xref: /linux/mm/kfence/kfence.h (revision f9bff0e31881d03badf191d3b0005839391f5f2b)
1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3  * Kernel Electric-Fence (KFENCE). For more info please see
4  * Documentation/dev-tools/kfence.rst.
5  *
6  * Copyright (C) 2020, Google LLC.
7  */
8 
9 #ifndef MM_KFENCE_KFENCE_H
10 #define MM_KFENCE_KFENCE_H
11 
12 #include <linux/mm.h>
13 #include <linux/slab.h>
14 #include <linux/spinlock.h>
15 #include <linux/types.h>
16 
17 #include "../slab.h" /* for struct kmem_cache */
18 
19 /*
20  * Get the canary byte pattern for @addr. Use a pattern that varies based on the
21  * lower 3 bits of the address, to detect memory corruptions with higher
22  * probability, where similar constants are used.
23  */
24 #define KFENCE_CANARY_PATTERN_U8(addr) ((u8)0xaa ^ (u8)((unsigned long)(addr) & 0x7))
25 
26 /*
27  * Define a continuous 8-byte canary starting from a multiple of 8. The canary
28  * of each byte is only related to the lowest three bits of its address, so the
29  * canary of every 8 bytes is the same. 64-bit memory can be filled and checked
30  * at a time instead of byte by byte to improve performance.
31  */
32 #define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(le64_to_cpu(0x0706050403020100)))
33 
34 /* Maximum stack depth for reports. */
35 #define KFENCE_STACK_DEPTH 64
36 
37 /* KFENCE object states. */
38 enum kfence_object_state {
39 	KFENCE_OBJECT_UNUSED,		/* Object is unused. */
40 	KFENCE_OBJECT_ALLOCATED,	/* Object is currently allocated. */
41 	KFENCE_OBJECT_FREED,		/* Object was allocated, and then freed. */
42 };
43 
44 /* Alloc/free tracking information. */
45 struct kfence_track {
46 	pid_t pid;
47 	int cpu;
48 	u64 ts_nsec;
49 	int num_stack_entries;
50 	unsigned long stack_entries[KFENCE_STACK_DEPTH];
51 };
52 
53 /* KFENCE metadata per guarded allocation. */
54 struct kfence_metadata {
55 	struct list_head list;		/* Freelist node; access under kfence_freelist_lock. */
56 	struct rcu_head rcu_head;	/* For delayed freeing. */
57 
58 	/*
59 	 * Lock protecting below data; to ensure consistency of the below data,
60 	 * since the following may execute concurrently: __kfence_alloc(),
61 	 * __kfence_free(), kfence_handle_page_fault(). However, note that we
62 	 * cannot grab the same metadata off the freelist twice, and multiple
63 	 * __kfence_alloc() cannot run concurrently on the same metadata.
64 	 */
65 	raw_spinlock_t lock;
66 
67 	/* The current state of the object; see above. */
68 	enum kfence_object_state state;
69 
70 	/*
71 	 * Allocated object address; cannot be calculated from size, because of
72 	 * alignment requirements.
73 	 *
74 	 * Invariant: ALIGN_DOWN(addr, PAGE_SIZE) is constant.
75 	 */
76 	unsigned long addr;
77 
78 	/*
79 	 * The size of the original allocation.
80 	 */
81 	size_t size;
82 
83 	/*
84 	 * The kmem_cache cache of the last allocation; NULL if never allocated
85 	 * or the cache has already been destroyed.
86 	 */
87 	struct kmem_cache *cache;
88 
89 	/*
90 	 * In case of an invalid access, the page that was unprotected; we
91 	 * optimistically only store one address.
92 	 */
93 	unsigned long unprotected_page;
94 
95 	/* Allocation and free stack information. */
96 	struct kfence_track alloc_track;
97 	struct kfence_track free_track;
98 	/* For updating alloc_covered on frees. */
99 	u32 alloc_stack_hash;
100 #ifdef CONFIG_MEMCG
101 	struct obj_cgroup *objcg;
102 #endif
103 };
104 
105 #define KFENCE_METADATA_SIZE PAGE_ALIGN(sizeof(struct kfence_metadata) * \
106 					CONFIG_KFENCE_NUM_OBJECTS)
107 
108 extern struct kfence_metadata *kfence_metadata;
109 
110 static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
111 {
112 	long index;
113 
114 	/* The checks do not affect performance; only called from slow-paths. */
115 
116 	if (!is_kfence_address((void *)addr))
117 		return NULL;
118 
119 	/*
120 	 * May be an invalid index if called with an address at the edge of
121 	 * __kfence_pool, in which case we would report an "invalid access"
122 	 * error.
123 	 */
124 	index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
125 	if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
126 		return NULL;
127 
128 	return &kfence_metadata[index];
129 }
130 
131 /* KFENCE error types for report generation. */
132 enum kfence_error_type {
133 	KFENCE_ERROR_OOB,		/* Detected a out-of-bounds access. */
134 	KFENCE_ERROR_UAF,		/* Detected a use-after-free access. */
135 	KFENCE_ERROR_CORRUPTION,	/* Detected a memory corruption on free. */
136 	KFENCE_ERROR_INVALID,		/* Invalid access of unknown type. */
137 	KFENCE_ERROR_INVALID_FREE,	/* Invalid free. */
138 };
139 
140 void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *regs,
141 			 const struct kfence_metadata *meta, enum kfence_error_type type);
142 
143 void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta);
144 
145 #endif /* MM_KFENCE_KFENCE_H */
146