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/atomic.h>
  21 #include <linux/jump_label.h>
  22 #include <asm/sections.h>
  23
  24 /*
  25  * Checks if a given pointer and length is contained by the current
  26  * stack frame (if possible).
  27  *
  28  * Returns:
  29  *      NOT_STACK: not at all on the stack
  30  *      GOOD_FRAME: fully within a valid stack frame
  31  *      GOOD_STACK: fully on the stack (when can't do frame-checking)
  32  *      BAD_STACK: error condition (invalid stack position or bad stack frame)
  33  */
  34 static noinline int check_stack_object(const void *obj, unsigned long len)
  35 {
  36         const void * const stack = task_stack_page(current);
  37         const void * const stackend = stack + THREAD_SIZE;
  38         int ret;
  39
  40         /* Object is not on the stack at all. */
  41         if (obj + len <= stack || stackend <= obj)
  42                 return NOT_STACK;
  43
  44         /*
  45          * Reject: object partially overlaps the stack (passing the
  46          * the check above means at least one end is within the stack,
  47          * so if this check fails, the other end is outside the stack).
  48          */
  49         if (obj < stack || stackend < obj + len)
  50                 return BAD_STACK;
  51
  52         /* Check if object is safely within a valid frame. */
  53         ret = arch_within_stack_frames(stack, stackend, obj, len);
  54         if (ret)
  55                 return ret;
  56
  57         return GOOD_STACK;
  58 }
  59
  60 /*
  61  * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
  62  * an unexpected state during a copy_from_user() or copy_to_user() call.
  63  * There are several checks being performed on the buffer by the
  64  * __check_object_size() function. Normal stack buffer usage should never
  65  * trip the checks, and kernel text addressing will always trip the check.
  66  * For cache objects, it is checking that only the whitelisted range of
  67  * bytes for a given cache is being accessed (via the cache's usersize and
  68  * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
  69  * kmem_cache_create_usercopy() function to create the cache (and
  70  * carefully audit the whitelist range).
  71  */
  72 void usercopy_warn(const char *name, const char *detail, bool to_user,
  73                    unsigned long offset, unsigned long len)
  74 {
  75         WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
  76                  to_user ? "exposure" : "overwrite",
  77                  to_user ? "from" : "to",
  78                  name ? : "unknown?!",
  79                  detail ? " '" : "", detail ? : "", detail ? "'" : "",
  80                  offset, len);
  81 }
  82
  83 void __noreturn usercopy_abort(const char *name, const char *detail,
  84                                bool to_user, unsigned long offset,
  85                                unsigned long len)
  86 {
  87         pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
  88                  to_user ? "exposure" : "overwrite",
  89                  to_user ? "from" : "to",
  90                  name ? : "unknown?!",
  91                  detail ? " '" : "", detail ? : "", detail ? "'" : "",
  92                  offset, len);
  93
  94         /*
  95          * For greater effect, it would be nice to do do_group_exit(),
  96          * but BUG() actually hooks all the lock-breaking and per-arch
  97          * Oops code, so that is used here instead.
  98          */
  99         BUG();
 100 }
 101
 102 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
 103 static bool overlaps(const unsigned long ptr, unsigned long n,
 104                      unsigned long low, unsigned long high)
 105 {
 106         const unsigned long check_low = ptr;
 107         unsigned long check_high = check_low + n;
 108
 109         /* Does not overlap if entirely above or entirely below. */
 110         if (check_low >= high || check_high <= low)
 111                 return false;
 112
 113         return true;
 114 }
 115
 116 /* Is this address range in the kernel text area? */
 117 static inline void check_kernel_text_object(const unsigned long ptr,
 118                                             unsigned long n, bool to_user)
 119 {
 120         unsigned long textlow = (unsigned long)_stext;
 121         unsigned long texthigh = (unsigned long)_etext;
 122         unsigned long textlow_linear, texthigh_linear;
 123
 124         if (overlaps(ptr, n, textlow, texthigh))
 125                 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
 126
 127         /*
 128          * Some architectures have virtual memory mappings with a secondary
 129          * mapping of the kernel text, i.e. there is more than one virtual
 130          * kernel address that points to the kernel image. It is usually
 131          * when there is a separate linear physical memory mapping, in that
 132          * __pa() is not just the reverse of __va(). This can be detected
 133          * and checked:
 134          */
 135         textlow_linear = (unsigned long)lm_alias(textlow);
 136         /* No different mapping: we're done. */
 137         if (textlow_linear == textlow)
 138                 return;
 139
 140         /* Check the secondary mapping... */
 141         texthigh_linear = (unsigned long)lm_alias(texthigh);
 142         if (overlaps(ptr, n, textlow_linear, texthigh_linear))
 143                 usercopy_abort("linear kernel text", NULL, to_user,
 144                                ptr - textlow_linear, n);
 145 }
 146
 147 static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 148                                        bool to_user)
 149 {
 150         /* Reject if object wraps past end of memory. */
 151         if (ptr + (n - 1) < ptr)
 152                 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
 153
 154         /* Reject if NULL or ZERO-allocation. */
 155         if (ZERO_OR_NULL_PTR(ptr))
 156                 usercopy_abort("null address", NULL, to_user, ptr, n);
 157 }
 158
 159 /* Checks for allocs that are marked in some way as spanning multiple pages. */
 160 static inline void check_page_span(const void *ptr, unsigned long n,
 161                                    struct page *page, bool to_user)
 162 {
 163 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
 164         const void *end = ptr + n - 1;
 165         struct page *endpage;
 166         bool is_reserved, is_cma;
 167
 168         /*
 169          * Sometimes the kernel data regions are not marked Reserved (see
 170          * check below). And sometimes [_sdata,_edata) does not cover
 171          * rodata and/or bss, so check each range explicitly.
 172          */
 173
 174         /* Allow reads of kernel rodata region (if not marked as Reserved). */
 175         if (ptr >= (const void *)__start_rodata &&
 176             end <= (const void *)__end_rodata) {
 177                 if (!to_user)
 178                         usercopy_abort("rodata", NULL, to_user, 0, n);
 179                 return;
 180         }
 181
 182         /* Allow kernel data region (if not marked as Reserved). */
 183         if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
 184                 return;
 185
 186         /* Allow kernel bss region (if not marked as Reserved). */
 187         if (ptr >= (const void *)__bss_start &&
 188             end <= (const void *)__bss_stop)
 189                 return;
 190
 191         /* Is the object wholly within one base page? */
 192         if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
 193                    ((unsigned long)end & (unsigned long)PAGE_MASK)))
 194                 return;
 195
 196         /* Allow if fully inside the same compound (__GFP_COMP) page. */
 197         endpage = virt_to_head_page(end);
 198         if (likely(endpage == page))
 199                 return;
 200
 201         /*
 202          * Reject if range is entirely either Reserved (i.e. special or
 203          * device memory), or CMA. Otherwise, reject since the object spans
 204          * several independently allocated pages.
 205          */
 206         is_reserved = PageReserved(page);
 207         is_cma = is_migrate_cma_page(page);
 208         if (!is_reserved && !is_cma)
 209                 usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
 210
 211         for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
 212                 page = virt_to_head_page(ptr);
 213                 if (is_reserved && !PageReserved(page))
 214                         usercopy_abort("spans Reserved and non-Reserved pages",
 215                                        NULL, to_user, 0, n);
 216                 if (is_cma && !is_migrate_cma_page(page))
 217                         usercopy_abort("spans CMA and non-CMA pages", NULL,
 218                                        to_user, 0, n);
 219         }
 220 #endif
 221 }
 222
 223 static inline void check_heap_object(const void *ptr, unsigned long n,
 224                                      bool to_user)
 225 {
 226         struct page *page;
 227
 228         if (!virt_addr_valid(ptr))
 229                 return;
 230
 231         /*
 232          * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
 233          * highmem page or fallback to virt_to_page(). The following
 234          * is effectively a highmem-aware virt_to_head_page().
 235          */
 236         page = compound_head(kmap_to_page((void *)ptr));
 237
 238         if (PageSlab(page)) {
 239                 /* Check slab allocator for flags and size. */
 240                 __check_heap_object(ptr, n, page, to_user);
 241         } else {
 242                 /* Verify object does not incorrectly span multiple pages. */
 243                 check_page_span(ptr, n, page, to_user);
 244         }
 245 }
 246
 247 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
 248
 249 /*
 250  * Validates that the given object is:
 251  * - not bogus address
 252  * - fully contained by stack (or stack frame, when available)
 253  * - fully within SLAB object (or object whitelist area, when available)
 254  * - not in kernel text
 255  */
 256 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 257 {
 258         if (static_branch_unlikely(&bypass_usercopy_checks))
 259                 return;
 260
 261         /* Skip all tests if size is zero. */
 262         if (!n)
 263                 return;
 264
 265         /* Check for invalid addresses. */
 266         check_bogus_address((const unsigned long)ptr, n, to_user);
 267
 268         /* Check for bad stack object. */
 269         switch (check_stack_object(ptr, n)) {
 270         case NOT_STACK:
 271                 /* Object is not touching the current process stack. */
 272                 break;
 273         case GOOD_FRAME:
 274         case GOOD_STACK:
 275                 /*
 276                  * Object is either in the correct frame (when it
 277                  * is possible to check) or just generally on the
 278                  * process stack (when frame checking not available).
 279                  */
 280                 return;
 281         default:
 282                 usercopy_abort("process stack", NULL, to_user, 0, n);
 283         }
 284
 285         /* Check for bad heap object. */
 286         check_heap_object(ptr, n, to_user);
 287
 288         /* Check for object in kernel to avoid text exposure. */
 289         check_kernel_text_object((const unsigned long)ptr, n, to_user);
 290 }
 291 EXPORT_SYMBOL(__check_object_size);
 292
 293 static bool enable_checks __initdata = true;
 294
 295 static int __init parse_hardened_usercopy(char *str)
 296 {
 297         return strtobool(str, &enable_checks);
 298 }
 299
 300 __setup("hardened_usercopy=", parse_hardened_usercopy);
 301
 302 static int __init set_hardened_usercopy(void)
 303 {
 304         if (enable_checks == false)
 305                 static_branch_enable(&bypass_usercopy_checks);
 306         return 1;
 307 }
 308
 309 late_initcall(set_hardened_usercopy);