include/linux/kfence.h

   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 extern unsigned long kfence_sample_interval;
  21
  22 /*
  23  * We allocate an even number of pages, as it simplifies calculations to map
  24  * address to metadata indices; effectively, the very first page serves as an
  25  * extended guard page, but otherwise has no special purpose.
  26  */
  27 #define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
  28 extern char *__kfence_pool;
  29
  30 DECLARE_STATIC_KEY_FALSE(kfence_allocation_key);
  31 extern atomic_t kfence_allocation_gate;
  32
  33 /**
  34  * is_kfence_address() - check if an address belongs to KFENCE pool
  35  * @addr: address to check
  36  *
  37  * Return: true or false depending on whether the address is within the KFENCE
  38  * object range.
  39  *
  40  * KFENCE objects live in a separate page range and are not to be intermixed
  41  * with regular heap objects (e.g. KFENCE objects must never be added to the
  42  * allocator freelists). Failing to do so may and will result in heap
  43  * corruptions, therefore is_kfence_address() must be used to check whether
  44  * an object requires specific handling.
  45  *
  46  * Note: This function may be used in fast-paths, and is performance critical.
  47  * Future changes should take this into account; for instance, we want to avoid
  48  * introducing another load and therefore need to keep KFENCE_POOL_SIZE a
  49  * constant (until immediate patching support is added to the kernel).
  50  */
  51 static __always_inline bool is_kfence_address(const void *addr)
  52 {
  53         /*
  54          * The __kfence_pool != NULL check is required to deal with the case
  55          * where __kfence_pool == NULL && addr < KFENCE_POOL_SIZE. Keep it in
  56          * the slow-path after the range-check!
  57          */
  58         return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && __kfence_pool);
  59 }
  60
  61 /**
  62  * kfence_alloc_pool_and_metadata() - allocate the KFENCE pool and KFENCE
  63  * metadata via memblock
  64  */
  65 void __init kfence_alloc_pool_and_metadata(void);
  66
  67 /**
  68  * kfence_init() - perform KFENCE initialization at boot time
  69  *
  70  * Requires that kfence_alloc_pool_and_metadata() was called before. This sets
  71  * up the allocation gate timer, and requires that workqueues are available.
  72  */
  73 void __init kfence_init(void);
  74
  75 /**
  76  * kfence_shutdown_cache() - handle shutdown_cache() for KFENCE objects
  77  * @s: cache being shut down
  78  *
  79  * Before shutting down a cache, one must ensure there are no remaining objects
  80  * allocated from it. Because KFENCE objects are not referenced from the cache
  81  * directly, we need to check them here.
  82  *
  83  * Note that shutdown_cache() is internal to SL*B, and kmem_cache_destroy() does
  84  * not return if allocated objects still exist: it prints an error message and
  85  * simply aborts destruction of a cache, leaking memory.
  86  *
  87  * If the only such objects are KFENCE objects, we will not leak the entire
  88  * cache, but instead try to provide more useful debug info by making allocated
  89  * objects "zombie allocations". Objects may then still be used or freed (which
  90  * is handled gracefully), but usage will result in showing KFENCE error reports
  91  * which include stack traces to the user of the object, the original allocation
  92  * site, and caller to shutdown_cache().
  93  */
  94 void kfence_shutdown_cache(struct kmem_cache *s);
  95
  96 /*
  97  * Allocate a KFENCE object. Allocators must not call this function directly,
  98  * use kfence_alloc() instead.
  99  */
 100 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags);
 101
 102 /**
 103  * kfence_alloc() - allocate a KFENCE object with a low probability
 104  * @s:     struct kmem_cache with object requirements
 105  * @size:  exact size of the object to allocate (can be less than @s->size
 106  *         e.g. for kmalloc caches)
 107  * @flags: GFP flags
 108  *
 109  * Return:
 110  * * NULL     - must proceed with allocating as usual,
 111  * * non-NULL - pointer to a KFENCE object.
 112  *
 113  * kfence_alloc() should be inserted into the heap allocation fast path,
 114  * allowing it to transparently return KFENCE-allocated objects with a low
 115  * probability using a static branch (the probability is controlled by the
 116  * kfence.sample_interval boot parameter).
 117  */
 118 static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 119 {
 120 #if defined(CONFIG_KFENCE_STATIC_KEYS) || CONFIG_KFENCE_SAMPLE_INTERVAL == 0
 121         if (!static_branch_unlikely(&kfence_allocation_key))
 122                 return NULL;
 123 #else
 124         if (!static_branch_likely(&kfence_allocation_key))
 125                 return NULL;
 126 #endif
 127         if (likely(atomic_read(&kfence_allocation_gate) > 0))
 128                 return NULL;
 129         return __kfence_alloc(s, size, flags);
 130 }
 131
 132 /**
 133  * kfence_ksize() - get actual amount of memory allocated for a KFENCE object
 134  * @addr: pointer to a heap object
 135  *
 136  * Return:
 137  * * 0     - not a KFENCE object, must call __ksize() instead,
 138  * * non-0 - this many bytes can be accessed without causing a memory error.
 139  *
 140  * kfence_ksize() returns the number of bytes requested for a KFENCE object at
 141  * allocation time. This number may be less than the object size of the
 142  * corresponding struct kmem_cache.
 143  */
 144 size_t kfence_ksize(const void *addr);
 145
 146 /**
 147  * kfence_object_start() - find the beginning of a KFENCE object
 148  * @addr: address within a KFENCE-allocated object
 149  *
 150  * Return: address of the beginning of the object.
 151  *
 152  * SL[AU]B-allocated objects are laid out within a page one by one, so it is
 153  * easy to calculate the beginning of an object given a pointer inside it and
 154  * the object size. The same is not true for KFENCE, which places a single
 155  * object at either end of the page. This helper function is used to find the
 156  * beginning of a KFENCE-allocated object.
 157  */
 158 void *kfence_object_start(const void *addr);
 159
 160 /**
 161  * __kfence_free() - release a KFENCE heap object to KFENCE pool
 162  * @addr: object to be freed
 163  *
 164  * Requires: is_kfence_address(addr)
 165  *
 166  * Release a KFENCE object and mark it as freed.
 167  */
 168 void __kfence_free(void *addr);
 169
 170 /**
 171  * kfence_free() - try to release an arbitrary heap object to KFENCE pool
 172  * @addr: object to be freed
 173  *
 174  * Return:
 175  * * false - object doesn't belong to KFENCE pool and was ignored,
 176  * * true  - object was released to KFENCE pool.
 177  *
 178  * Release a KFENCE object and mark it as freed. May be called on any object,
 179  * even non-KFENCE objects, to simplify integration of the hooks into the
 180  * allocator's free codepath. The allocator must check the return value to
 181  * determine if it was a KFENCE object or not.
 182  */
 183 static __always_inline __must_check bool kfence_free(void *addr)
 184 {
 185         if (!is_kfence_address(addr))
 186                 return false;
 187         __kfence_free(addr);
 188         return true;
 189 }
 190
 191 /**
 192  * kfence_handle_page_fault() - perform page fault handling for KFENCE pages
 193  * @addr: faulting address
 194  * @is_write: is access a write
 195  * @regs: current struct pt_regs (can be NULL, but shows full stack trace)
 196  *
 197  * Return:
 198  * * false - address outside KFENCE pool,
 199  * * true  - page fault handled by KFENCE, no additional handling required.
 200  *
 201  * A page fault inside KFENCE pool indicates a memory error, such as an
 202  * out-of-bounds access, a use-after-free or an invalid memory access. In these
 203  * cases KFENCE prints an error message and marks the offending page as
 204  * present, so that the kernel can proceed.
 205  */
 206 bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs);
 207
 208 #ifdef CONFIG_PRINTK
 209 struct kmem_obj_info;
 210 /**
 211  * __kfence_obj_info() - fill kmem_obj_info struct
 212  * @kpp: kmem_obj_info to be filled
 213  * @object: the object
 214  *
 215  * Return:
 216  * * false - not a KFENCE object
 217  * * true - a KFENCE object, filled @kpp
 218  *
 219  * Copies information to @kpp for KFENCE objects.
 220  */
 221 bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab);
 222 #endif
 223
 224 #else /* CONFIG_KFENCE */
 225
 226 #define kfence_sample_interval  (0)
 227
 228 static inline bool is_kfence_address(const void *addr) { return false; }
 229 static inline void kfence_alloc_pool_and_metadata(void) { }
 230 static inline void kfence_init(void) { }
 231 static inline void kfence_shutdown_cache(struct kmem_cache *s) { }
 232 static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
 233 static inline size_t kfence_ksize(const void *addr) { return 0; }
 234 static inline void *kfence_object_start(const void *addr) { return NULL; }
 235 static inline void __kfence_free(void *addr) { }
 236 static inline bool __must_check kfence_free(void *addr) { return false; }
 237 static inline bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write,
 238                                                          struct pt_regs *regs)
 239 {
 240         return false;
 241 }
 242
 243 #ifdef CONFIG_PRINTK
 244 struct kmem_obj_info;
 245 static inline bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab)
 246 {
 247         return false;
 248 }
 249 #endif
 250
 251 #endif
 252
 253 #endif /* _LINUX_KFENCE_H */