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 static void report_usercopy(const void *ptr, unsigned long len, 62 bool to_user, const char *type) 63 { 64 pr_emerg("kernel memory %s attempt detected %s %p (%s) (%lu bytes)\n", 65 to_user ? "exposure" : "overwrite", 66 to_user ? "from" : "to", ptr, type ? : "unknown", len); 67 /* 68 * For greater effect, it would be nice to do do_group_exit(), 69 * but BUG() actually hooks all the lock-breaking and per-arch 70 * Oops code, so that is used here instead. 71 */ 72 BUG(); 73 } 74 75 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */ 76 static bool overlaps(const void *ptr, unsigned long n, unsigned long low, 77 unsigned long high) 78 { 79 unsigned long check_low = (uintptr_t)ptr; 80 unsigned long check_high = check_low + n; 81 82 /* Does not overlap if entirely above or entirely below. */ 83 if (check_low >= high || check_high <= low) 84 return false; 85 86 return true; 87 } 88 89 /* Is this address range in the kernel text area? */ 90 static inline const char *check_kernel_text_object(const void *ptr, 91 unsigned long n) 92 { 93 unsigned long textlow = (unsigned long)_stext; 94 unsigned long texthigh = (unsigned long)_etext; 95 unsigned long textlow_linear, texthigh_linear; 96 97 if (overlaps(ptr, n, textlow, texthigh)) 98 return "<kernel text>"; 99 100 /* 101 * Some architectures have virtual memory mappings with a secondary 102 * mapping of the kernel text, i.e. there is more than one virtual 103 * kernel address that points to the kernel image. It is usually 104 * when there is a separate linear physical memory mapping, in that 105 * __pa() is not just the reverse of __va(). This can be detected 106 * and checked: 107 */ 108 textlow_linear = (unsigned long)lm_alias(textlow); 109 /* No different mapping: we're done. */ 110 if (textlow_linear == textlow) 111 return NULL; 112 113 /* Check the secondary mapping... */ 114 texthigh_linear = (unsigned long)lm_alias(texthigh); 115 if (overlaps(ptr, n, textlow_linear, texthigh_linear)) 116 return "<linear kernel text>"; 117 118 return NULL; 119 } 120 121 static inline const char *check_bogus_address(const void *ptr, unsigned long n) 122 { 123 /* Reject if object wraps past end of memory. */ 124 if ((unsigned long)ptr + n < (unsigned long)ptr) 125 return "<wrapped address>"; 126 127 /* Reject if NULL or ZERO-allocation. */ 128 if (ZERO_OR_NULL_PTR(ptr)) 129 return "<null>"; 130 131 return NULL; 132 } 133 134 /* Checks for allocs that are marked in some way as spanning multiple pages. */ 135 static inline const char *check_page_span(const void *ptr, unsigned long n, 136 struct page *page, bool to_user) 137 { 138 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN 139 const void *end = ptr + n - 1; 140 struct page *endpage; 141 bool is_reserved, is_cma; 142 143 /* 144 * Sometimes the kernel data regions are not marked Reserved (see 145 * check below). And sometimes [_sdata,_edata) does not cover 146 * rodata and/or bss, so check each range explicitly. 147 */ 148 149 /* Allow reads of kernel rodata region (if not marked as Reserved). */ 150 if (ptr >= (const void *)__start_rodata && 151 end <= (const void *)__end_rodata) { 152 if (!to_user) 153 return "<rodata>"; 154 return NULL; 155 } 156 157 /* Allow kernel data region (if not marked as Reserved). */ 158 if (ptr >= (const void *)_sdata && end <= (const void *)_edata) 159 return NULL; 160 161 /* Allow kernel bss region (if not marked as Reserved). */ 162 if (ptr >= (const void *)__bss_start && 163 end <= (const void *)__bss_stop) 164 return NULL; 165 166 /* Is the object wholly within one base page? */ 167 if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == 168 ((unsigned long)end & (unsigned long)PAGE_MASK))) 169 return NULL; 170 171 /* Allow if fully inside the same compound (__GFP_COMP) page. */ 172 endpage = virt_to_head_page(end); 173 if (likely(endpage == page)) 174 return NULL; 175 176 /* 177 * Reject if range is entirely either Reserved (i.e. special or 178 * device memory), or CMA. Otherwise, reject since the object spans 179 * several independently allocated pages. 180 */ 181 is_reserved = PageReserved(page); 182 is_cma = is_migrate_cma_page(page); 183 if (!is_reserved && !is_cma) 184 return "<spans multiple pages>"; 185 186 for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) { 187 page = virt_to_head_page(ptr); 188 if (is_reserved && !PageReserved(page)) 189 return "<spans Reserved and non-Reserved pages>"; 190 if (is_cma && !is_migrate_cma_page(page)) 191 return "<spans CMA and non-CMA pages>"; 192 } 193 #endif 194 195 return NULL; 196 } 197 198 static inline const char *check_heap_object(const void *ptr, unsigned long n, 199 bool to_user) 200 { 201 struct page *page; 202 203 if (!virt_addr_valid(ptr)) 204 return NULL; 205 206 page = virt_to_head_page(ptr); 207 208 /* Check slab allocator for flags and size. */ 209 if (PageSlab(page)) 210 return __check_heap_object(ptr, n, page); 211 212 /* Verify object does not incorrectly span multiple pages. */ 213 return check_page_span(ptr, n, page, to_user); 214 } 215 216 /* 217 * Validates that the given object is: 218 * - not bogus address 219 * - known-safe heap or stack object 220 * - not in kernel text 221 */ 222 void __check_object_size(const void *ptr, unsigned long n, bool to_user) 223 { 224 const char *err; 225 226 /* Skip all tests if size is zero. */ 227 if (!n) 228 return; 229 230 /* Check for invalid addresses. */ 231 err = check_bogus_address(ptr, n); 232 if (err) 233 goto report; 234 235 /* Check for bad heap object. */ 236 err = check_heap_object(ptr, n, to_user); 237 if (err) 238 goto report; 239 240 /* Check for bad stack object. */ 241 switch (check_stack_object(ptr, n)) { 242 case NOT_STACK: 243 /* Object is not touching the current process stack. */ 244 break; 245 case GOOD_FRAME: 246 case GOOD_STACK: 247 /* 248 * Object is either in the correct frame (when it 249 * is possible to check) or just generally on the 250 * process stack (when frame checking not available). 251 */ 252 return; 253 default: 254 err = "<process stack>"; 255 goto report; 256 } 257 258 /* Check for object in kernel to avoid text exposure. */ 259 err = check_kernel_text_object(ptr, n); 260 if (!err) 261 return; 262 263 report: 264 report_usercopy(ptr, n, to_user, err); 265 } 266 EXPORT_SYMBOL(__check_object_size); 267