1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * Kernel Electric-Fence (KFENCE). Public interface for allocator and fault 4 * handler integration. For more info see Documentation/dev-tools/kfence.rst. 5 * 6 * Copyright (C) 2020, Google LLC. 7 */ 8 9 #ifndef _LINUX_KFENCE_H 10 #define _LINUX_KFENCE_H 11 12 #include <linux/mm.h> 13 #include <linux/types.h> 14 15 #ifdef CONFIG_KFENCE 16 17 #include <linux/atomic.h> 18 #include <linux/static_key.h> 19 20 /* 21 * We allocate an even number of pages, as it simplifies calculations to map 22 * address to metadata indices; effectively, the very first page serves as an 23 * extended guard page, but otherwise has no special purpose. 24 */ 25 #define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE) 26 extern char *__kfence_pool; 27 28 DECLARE_STATIC_KEY_FALSE(kfence_allocation_key); 29 extern atomic_t kfence_allocation_gate; 30 31 /** 32 * is_kfence_address() - check if an address belongs to KFENCE pool 33 * @addr: address to check 34 * 35 * Return: true or false depending on whether the address is within the KFENCE 36 * object range. 37 * 38 * KFENCE objects live in a separate page range and are not to be intermixed 39 * with regular heap objects (e.g. KFENCE objects must never be added to the 40 * allocator freelists). Failing to do so may and will result in heap 41 * corruptions, therefore is_kfence_address() must be used to check whether 42 * an object requires specific handling. 43 * 44 * Note: This function may be used in fast-paths, and is performance critical. 45 * Future changes should take this into account; for instance, we want to avoid 46 * introducing another load and therefore need to keep KFENCE_POOL_SIZE a 47 * constant (until immediate patching support is added to the kernel). 48 */ 49 static __always_inline bool is_kfence_address(const void *addr) 50 { 51 /* 52 * The __kfence_pool != NULL check is required to deal with the case 53 * where __kfence_pool == NULL && addr < KFENCE_POOL_SIZE. Keep it in 54 * the slow-path after the range-check! 55 */ 56 return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && __kfence_pool); 57 } 58 59 /** 60 * kfence_alloc_pool() - allocate the KFENCE pool via memblock 61 */ 62 void __init kfence_alloc_pool(void); 63 64 /** 65 * kfence_init() - perform KFENCE initialization at boot time 66 * 67 * Requires that kfence_alloc_pool() was called before. This sets up the 68 * allocation gate timer, and requires that workqueues are available. 69 */ 70 void __init kfence_init(void); 71 72 /** 73 * kfence_shutdown_cache() - handle shutdown_cache() for KFENCE objects 74 * @s: cache being shut down 75 * 76 * Before shutting down a cache, one must ensure there are no remaining objects 77 * allocated from it. Because KFENCE objects are not referenced from the cache 78 * directly, we need to check them here. 79 * 80 * Note that shutdown_cache() is internal to SL*B, and kmem_cache_destroy() does 81 * not return if allocated objects still exist: it prints an error message and 82 * simply aborts destruction of a cache, leaking memory. 83 * 84 * If the only such objects are KFENCE objects, we will not leak the entire 85 * cache, but instead try to provide more useful debug info by making allocated 86 * objects "zombie allocations". Objects may then still be used or freed (which 87 * is handled gracefully), but usage will result in showing KFENCE error reports 88 * which include stack traces to the user of the object, the original allocation 89 * site, and caller to shutdown_cache(). 90 */ 91 void kfence_shutdown_cache(struct kmem_cache *s); 92 93 /* 94 * Allocate a KFENCE object. Allocators must not call this function directly, 95 * use kfence_alloc() instead. 96 */ 97 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags); 98 99 /** 100 * kfence_alloc() - allocate a KFENCE object with a low probability 101 * @s: struct kmem_cache with object requirements 102 * @size: exact size of the object to allocate (can be less than @s->size 103 * e.g. for kmalloc caches) 104 * @flags: GFP flags 105 * 106 * Return: 107 * * NULL - must proceed with allocating as usual, 108 * * non-NULL - pointer to a KFENCE object. 109 * 110 * kfence_alloc() should be inserted into the heap allocation fast path, 111 * allowing it to transparently return KFENCE-allocated objects with a low 112 * probability using a static branch (the probability is controlled by the 113 * kfence.sample_interval boot parameter). 114 */ 115 static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) 116 { 117 #if defined(CONFIG_KFENCE_STATIC_KEYS) || CONFIG_KFENCE_SAMPLE_INTERVAL == 0 118 if (!static_branch_unlikely(&kfence_allocation_key)) 119 return NULL; 120 #else 121 if (!static_branch_likely(&kfence_allocation_key)) 122 return NULL; 123 #endif 124 if (likely(atomic_read(&kfence_allocation_gate))) 125 return NULL; 126 return __kfence_alloc(s, size, flags); 127 } 128 129 /** 130 * kfence_ksize() - get actual amount of memory allocated for a KFENCE object 131 * @addr: pointer to a heap object 132 * 133 * Return: 134 * * 0 - not a KFENCE object, must call __ksize() instead, 135 * * non-0 - this many bytes can be accessed without causing a memory error. 136 * 137 * kfence_ksize() returns the number of bytes requested for a KFENCE object at 138 * allocation time. This number may be less than the object size of the 139 * corresponding struct kmem_cache. 140 */ 141 size_t kfence_ksize(const void *addr); 142 143 /** 144 * kfence_object_start() - find the beginning of a KFENCE object 145 * @addr: address within a KFENCE-allocated object 146 * 147 * Return: address of the beginning of the object. 148 * 149 * SL[AU]B-allocated objects are laid out within a page one by one, so it is 150 * easy to calculate the beginning of an object given a pointer inside it and 151 * the object size. The same is not true for KFENCE, which places a single 152 * object at either end of the page. This helper function is used to find the 153 * beginning of a KFENCE-allocated object. 154 */ 155 void *kfence_object_start(const void *addr); 156 157 /** 158 * __kfence_free() - release a KFENCE heap object to KFENCE pool 159 * @addr: object to be freed 160 * 161 * Requires: is_kfence_address(addr) 162 * 163 * Release a KFENCE object and mark it as freed. 164 */ 165 void __kfence_free(void *addr); 166 167 /** 168 * kfence_free() - try to release an arbitrary heap object to KFENCE pool 169 * @addr: object to be freed 170 * 171 * Return: 172 * * false - object doesn't belong to KFENCE pool and was ignored, 173 * * true - object was released to KFENCE pool. 174 * 175 * Release a KFENCE object and mark it as freed. May be called on any object, 176 * even non-KFENCE objects, to simplify integration of the hooks into the 177 * allocator's free codepath. The allocator must check the return value to 178 * determine if it was a KFENCE object or not. 179 */ 180 static __always_inline __must_check bool kfence_free(void *addr) 181 { 182 if (!is_kfence_address(addr)) 183 return false; 184 __kfence_free(addr); 185 return true; 186 } 187 188 /** 189 * kfence_handle_page_fault() - perform page fault handling for KFENCE pages 190 * @addr: faulting address 191 * @is_write: is access a write 192 * @regs: current struct pt_regs (can be NULL, but shows full stack trace) 193 * 194 * Return: 195 * * false - address outside KFENCE pool, 196 * * true - page fault handled by KFENCE, no additional handling required. 197 * 198 * A page fault inside KFENCE pool indicates a memory error, such as an 199 * out-of-bounds access, a use-after-free or an invalid memory access. In these 200 * cases KFENCE prints an error message and marks the offending page as 201 * present, so that the kernel can proceed. 202 */ 203 bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs); 204 205 #else /* CONFIG_KFENCE */ 206 207 static inline bool is_kfence_address(const void *addr) { return false; } 208 static inline void kfence_alloc_pool(void) { } 209 static inline void kfence_init(void) { } 210 static inline void kfence_shutdown_cache(struct kmem_cache *s) { } 211 static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; } 212 static inline size_t kfence_ksize(const void *addr) { return 0; } 213 static inline void *kfence_object_start(const void *addr) { return NULL; } 214 static inline void __kfence_free(void *addr) { } 215 static inline bool __must_check kfence_free(void *addr) { return false; } 216 static inline bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write, 217 struct pt_regs *regs) 218 { 219 return false; 220 } 221 222 #endif 223 224 #endif /* _LINUX_KFENCE_H */ 225