1 /* 2 * This implements the various checks for CONFIG_HARDENED_USERCOPY*, 3 * which are designed to protect kernel memory from needless exposure 4 * and overwrite under many unintended conditions. This code is based 5 * on PAX_USERCOPY, which is: 6 * 7 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source 8 * Security Inc. 9 * 10 * This program is free software; you can redistribute it and/or modify 11 * it under the terms of the GNU General Public License version 2 as 12 * published by the Free Software Foundation. 13 * 14 */ 15 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt 16 17 #include <linux/mm.h> 18 #include <linux/slab.h> 19 #include <linux/sched.h> 20 #include <linux/sched/task.h> 21 #include <linux/sched/task_stack.h> 22 #include <linux/thread_info.h> 23 #include <asm/sections.h> 24 25 /* 26 * Checks if a given pointer and length is contained by the current 27 * stack frame (if possible). 28 * 29 * Returns: 30 * NOT_STACK: not at all on the stack 31 * GOOD_FRAME: fully within a valid stack frame 32 * GOOD_STACK: fully on the stack (when can't do frame-checking) 33 * BAD_STACK: error condition (invalid stack position or bad stack frame) 34 */ 35 static noinline int check_stack_object(const void *obj, unsigned long len) 36 { 37 const void * const stack = task_stack_page(current); 38 const void * const stackend = stack + THREAD_SIZE; 39 int ret; 40 41 /* Object is not on the stack at all. */ 42 if (obj + len <= stack || stackend <= obj) 43 return NOT_STACK; 44 45 /* 46 * Reject: object partially overlaps the stack (passing the 47 * the check above means at least one end is within the stack, 48 * so if this check fails, the other end is outside the stack). 49 */ 50 if (obj < stack || stackend < obj + len) 51 return BAD_STACK; 52 53 /* Check if object is safely within a valid frame. */ 54 ret = arch_within_stack_frames(stack, stackend, obj, len); 55 if (ret) 56 return ret; 57 58 return GOOD_STACK; 59 } 60 61 /* 62 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found 63 * an unexpected state during a copy_from_user() or copy_to_user() call. 64 * There are several checks being performed on the buffer by the 65 * __check_object_size() function. Normal stack buffer usage should never 66 * trip the checks, and kernel text addressing will always trip the check. 67 * For cache objects, it is checking that only the whitelisted range of 68 * bytes for a given cache is being accessed (via the cache's usersize and 69 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware 70 * kmem_cache_create_usercopy() function to create the cache (and 71 * carefully audit the whitelist range). 72 */ 73 void usercopy_warn(const char *name, const char *detail, bool to_user, 74 unsigned long offset, unsigned long len) 75 { 76 WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 77 to_user ? "exposure" : "overwrite", 78 to_user ? "from" : "to", 79 name ? : "unknown?!", 80 detail ? " '" : "", detail ? : "", detail ? "'" : "", 81 offset, len); 82 } 83 84 void __noreturn usercopy_abort(const char *name, const char *detail, 85 bool to_user, unsigned long offset, 86 unsigned long len) 87 { 88 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n", 89 to_user ? "exposure" : "overwrite", 90 to_user ? "from" : "to", 91 name ? : "unknown?!", 92 detail ? " '" : "", detail ? : "", detail ? "'" : "", 93 offset, len); 94 95 /* 96 * For greater effect, it would be nice to do do_group_exit(), 97 * but BUG() actually hooks all the lock-breaking and per-arch 98 * Oops code, so that is used here instead. 99 */ 100 BUG(); 101 } 102 103 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */ 104 static bool overlaps(const unsigned long ptr, unsigned long n, 105 unsigned long low, unsigned long high) 106 { 107 const unsigned long check_low = ptr; 108 unsigned long check_high = check_low + n; 109 110 /* Does not overlap if entirely above or entirely below. */ 111 if (check_low >= high || check_high <= low) 112 return false; 113 114 return true; 115 } 116 117 /* Is this address range in the kernel text area? */ 118 static inline void check_kernel_text_object(const unsigned long ptr, 119 unsigned long n, bool to_user) 120 { 121 unsigned long textlow = (unsigned long)_stext; 122 unsigned long texthigh = (unsigned long)_etext; 123 unsigned long textlow_linear, texthigh_linear; 124 125 if (overlaps(ptr, n, textlow, texthigh)) 126 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n); 127 128 /* 129 * Some architectures have virtual memory mappings with a secondary 130 * mapping of the kernel text, i.e. there is more than one virtual 131 * kernel address that points to the kernel image. It is usually 132 * when there is a separate linear physical memory mapping, in that 133 * __pa() is not just the reverse of __va(). This can be detected 134 * and checked: 135 */ 136 textlow_linear = (unsigned long)lm_alias(textlow); 137 /* No different mapping: we're done. */ 138 if (textlow_linear == textlow) 139 return; 140 141 /* Check the secondary mapping... */ 142 texthigh_linear = (unsigned long)lm_alias(texthigh); 143 if (overlaps(ptr, n, textlow_linear, texthigh_linear)) 144 usercopy_abort("linear kernel text", NULL, to_user, 145 ptr - textlow_linear, n); 146 } 147 148 static inline void check_bogus_address(const unsigned long ptr, unsigned long n, 149 bool to_user) 150 { 151 /* Reject if object wraps past end of memory. */ 152 if (ptr + n < ptr) 153 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); 154 155 /* Reject if NULL or ZERO-allocation. */ 156 if (ZERO_OR_NULL_PTR(ptr)) 157 usercopy_abort("null address", NULL, to_user, ptr, n); 158 } 159 160 /* Checks for allocs that are marked in some way as spanning multiple pages. */ 161 static inline void check_page_span(const void *ptr, unsigned long n, 162 struct page *page, bool to_user) 163 { 164 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN 165 const void *end = ptr + n - 1; 166 struct page *endpage; 167 bool is_reserved, is_cma; 168 169 /* 170 * Sometimes the kernel data regions are not marked Reserved (see 171 * check below). And sometimes [_sdata,_edata) does not cover 172 * rodata and/or bss, so check each range explicitly. 173 */ 174 175 /* Allow reads of kernel rodata region (if not marked as Reserved). */ 176 if (ptr >= (const void *)__start_rodata && 177 end <= (const void *)__end_rodata) { 178 if (!to_user) 179 usercopy_abort("rodata", NULL, to_user, 0, n); 180 return; 181 } 182 183 /* Allow kernel data region (if not marked as Reserved). */ 184 if (ptr >= (const void *)_sdata && end <= (const void *)_edata) 185 return; 186 187 /* Allow kernel bss region (if not marked as Reserved). */ 188 if (ptr >= (const void *)__bss_start && 189 end <= (const void *)__bss_stop) 190 return; 191 192 /* Is the object wholly within one base page? */ 193 if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == 194 ((unsigned long)end & (unsigned long)PAGE_MASK))) 195 return; 196 197 /* Allow if fully inside the same compound (__GFP_COMP) page. */ 198 endpage = virt_to_head_page(end); 199 if (likely(endpage == page)) 200 return; 201 202 /* 203 * Reject if range is entirely either Reserved (i.e. special or 204 * device memory), or CMA. Otherwise, reject since the object spans 205 * several independently allocated pages. 206 */ 207 is_reserved = PageReserved(page); 208 is_cma = is_migrate_cma_page(page); 209 if (!is_reserved && !is_cma) 210 usercopy_abort("spans multiple pages", NULL, to_user, 0, n); 211 212 for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) { 213 page = virt_to_head_page(ptr); 214 if (is_reserved && !PageReserved(page)) 215 usercopy_abort("spans Reserved and non-Reserved pages", 216 NULL, to_user, 0, n); 217 if (is_cma && !is_migrate_cma_page(page)) 218 usercopy_abort("spans CMA and non-CMA pages", NULL, 219 to_user, 0, n); 220 } 221 #endif 222 } 223 224 static inline void check_heap_object(const void *ptr, unsigned long n, 225 bool to_user) 226 { 227 struct page *page; 228 229 if (!virt_addr_valid(ptr)) 230 return; 231 232 page = virt_to_head_page(ptr); 233 234 if (PageSlab(page)) { 235 /* Check slab allocator for flags and size. */ 236 __check_heap_object(ptr, n, page, to_user); 237 } else { 238 /* Verify object does not incorrectly span multiple pages. */ 239 check_page_span(ptr, n, page, to_user); 240 } 241 } 242 243 /* 244 * Validates that the given object is: 245 * - not bogus address 246 * - known-safe heap or stack object 247 * - not in kernel text 248 */ 249 void __check_object_size(const void *ptr, unsigned long n, bool to_user) 250 { 251 /* Skip all tests if size is zero. */ 252 if (!n) 253 return; 254 255 /* Check for invalid addresses. */ 256 check_bogus_address((const unsigned long)ptr, n, to_user); 257 258 /* Check for bad heap object. */ 259 check_heap_object(ptr, n, to_user); 260 261 /* Check for bad stack object. */ 262 switch (check_stack_object(ptr, n)) { 263 case NOT_STACK: 264 /* Object is not touching the current process stack. */ 265 break; 266 case GOOD_FRAME: 267 case GOOD_STACK: 268 /* 269 * Object is either in the correct frame (when it 270 * is possible to check) or just generally on the 271 * process stack (when frame checking not available). 272 */ 273 return; 274 default: 275 usercopy_abort("process stack", NULL, to_user, 0, n); 276 } 277 278 /* Check for object in kernel to avoid text exposure. */ 279 check_kernel_text_object((const unsigned long)ptr, n, to_user); 280 } 281 EXPORT_SYMBOL(__check_object_size); 282