1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * This implements the various checks for CONFIG_HARDENED_USERCOPY*, 4 * which are designed to protect kernel memory from needless exposure 5 * and overwrite under many unintended conditions. This code is based 6 * on PAX_USERCOPY, which is: 7 * 8 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source 9 * Security Inc. 10 */ 11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt 12 13 #include <linux/mm.h> 14 #include <linux/highmem.h> 15 #include <linux/slab.h> 16 #include <linux/sched.h> 17 #include <linux/sched/task.h> 18 #include <linux/sched/task_stack.h> 19 #include <linux/thread_info.h> 20 #include <linux/vmalloc.h> 21 #include <linux/atomic.h> 22 #include <linux/jump_label.h> 23 #include <asm/sections.h> 24 #include "slab.h" 25 26 /* 27 * Checks if a given pointer and length is contained by the current 28 * stack frame (if possible). 29 * 30 * Returns: 31 * NOT_STACK: not at all on the stack 32 * GOOD_FRAME: fully within a valid stack frame 33 * GOOD_STACK: within the current stack (when can't frame-check exactly) 34 * BAD_STACK: error condition (invalid stack position or bad stack frame) 35 */ 36 static noinline int check_stack_object(const void *obj, unsigned long len) 37 { 38 const void * const stack = task_stack_page(current); 39 const void * const stackend = stack + THREAD_SIZE; 40 int ret; 41 42 /* Object is not on the stack at all. */ 43 if (obj + len <= stack || stackend <= obj) 44 return NOT_STACK; 45 46 /* 47 * Reject: object partially overlaps the stack (passing the 48 * check above means at least one end is within the stack, 49 * so if this check fails, the other end is outside the stack). 50 */ 51 if (obj < stack || stackend < obj + len) 52 return BAD_STACK; 53 54 /* Check if object is safely within a valid frame. */ 55 ret = arch_within_stack_frames(stack, stackend, obj, len); 56 if (ret) 57 return ret; 58 59 /* Finally, check stack depth if possible. */ 60 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER 61 if (IS_ENABLED(CONFIG_STACK_GROWSUP)) { 62 if ((void *)current_stack_pointer < obj + len) 63 return BAD_STACK; 64 } else { 65 if (obj < (void *)current_stack_pointer) 66 return BAD_STACK; 67 } 68 #endif 69 70 return GOOD_STACK; 71 } 72 73 /* 74 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found 75 * an unexpected state during a copy_from_user() or copy_to_user() call. 76 * There are several checks being performed on the buffer by the 77 * __check_object_size() function. Normal stack buffer usage should never 78 * trip the checks, and kernel text addressing will always trip the check. 79 * For cache objects, it is checking that only the whitelisted range of 80 * bytes for a given cache is being accessed (via the cache's usersize and 81 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware 82 * kmem_cache_create_usercopy() function to create the cache (and 83 * carefully audit the whitelist range). 84 */ 85 void __noreturn usercopy_abort(const char *name, const char *detail, 86 bool to_user, unsigned long offset, 87 unsigned long len) 88 { 89 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 90 to_user ? "exposure" : "overwrite", 91 to_user ? "from" : "to", 92 name ? : "unknown?!", 93 detail ? " '" : "", detail ? : "", detail ? "'" : "", 94 offset, len); 95 96 /* 97 * For greater effect, it would be nice to do do_group_exit(), 98 * but BUG() actually hooks all the lock-breaking and per-arch 99 * Oops code, so that is used here instead. 100 */ 101 BUG(); 102 } 103 104 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */ 105 static bool overlaps(const unsigned long ptr, unsigned long n, 106 unsigned long low, unsigned long high) 107 { 108 const unsigned long check_low = ptr; 109 unsigned long check_high = check_low + n; 110 111 /* Does not overlap if entirely above or entirely below. */ 112 if (check_low >= high || check_high <= low) 113 return false; 114 115 return true; 116 } 117 118 /* Is this address range in the kernel text area? */ 119 static inline void check_kernel_text_object(const unsigned long ptr, 120 unsigned long n, bool to_user) 121 { 122 unsigned long textlow = (unsigned long)_stext; 123 unsigned long texthigh = (unsigned long)_etext; 124 unsigned long textlow_linear, texthigh_linear; 125 126 if (overlaps(ptr, n, textlow, texthigh)) 127 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n); 128 129 /* 130 * Some architectures have virtual memory mappings with a secondary 131 * mapping of the kernel text, i.e. there is more than one virtual 132 * kernel address that points to the kernel image. It is usually 133 * when there is a separate linear physical memory mapping, in that 134 * __pa() is not just the reverse of __va(). This can be detected 135 * and checked: 136 */ 137 textlow_linear = (unsigned long)lm_alias(textlow); 138 /* No different mapping: we're done. */ 139 if (textlow_linear == textlow) 140 return; 141 142 /* Check the secondary mapping... */ 143 texthigh_linear = (unsigned long)lm_alias(texthigh); 144 if (overlaps(ptr, n, textlow_linear, texthigh_linear)) 145 usercopy_abort("linear kernel text", NULL, to_user, 146 ptr - textlow_linear, n); 147 } 148 149 static inline void check_bogus_address(const unsigned long ptr, unsigned long n, 150 bool to_user) 151 { 152 /* Reject if object wraps past end of memory. */ 153 if (ptr + (n - 1) < ptr) 154 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); 155 156 /* Reject if NULL or ZERO-allocation. */ 157 if (ZERO_OR_NULL_PTR(ptr)) 158 usercopy_abort("null address", NULL, to_user, ptr, n); 159 } 160 161 static inline void check_heap_object(const void *ptr, unsigned long n, 162 bool to_user) 163 { 164 struct folio *folio; 165 166 if (is_kmap_addr(ptr)) { 167 unsigned long page_end = (unsigned long)ptr | (PAGE_SIZE - 1); 168 169 if ((unsigned long)ptr + n - 1 > page_end) 170 usercopy_abort("kmap", NULL, to_user, 171 offset_in_page(ptr), n); 172 return; 173 } 174 175 if (is_vmalloc_addr(ptr)) { 176 struct vm_struct *area = find_vm_area(ptr); 177 unsigned long offset; 178 179 if (!area) { 180 usercopy_abort("vmalloc", "no area", to_user, 0, n); 181 return; 182 } 183 184 offset = ptr - area->addr; 185 if (offset + n > get_vm_area_size(area)) 186 usercopy_abort("vmalloc", NULL, to_user, offset, n); 187 return; 188 } 189 190 if (!virt_addr_valid(ptr)) 191 return; 192 193 folio = virt_to_folio(ptr); 194 195 if (folio_test_slab(folio)) { 196 /* Check slab allocator for flags and size. */ 197 __check_heap_object(ptr, n, folio_slab(folio), to_user); 198 } else if (folio_test_large(folio)) { 199 unsigned long offset = ptr - folio_address(folio); 200 if (offset + n > folio_size(folio)) 201 usercopy_abort("page alloc", NULL, to_user, offset, n); 202 } 203 } 204 205 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks); 206 207 /* 208 * Validates that the given object is: 209 * - not bogus address 210 * - fully contained by stack (or stack frame, when available) 211 * - fully within SLAB object (or object whitelist area, when available) 212 * - not in kernel text 213 */ 214 void __check_object_size(const void *ptr, unsigned long n, bool to_user) 215 { 216 if (static_branch_unlikely(&bypass_usercopy_checks)) 217 return; 218 219 /* Skip all tests if size is zero. */ 220 if (!n) 221 return; 222 223 /* Check for invalid addresses. */ 224 check_bogus_address((const unsigned long)ptr, n, to_user); 225 226 /* Check for bad stack object. */ 227 switch (check_stack_object(ptr, n)) { 228 case NOT_STACK: 229 /* Object is not touching the current process stack. */ 230 break; 231 case GOOD_FRAME: 232 case GOOD_STACK: 233 /* 234 * Object is either in the correct frame (when it 235 * is possible to check) or just generally on the 236 * process stack (when frame checking not available). 237 */ 238 return; 239 default: 240 usercopy_abort("process stack", NULL, to_user, 241 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER 242 IS_ENABLED(CONFIG_STACK_GROWSUP) ? 243 ptr - (void *)current_stack_pointer : 244 (void *)current_stack_pointer - ptr, 245 #else 246 0, 247 #endif 248 n); 249 } 250 251 /* Check for bad heap object. */ 252 check_heap_object(ptr, n, to_user); 253 254 /* Check for object in kernel to avoid text exposure. */ 255 check_kernel_text_object((const unsigned long)ptr, n, to_user); 256 } 257 EXPORT_SYMBOL(__check_object_size); 258 259 static bool enable_checks __initdata = true; 260 261 static int __init parse_hardened_usercopy(char *str) 262 { 263 if (strtobool(str, &enable_checks)) 264 pr_warn("Invalid option string for hardened_usercopy: '%s'\n", 265 str); 266 return 1; 267 } 268 269 __setup("hardened_usercopy=", parse_hardened_usercopy); 270 271 static int __init set_hardened_usercopy(void) 272 { 273 if (enable_checks == false) 274 static_branch_enable(&bypass_usercopy_checks); 275 return 1; 276 } 277 278 late_initcall(set_hardened_usercopy); 279