mm/usercopy.c

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