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/atomic.h> 21 #include <linux/jump_label.h> 22 #include <asm/sections.h> 23 24 /* 25 * Checks if a given pointer and length is contained by the current 26 * stack frame (if possible). 27 * 28 * Returns: 29 * NOT_STACK: not at all on the stack 30 * GOOD_FRAME: fully within a valid stack frame 31 * GOOD_STACK: fully on the stack (when can't do frame-checking) 32 * BAD_STACK: error condition (invalid stack position or bad stack frame) 33 */ 34 static noinline int check_stack_object(const void *obj, unsigned long len) 35 { 36 const void * const stack = task_stack_page(current); 37 const void * const stackend = stack + THREAD_SIZE; 38 int ret; 39 40 /* Object is not on the stack at all. */ 41 if (obj + len <= stack || stackend <= obj) 42 return NOT_STACK; 43 44 /* 45 * Reject: object partially overlaps the stack (passing the 46 * check above means at least one end is within the stack, 47 * so if this check fails, the other end is outside the stack). 48 */ 49 if (obj < stack || stackend < obj + len) 50 return BAD_STACK; 51 52 /* Check if object is safely within a valid frame. */ 53 ret = arch_within_stack_frames(stack, stackend, obj, len); 54 if (ret) 55 return ret; 56 57 return GOOD_STACK; 58 } 59 60 /* 61 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found 62 * an unexpected state during a copy_from_user() or copy_to_user() call. 63 * There are several checks being performed on the buffer by the 64 * __check_object_size() function. Normal stack buffer usage should never 65 * trip the checks, and kernel text addressing will always trip the check. 66 * For cache objects, it is checking that only the whitelisted range of 67 * bytes for a given cache is being accessed (via the cache's usersize and 68 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware 69 * kmem_cache_create_usercopy() function to create the cache (and 70 * carefully audit the whitelist range). 71 */ 72 void usercopy_warn(const char *name, const char *detail, bool to_user, 73 unsigned long offset, unsigned long len) 74 { 75 WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 76 to_user ? "exposure" : "overwrite", 77 to_user ? "from" : "to", 78 name ? : "unknown?!", 79 detail ? " '" : "", detail ? : "", detail ? "'" : "", 80 offset, len); 81 } 82 83 void __noreturn usercopy_abort(const char *name, const char *detail, 84 bool to_user, unsigned long offset, 85 unsigned long len) 86 { 87 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 88 to_user ? "exposure" : "overwrite", 89 to_user ? "from" : "to", 90 name ? : "unknown?!", 91 detail ? " '" : "", detail ? : "", detail ? "'" : "", 92 offset, len); 93 94 /* 95 * For greater effect, it would be nice to do do_group_exit(), 96 * but BUG() actually hooks all the lock-breaking and per-arch 97 * Oops code, so that is used here instead. 98 */ 99 BUG(); 100 } 101 102 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */ 103 static bool overlaps(const unsigned long ptr, unsigned long n, 104 unsigned long low, unsigned long high) 105 { 106 const unsigned long check_low = ptr; 107 unsigned long check_high = check_low + n; 108 109 /* Does not overlap if entirely above or entirely below. */ 110 if (check_low >= high || check_high <= low) 111 return false; 112 113 return true; 114 } 115 116 /* Is this address range in the kernel text area? */ 117 static inline void check_kernel_text_object(const unsigned long ptr, 118 unsigned long n, bool to_user) 119 { 120 unsigned long textlow = (unsigned long)_stext; 121 unsigned long texthigh = (unsigned long)_etext; 122 unsigned long textlow_linear, texthigh_linear; 123 124 if (overlaps(ptr, n, textlow, texthigh)) 125 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n); 126 127 /* 128 * Some architectures have virtual memory mappings with a secondary 129 * mapping of the kernel text, i.e. there is more than one virtual 130 * kernel address that points to the kernel image. It is usually 131 * when there is a separate linear physical memory mapping, in that 132 * __pa() is not just the reverse of __va(). This can be detected 133 * and checked: 134 */ 135 textlow_linear = (unsigned long)lm_alias(textlow); 136 /* No different mapping: we're done. */ 137 if (textlow_linear == textlow) 138 return; 139 140 /* Check the secondary mapping... */ 141 texthigh_linear = (unsigned long)lm_alias(texthigh); 142 if (overlaps(ptr, n, textlow_linear, texthigh_linear)) 143 usercopy_abort("linear kernel text", NULL, to_user, 144 ptr - textlow_linear, n); 145 } 146 147 static inline void check_bogus_address(const unsigned long ptr, unsigned long n, 148 bool to_user) 149 { 150 /* Reject if object wraps past end of memory. */ 151 if (ptr + (n - 1) < ptr) 152 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); 153 154 /* Reject if NULL or ZERO-allocation. */ 155 if (ZERO_OR_NULL_PTR(ptr)) 156 usercopy_abort("null address", NULL, to_user, ptr, n); 157 } 158 159 /* Checks for allocs that are marked in some way as spanning multiple pages. */ 160 static inline void check_page_span(const void *ptr, unsigned long n, 161 struct page *page, bool to_user) 162 { 163 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN 164 const void *end = ptr + n - 1; 165 struct page *endpage; 166 bool is_reserved, is_cma; 167 168 /* 169 * Sometimes the kernel data regions are not marked Reserved (see 170 * check below). And sometimes [_sdata,_edata) does not cover 171 * rodata and/or bss, so check each range explicitly. 172 */ 173 174 /* Allow reads of kernel rodata region (if not marked as Reserved). */ 175 if (ptr >= (const void *)__start_rodata && 176 end <= (const void *)__end_rodata) { 177 if (!to_user) 178 usercopy_abort("rodata", NULL, to_user, 0, n); 179 return; 180 } 181 182 /* Allow kernel data region (if not marked as Reserved). */ 183 if (ptr >= (const void *)_sdata && end <= (const void *)_edata) 184 return; 185 186 /* Allow kernel bss region (if not marked as Reserved). */ 187 if (ptr >= (const void *)__bss_start && 188 end <= (const void *)__bss_stop) 189 return; 190 191 /* Is the object wholly within one base page? */ 192 if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == 193 ((unsigned long)end & (unsigned long)PAGE_MASK))) 194 return; 195 196 /* Allow if fully inside the same compound (__GFP_COMP) page. */ 197 endpage = virt_to_head_page(end); 198 if (likely(endpage == page)) 199 return; 200 201 /* 202 * Reject if range is entirely either Reserved (i.e. special or 203 * device memory), or CMA. Otherwise, reject since the object spans 204 * several independently allocated pages. 205 */ 206 is_reserved = PageReserved(page); 207 is_cma = is_migrate_cma_page(page); 208 if (!is_reserved && !is_cma) 209 usercopy_abort("spans multiple pages", NULL, to_user, 0, n); 210 211 for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) { 212 page = virt_to_head_page(ptr); 213 if (is_reserved && !PageReserved(page)) 214 usercopy_abort("spans Reserved and non-Reserved pages", 215 NULL, to_user, 0, n); 216 if (is_cma && !is_migrate_cma_page(page)) 217 usercopy_abort("spans CMA and non-CMA pages", NULL, 218 to_user, 0, n); 219 } 220 #endif 221 } 222 223 static inline void check_heap_object(const void *ptr, unsigned long n, 224 bool to_user) 225 { 226 struct page *page; 227 228 if (!virt_addr_valid(ptr)) 229 return; 230 231 /* 232 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the 233 * highmem page or fallback to virt_to_page(). The following 234 * is effectively a highmem-aware virt_to_head_page(). 235 */ 236 page = compound_head(kmap_to_page((void *)ptr)); 237 238 if (PageSlab(page)) { 239 /* Check slab allocator for flags and size. */ 240 __check_heap_object(ptr, n, page, to_user); 241 } else { 242 /* Verify object does not incorrectly span multiple pages. */ 243 check_page_span(ptr, n, page, to_user); 244 } 245 } 246 247 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks); 248 249 /* 250 * Validates that the given object is: 251 * - not bogus address 252 * - fully contained by stack (or stack frame, when available) 253 * - fully within SLAB object (or object whitelist area, when available) 254 * - not in kernel text 255 */ 256 void __check_object_size(const void *ptr, unsigned long n, bool to_user) 257 { 258 if (static_branch_unlikely(&bypass_usercopy_checks)) 259 return; 260 261 /* Skip all tests if size is zero. */ 262 if (!n) 263 return; 264 265 /* Check for invalid addresses. */ 266 check_bogus_address((const unsigned long)ptr, n, to_user); 267 268 /* Check for bad stack object. */ 269 switch (check_stack_object(ptr, n)) { 270 case NOT_STACK: 271 /* Object is not touching the current process stack. */ 272 break; 273 case GOOD_FRAME: 274 case GOOD_STACK: 275 /* 276 * Object is either in the correct frame (when it 277 * is possible to check) or just generally on the 278 * process stack (when frame checking not available). 279 */ 280 return; 281 default: 282 usercopy_abort("process stack", NULL, to_user, 0, n); 283 } 284 285 /* Check for bad heap object. */ 286 check_heap_object(ptr, n, to_user); 287 288 /* Check for object in kernel to avoid text exposure. */ 289 check_kernel_text_object((const unsigned long)ptr, n, to_user); 290 } 291 EXPORT_SYMBOL(__check_object_size); 292 293 static bool enable_checks __initdata = true; 294 295 static int __init parse_hardened_usercopy(char *str) 296 { 297 return strtobool(str, &enable_checks); 298 } 299 300 __setup("hardened_usercopy=", parse_hardened_usercopy); 301 302 static int __init set_hardened_usercopy(void) 303 { 304 if (enable_checks == false) 305 static_branch_enable(&bypass_usercopy_checks); 306 return 1; 307 } 308 309 late_initcall(set_hardened_usercopy); 310