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 /*
  18  * We allocate an even number of pages, as it simplifies calculations to map
  19  * address to metadata indices; effectively, the very first page serves as an
  20  * extended guard page, but otherwise has no special purpose.
  21  */
  22 #define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
  23 extern char *__kfence_pool;
  24
  25 #ifdef CONFIG_KFENCE_STATIC_KEYS
  26 #include <linux/static_key.h>
  27 DECLARE_STATIC_KEY_FALSE(kfence_allocation_key);
  28 #else
  29 #include <linux/atomic.h>
  30 extern atomic_t kfence_allocation_gate;
  31 #endif
  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() - allocate the KFENCE pool via memblock
  63  */
  64 void __init kfence_alloc_pool(void);
  65
  66 /**
  67  * kfence_init() - perform KFENCE initialization at boot time
  68  *
  69  * Requires that kfence_alloc_pool() was called before. This sets up the
  70  * allocation gate timer, and requires that workqueues are available.
  71  */
  72 void __init kfence_init(void);
  73
  74 /**
  75  * kfence_shutdown_cache() - handle shutdown_cache() for KFENCE objects
  76  * @s: cache being shut down
  77  *
  78  * Before shutting down a cache, one must ensure there are no remaining objects
  79  * allocated from it. Because KFENCE objects are not referenced from the cache
  80  * directly, we need to check them here.
  81  *
  82  * Note that shutdown_cache() is internal to SL*B, and kmem_cache_destroy() does
  83  * not return if allocated objects still exist: it prints an error message and
  84  * simply aborts destruction of a cache, leaking memory.
  85  *
  86  * If the only such objects are KFENCE objects, we will not leak the entire
  87  * cache, but instead try to provide more useful debug info by making allocated
  88  * objects "zombie allocations". Objects may then still be used or freed (which
  89  * is handled gracefully), but usage will result in showing KFENCE error reports
  90  * which include stack traces to the user of the object, the original allocation
  91  * site, and caller to shutdown_cache().
  92  */
  93 void kfence_shutdown_cache(struct kmem_cache *s);
  94
  95 /*
  96  * Allocate a KFENCE object. Allocators must not call this function directly,
  97  * use kfence_alloc() instead.
  98  */
  99 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags);
 100
 101 /**
 102  * kfence_alloc() - allocate a KFENCE object with a low probability
 103  * @s:     struct kmem_cache with object requirements
 104  * @size:  exact size of the object to allocate (can be less than @s->size
 105  *         e.g. for kmalloc caches)
 106  * @flags: GFP flags
 107  *
 108  * Return:
 109  * * NULL     - must proceed with allocating as usual,
 110  * * non-NULL - pointer to a KFENCE object.
 111  *
 112  * kfence_alloc() should be inserted into the heap allocation fast path,
 113  * allowing it to transparently return KFENCE-allocated objects with a low
 114  * probability using a static branch (the probability is controlled by the
 115  * kfence.sample_interval boot parameter).
 116  */
 117 static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 118 {
 119 #ifdef CONFIG_KFENCE_STATIC_KEYS
 120         if (static_branch_unlikely(&kfence_allocation_key))
 121 #else
 122         if (unlikely(!atomic_read(&kfence_allocation_gate)))
 123 #endif
 124                 return __kfence_alloc(s, size, flags);
 125         return NULL;
 126 }
 127
 128 /**
 129  * kfence_ksize() - get actual amount of memory allocated for a KFENCE object
 130  * @addr: pointer to a heap object
 131  *
 132  * Return:
 133  * * 0     - not a KFENCE object, must call __ksize() instead,
 134  * * non-0 - this many bytes can be accessed without causing a memory error.
 135  *
 136  * kfence_ksize() returns the number of bytes requested for a KFENCE object at
 137  * allocation time. This number may be less than the object size of the
 138  * corresponding struct kmem_cache.
 139  */
 140 size_t kfence_ksize(const void *addr);
 141
 142 /**
 143  * kfence_object_start() - find the beginning of a KFENCE object
 144  * @addr: address within a KFENCE-allocated object
 145  *
 146  * Return: address of the beginning of the object.
 147  *
 148  * SL[AU]B-allocated objects are laid out within a page one by one, so it is
 149  * easy to calculate the beginning of an object given a pointer inside it and
 150  * the object size. The same is not true for KFENCE, which places a single
 151  * object at either end of the page. This helper function is used to find the
 152  * beginning of a KFENCE-allocated object.
 153  */
 154 void *kfence_object_start(const void *addr);
 155
 156 /**
 157  * __kfence_free() - release a KFENCE heap object to KFENCE pool
 158  * @addr: object to be freed
 159  *
 160  * Requires: is_kfence_address(addr)
 161  *
 162  * Release a KFENCE object and mark it as freed.
 163  */
 164 void __kfence_free(void *addr);
 165
 166 /**
 167  * kfence_free() - try to release an arbitrary heap object to KFENCE pool
 168  * @addr: object to be freed
 169  *
 170  * Return:
 171  * * false - object doesn't belong to KFENCE pool and was ignored,
 172  * * true  - object was released to KFENCE pool.
 173  *
 174  * Release a KFENCE object and mark it as freed. May be called on any object,
 175  * even non-KFENCE objects, to simplify integration of the hooks into the
 176  * allocator's free codepath. The allocator must check the return value to
 177  * determine if it was a KFENCE object or not.
 178  */
 179 static __always_inline __must_check bool kfence_free(void *addr)
 180 {
 181         if (!is_kfence_address(addr))
 182                 return false;
 183         __kfence_free(addr);
 184         return true;
 185 }
 186
 187 /**
 188  * kfence_handle_page_fault() - perform page fault handling for KFENCE pages
 189  * @addr: faulting address
 190  * @is_write: is access a write
 191  * @regs: current struct pt_regs (can be NULL, but shows full stack trace)
 192  *
 193  * Return:
 194  * * false - address outside KFENCE pool,
 195  * * true  - page fault handled by KFENCE, no additional handling required.
 196  *
 197  * A page fault inside KFENCE pool indicates a memory error, such as an
 198  * out-of-bounds access, a use-after-free or an invalid memory access. In these
 199  * cases KFENCE prints an error message and marks the offending page as
 200  * present, so that the kernel can proceed.
 201  */
 202 bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs);
 203
 204 #else /* CONFIG_KFENCE */
 205
 206 static inline bool is_kfence_address(const void *addr) { return false; }
 207 static inline void kfence_alloc_pool(void) { }
 208 static inline void kfence_init(void) { }
 209 static inline void kfence_shutdown_cache(struct kmem_cache *s) { }
 210 static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
 211 static inline size_t kfence_ksize(const void *addr) { return 0; }
 212 static inline void *kfence_object_start(const void *addr) { return NULL; }
 213 static inline void __kfence_free(void *addr) { }
 214 static inline bool __must_check kfence_free(void *addr) { return false; }
 215 static inline bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write,
 216                                                          struct pt_regs *regs)
 217 {
 218         return false;
 219 }
 220
 221 #endif
 222
 223 #endif /* _LINUX_KFENCE_H */