linux/mm/usercopy.c
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   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 */
  34static 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         * 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 */
  72void 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
  83void __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). */
 103static 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? */
 117static 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
 147static 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. */
 160static 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
 223static 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
 247static 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 */
 256void __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}
 291EXPORT_SYMBOL(__check_object_size);
 292
 293static bool enable_checks __initdata = true;
 294
 295static int __init parse_hardened_usercopy(char *str)
 296{
 297        return strtobool(str, &enable_checks);
 298}
 299
 300__setup("hardened_usercopy=", parse_hardened_usercopy);
 301
 302static int __init set_hardened_usercopy(void)
 303{
 304        if (enable_checks == false)
 305                static_branch_enable(&bypass_usercopy_checks);
 306        return 1;
 307}
 308
 309late_initcall(set_hardened_usercopy);
 310
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