mm/usercopy.c

   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);