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/slab.h>
  15 #include <linux/sched.h>
  16 #include <linux/sched/task.h>
  17 #include <linux/sched/task_stack.h>
  18 #include <linux/thread_info.h>
  19 #include <linux/atomic.h>
  20 #include <linux/jump_label.h>
  21 #include <asm/sections.h>
  22
  23 /*
  24  * Checks if a given pointer and length is contained by the current
  25  * stack frame (if possible).
  26  *
  27  * Returns:
  28  *      NOT_STACK: not at all on the stack
  29  *      GOOD_FRAME: fully within a valid stack frame
  30  *      GOOD_STACK: fully on the stack (when can't do frame-checking)
  31  *      BAD_STACK: error condition (invalid stack position or bad stack frame)
  32  */
  33 static noinline int check_stack_object(const void *obj, unsigned long len)
  34 {
  35         const void * const stack = task_stack_page(current);
  36         const void * const stackend = stack + THREAD_SIZE;
  37         int ret;
  38
  39         /* Object is not on the stack at all. */
  40         if (obj + len <= stack || stackend <= obj)
  41                 return NOT_STACK;
  42
  43         /*
  44          * Reject: object partially overlaps the stack (passing the
  45          * the check above means at least one end is within the stack,
  46          * so if this check fails, the other end is outside the stack).
  47          */
  48         if (obj < stack || stackend < obj + len)
  49                 return BAD_STACK;
  50
  51         /* Check if object is safely within a valid frame. */
  52         ret = arch_within_stack_frames(stack, stackend, obj, len);
  53         if (ret)
  54                 return ret;
  55
  56         return GOOD_STACK;
  57 }
  58
  59 /*
  60  * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
  61  * an unexpected state during a copy_from_user() or copy_to_user() call.
  62  * There are several checks being performed on the buffer by the
  63  * __check_object_size() function. Normal stack buffer usage should never
  64  * trip the checks, and kernel text addressing will always trip the check.
  65  * For cache objects, it is checking that only the whitelisted range of
  66  * bytes for a given cache is being accessed (via the cache's usersize and
  67  * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
  68  * kmem_cache_create_usercopy() function to create the cache (and
  69  * carefully audit the whitelist range).
  70  */
  71 void usercopy_warn(const char *name, const char *detail, bool to_user,
  72                    unsigned long offset, unsigned long len)
  73 {
  74         WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
  75                  to_user ? "exposure" : "overwrite",
  76                  to_user ? "from" : "to",
  77                  name ? : "unknown?!",
  78                  detail ? " '" : "", detail ? : "", detail ? "'" : "",
  79                  offset, len);
  80 }
  81
  82 void __noreturn usercopy_abort(const char *name, const char *detail,
  83                                bool to_user, unsigned long offset,
  84                                unsigned long len)
  85 {
  86         pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
  87                  to_user ? "exposure" : "overwrite",
  88                  to_user ? "from" : "to",
  89                  name ? : "unknown?!",
  90                  detail ? " '" : "", detail ? : "", detail ? "'" : "",
  91                  offset, len);
  92
  93         /*
  94          * For greater effect, it would be nice to do do_group_exit(),
  95          * but BUG() actually hooks all the lock-breaking and per-arch
  96          * Oops code, so that is used here instead.
  97          */
  98         BUG();
  99 }
 100
 101 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
 102 static bool overlaps(const unsigned long ptr, unsigned long n,
 103                      unsigned long low, unsigned long high)
 104 {
 105         const unsigned long check_low = ptr;
 106         unsigned long check_high = check_low + n;
 107
 108         /* Does not overlap if entirely above or entirely below. */
 109         if (check_low >= high || check_high <= low)
 110                 return false;
 111
 112         return true;
 113 }
 114
 115 /* Is this address range in the kernel text area? */
 116 static inline void check_kernel_text_object(const unsigned long ptr,
 117                                             unsigned long n, bool to_user)
 118 {
 119         unsigned long textlow = (unsigned long)_stext;
 120         unsigned long texthigh = (unsigned long)_etext;
 121         unsigned long textlow_linear, texthigh_linear;
 122
 123         if (overlaps(ptr, n, textlow, texthigh))
 124                 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
 125
 126         /*
 127          * Some architectures have virtual memory mappings with a secondary
 128          * mapping of the kernel text, i.e. there is more than one virtual
 129          * kernel address that points to the kernel image. It is usually
 130          * when there is a separate linear physical memory mapping, in that
 131          * __pa() is not just the reverse of __va(). This can be detected
 132          * and checked:
 133          */
 134         textlow_linear = (unsigned long)lm_alias(textlow);
 135         /* No different mapping: we're done. */
 136         if (textlow_linear == textlow)
 137                 return;
 138
 139         /* Check the secondary mapping... */
 140         texthigh_linear = (unsigned long)lm_alias(texthigh);
 141         if (overlaps(ptr, n, textlow_linear, texthigh_linear))
 142                 usercopy_abort("linear kernel text", NULL, to_user,
 143                                ptr - textlow_linear, n);
 144 }
 145
 146 static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 147                                        bool to_user)
 148 {
 149         /* Reject if object wraps past end of memory. */
 150         if (ptr + (n - 1) < ptr)
 151                 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
 152
 153         /* Reject if NULL or ZERO-allocation. */
 154         if (ZERO_OR_NULL_PTR(ptr))
 155                 usercopy_abort("null address", NULL, to_user, ptr, n);
 156 }
 157
 158 /* Checks for allocs that are marked in some way as spanning multiple pages. */
 159 static inline void check_page_span(const void *ptr, unsigned long n,
 160                                    struct page *page, bool to_user)
 161 {
 162 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
 163         const void *end = ptr + n - 1;
 164         struct page *endpage;
 165         bool is_reserved, is_cma;
 166
 167         /*
 168          * Sometimes the kernel data regions are not marked Reserved (see
 169          * check below). And sometimes [_sdata,_edata) does not cover
 170          * rodata and/or bss, so check each range explicitly.
 171          */
 172
 173         /* Allow reads of kernel rodata region (if not marked as Reserved). */
 174         if (ptr >= (const void *)__start_rodata &&
 175             end <= (const void *)__end_rodata) {
 176                 if (!to_user)
 177                         usercopy_abort("rodata", NULL, to_user, 0, n);
 178                 return;
 179         }
 180
 181         /* Allow kernel data region (if not marked as Reserved). */
 182         if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
 183                 return;
 184
 185         /* Allow kernel bss region (if not marked as Reserved). */
 186         if (ptr >= (const void *)__bss_start &&
 187             end <= (const void *)__bss_stop)
 188                 return;
 189
 190         /* Is the object wholly within one base page? */
 191         if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
 192                    ((unsigned long)end & (unsigned long)PAGE_MASK)))
 193                 return;
 194
 195         /* Allow if fully inside the same compound (__GFP_COMP) page. */
 196         endpage = virt_to_head_page(end);
 197         if (likely(endpage == page))
 198                 return;
 199
 200         /*
 201          * Reject if range is entirely either Reserved (i.e. special or
 202          * device memory), or CMA. Otherwise, reject since the object spans
 203          * several independently allocated pages.
 204          */
 205         is_reserved = PageReserved(page);
 206         is_cma = is_migrate_cma_page(page);
 207         if (!is_reserved && !is_cma)
 208                 usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
 209
 210         for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
 211                 page = virt_to_head_page(ptr);
 212                 if (is_reserved && !PageReserved(page))
 213                         usercopy_abort("spans Reserved and non-Reserved pages",
 214                                        NULL, to_user, 0, n);
 215                 if (is_cma && !is_migrate_cma_page(page))
 216                         usercopy_abort("spans CMA and non-CMA pages", NULL,
 217                                        to_user, 0, n);
 218         }
 219 #endif
 220 }
 221
 222 static inline void check_heap_object(const void *ptr, unsigned long n,
 223                                      bool to_user)
 224 {
 225         struct page *page;
 226
 227         if (!virt_addr_valid(ptr))
 228                 return;
 229
 230         page = virt_to_head_page(ptr);
 231
 232         if (PageSlab(page)) {
 233                 /* Check slab allocator for flags and size. */
 234                 __check_heap_object(ptr, n, page, to_user);
 235         } else {
 236                 /* Verify object does not incorrectly span multiple pages. */
 237                 check_page_span(ptr, n, page, to_user);
 238         }
 239 }
 240
 241 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
 242
 243 /*
 244  * Validates that the given object is:
 245  * - not bogus address
 246  * - fully contained by stack (or stack frame, when available)
 247  * - fully within SLAB object (or object whitelist area, when available)
 248  * - not in kernel text
 249  */
 250 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 251 {
 252         if (static_branch_unlikely(&bypass_usercopy_checks))
 253                 return;
 254
 255         /* Skip all tests if size is zero. */
 256         if (!n)
 257                 return;
 258
 259         /* Check for invalid addresses. */
 260         check_bogus_address((const unsigned long)ptr, n, to_user);
 261
 262         /* Check for bad stack object. */
 263         switch (check_stack_object(ptr, n)) {
 264         case NOT_STACK:
 265                 /* Object is not touching the current process stack. */
 266                 break;
 267         case GOOD_FRAME:
 268         case GOOD_STACK:
 269                 /*
 270                  * Object is either in the correct frame (when it
 271                  * is possible to check) or just generally on the
 272                  * process stack (when frame checking not available).
 273                  */
 274                 return;
 275         default:
 276                 usercopy_abort("process stack", NULL, to_user, 0, n);
 277         }
 278
 279         /* Check for bad heap object. */
 280         check_heap_object(ptr, n, to_user);
 281
 282         /* Check for object in kernel to avoid text exposure. */
 283         check_kernel_text_object((const unsigned long)ptr, n, to_user);
 284 }
 285 EXPORT_SYMBOL(__check_object_size);
 286
 287 static bool enable_checks __initdata = true;
 288
 289 static int __init parse_hardened_usercopy(char *str)
 290 {
 291         return strtobool(str, &enable_checks);
 292 }
 293
 294 __setup("hardened_usercopy=", parse_hardened_usercopy);
 295
 296 static int __init set_hardened_usercopy(void)
 297 {
 298         if (enable_checks == false)
 299                 static_branch_enable(&bypass_usercopy_checks);
 300         return 1;
 301 }
 302
 303 late_initcall(set_hardened_usercopy);