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 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);