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/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);