73  *   will we be in user mode?"
74  * - Reduce the lifetime of the new offset sitting in memory during
75  *   kernel mode execution. Exposure of "thread-local" memory content
76  *   (e.g. current, percpu, etc) tends to be easier than arbitrary
77  *   location memory exposure.
78  */
79 #define choose_random_kstack_offset(rand) do {				\
80 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
81 				&randomize_kstack_offset)) {		\
82 		u32 offset = raw_cpu_read(kstack_offset);		\
83 		offset ^= (rand);					\
84 		raw_cpu_write(kstack_offset, offset);			\
85 	}								\
86 } while (0)
87 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
88 #define add_random_kstack_offset()		do { } while (0)
89 #define choose_random_kstack_offset(rand)	do { } while (0)
90 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
91 
92 #endif
93