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/slab.h> 15 #include <linux/sched.h> 16 #include <linux/sched/task.h> 17 #include <linux/sched/task_stack.h> 18 #include <linux/thread_info.h> 19 #include <linux/atomic.h> 20 #include <linux/jump_label.h> 21 #include <asm/sections.h> 22 23 /* 24 * Checks if a given pointer and length is contained by the current 25 * stack frame (if possible). 26 * 27 * Returns: 28 * NOT_STACK: not at all on the stack 29 * GOOD_FRAME: fully within a valid stack frame 30 * GOOD_STACK: fully on the stack (when can't do frame-checking) 31 * BAD_STACK: error condition (invalid stack position or bad stack frame) 32 */ 33 static noinline int check_stack_object(const void *obj, unsigned long len) 34 { 35 const void * const stack = task_stack_page(current); 36 const void * const stackend = stack + THREAD_SIZE; 37 int ret; 38 39 /* Object is not on the stack at all. */ 40 if (obj + len <= stack || stackend <= obj) 41 return NOT_STACK; 42 43 /* 44 * Reject: object partially overlaps the stack (passing the 45 * the check above means at least one end is within the stack, 46 * so if this check fails, the other end is outside the stack). 47 */ 48 if (obj < stack || stackend < obj + len) 49 return BAD_STACK; 50 51 /* Check if object is safely within a valid frame. */ 52 ret = arch_within_stack_frames(stack, stackend, obj, len); 53 if (ret) 54 return ret; 55 56 return GOOD_STACK; 57 } 58 59 /* 60 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found 61 * an unexpected state during a copy_from_user() or copy_to_user() call. 62 * There are several checks being performed on the buffer by the 63 * __check_object_size() function. Normal stack buffer usage should never 64 * trip the checks, and kernel text addressing will always trip the check. 65 * For cache objects, it is checking that only the whitelisted range of 66 * bytes for a given cache is being accessed (via the cache's usersize and 67 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware 68 * kmem_cache_create_usercopy() function to create the cache (and 69 * carefully audit the whitelist range). 70 */ 71 void usercopy_warn(const char *name, const char *detail, bool to_user, 72 unsigned long offset, unsigned long len) 73 { 74 WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 75 to_user ? "exposure" : "overwrite", 76 to_user ? "from" : "to", 77 name ? : "unknown?!", 78 detail ? " '" : "", detail ? : "", detail ? "'" : "", 79 offset, len); 80 } 81 82 void __noreturn usercopy_abort(const char *name, const char *detail, 83 bool to_user, unsigned long offset, 84 unsigned long len) 85 { 86 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 87 to_user ? "exposure" : "overwrite", 88 to_user ? "from" : "to", 89 name ? : "unknown?!", 90 detail ? " '" : "", detail ? : "", detail ? "'" : "", 91 offset, len); 92 93 /* 94 * For greater effect, it would be nice to do do_group_exit(), 95 * but BUG() actually hooks all the lock-breaking and per-arch 96 * Oops code, so that is used here instead. 97 */ 98 BUG(); 99 } 100 101 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */ 102 static bool overlaps(const unsigned long ptr, unsigned long n, 103 unsigned long low, unsigned long high) 104 { 105 const unsigned long check_low = ptr; 106 unsigned long check_high = check_low + n; 107 108 /* Does not overlap if entirely above or entirely below. */ 109 if (check_low >= high || check_high <= low) 110 return false; 111 112 return true; 113 } 114 115 /* Is this address range in the kernel text area? */ 116 static inline void check_kernel_text_object(const unsigned long ptr, 117 unsigned long n, bool to_user) 118 { 119 unsigned long textlow = (unsigned long)_stext; 120 unsigned long texthigh = (unsigned long)_etext; 121 unsigned long textlow_linear, texthigh_linear; 122 123 if (overlaps(ptr, n, textlow, texthigh)) 124 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n); 125 126 /* 127 * Some architectures have virtual memory mappings with a secondary 128 * mapping of the kernel text, i.e. there is more than one virtual 129 * kernel address that points to the kernel image. It is usually 130 * when there is a separate linear physical memory mapping, in that 131 * __pa() is not just the reverse of __va(). This can be detected 132 * and checked: 133 */ 134 textlow_linear = (unsigned long)lm_alias(textlow); 135 /* No different mapping: we're done. */ 136 if (textlow_linear == textlow) 137 return; 138 139 /* Check the secondary mapping... */ 140 texthigh_linear = (unsigned long)lm_alias(texthigh); 141 if (overlaps(ptr, n, textlow_linear, texthigh_linear)) 142 usercopy_abort("linear kernel text", NULL, to_user, 143 ptr - textlow_linear, n); 144 } 145 146 static inline void check_bogus_address(const unsigned long ptr, unsigned long n, 147 bool to_user) 148 { 149 /* Reject if object wraps past end of memory. */ 150 if (ptr + n < ptr) 151 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); 152 153 /* Reject if NULL or ZERO-allocation. */ 154 if (ZERO_OR_NULL_PTR(ptr)) 155 usercopy_abort("null address", NULL, to_user, ptr, n); 156 } 157 158 /* Checks for allocs that are marked in some way as spanning multiple pages. */ 159 static inline void check_page_span(const void *ptr, unsigned long n, 160 struct page *page, bool to_user) 161 { 162 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN 163 const void *end = ptr + n - 1; 164 struct page *endpage; 165 bool is_reserved, is_cma; 166 167 /* 168 * Sometimes the kernel data regions are not marked Reserved (see 169 * check below). And sometimes [_sdata,_edata) does not cover 170 * rodata and/or bss, so check each range explicitly. 171 */ 172 173 /* Allow reads of kernel rodata region (if not marked as Reserved). */ 174 if (ptr >= (const void *)__start_rodata && 175 end <= (const void *)__end_rodata) { 176 if (!to_user) 177 usercopy_abort("rodata", NULL, to_user, 0, n); 178 return; 179 } 180 181 /* Allow kernel data region (if not marked as Reserved). */ 182 if (ptr >= (const void *)_sdata && end <= (const void *)_edata) 183 return; 184 185 /* Allow kernel bss region (if not marked as Reserved). */ 186 if (ptr >= (const void *)__bss_start && 187 end <= (const void *)__bss_stop) 188 return; 189 190 /* Is the object wholly within one base page? */ 191 if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == 192 ((unsigned long)end & (unsigned long)PAGE_MASK))) 193 return; 194 195 /* Allow if fully inside the same compound (__GFP_COMP) page. */ 196 endpage = virt_to_head_page(end); 197 if (likely(endpage == page)) 198 return; 199 200 /* 201 * Reject if range is entirely either Reserved (i.e. special or 202 * device memory), or CMA. Otherwise, reject since the object spans 203 * several independently allocated pages. 204 */ 205 is_reserved = PageReserved(page); 206 is_cma = is_migrate_cma_page(page); 207 if (!is_reserved && !is_cma) 208 usercopy_abort("spans multiple pages", NULL, to_user, 0, n); 209 210 for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) { 211 page = virt_to_head_page(ptr); 212 if (is_reserved && !PageReserved(page)) 213 usercopy_abort("spans Reserved and non-Reserved pages", 214 NULL, to_user, 0, n); 215 if (is_cma && !is_migrate_cma_page(page)) 216 usercopy_abort("spans CMA and non-CMA pages", NULL, 217 to_user, 0, n); 218 } 219 #endif 220 } 221 222 static inline void check_heap_object(const void *ptr, unsigned long n, 223 bool to_user) 224 { 225 struct page *page; 226 227 if (!virt_addr_valid(ptr)) 228 return; 229 230 page = virt_to_head_page(ptr); 231 232 if (PageSlab(page)) { 233 /* Check slab allocator for flags and size. */ 234 __check_heap_object(ptr, n, page, to_user); 235 } else { 236 /* Verify object does not incorrectly span multiple pages. */ 237 check_page_span(ptr, n, page, to_user); 238 } 239 } 240 241 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks); 242 243 /* 244 * Validates that the given object is: 245 * - not bogus address 246 * - fully contained by stack (or stack frame, when available) 247 * - fully within SLAB object (or object whitelist area, when available) 248 * - not in kernel text 249 */ 250 void __check_object_size(const void *ptr, unsigned long n, bool to_user) 251 { 252 if (static_branch_unlikely(&bypass_usercopy_checks)) 253 return; 254 255 /* Skip all tests if size is zero. */ 256 if (!n) 257 return; 258 259 /* Check for invalid addresses. */ 260 check_bogus_address((const unsigned long)ptr, n, to_user); 261 262 /* Check for bad stack object. */ 263 switch (check_stack_object(ptr, n)) { 264 case NOT_STACK: 265 /* Object is not touching the current process stack. */ 266 break; 267 case GOOD_FRAME: 268 case GOOD_STACK: 269 /* 270 * Object is either in the correct frame (when it 271 * is possible to check) or just generally on the 272 * process stack (when frame checking not available). 273 */ 274 return; 275 default: 276 usercopy_abort("process stack", NULL, to_user, 0, n); 277 } 278 279 /* Check for bad heap object. */ 280 check_heap_object(ptr, n, to_user); 281 282 /* Check for object in kernel to avoid text exposure. */ 283 check_kernel_text_object((const unsigned long)ptr, n, to_user); 284 } 285 EXPORT_SYMBOL(__check_object_size); 286 287 static bool enable_checks __initdata = true; 288 289 static int __init parse_hardened_usercopy(char *str) 290 { 291 return strtobool(str, &enable_checks); 292 } 293 294 __setup("hardened_usercopy=", parse_hardened_usercopy); 295 296 static int __init set_hardened_usercopy(void) 297 { 298 if (enable_checks == false) 299 static_branch_enable(&bypass_usercopy_checks); 300 return 1; 301 } 302 303 late_initcall(set_hardened_usercopy); 304