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