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// SPDX-License-Identifier: GPL-2.0-only
* This implements the various checks for CONFIG_HARDENED_USERCOPY*,
* which are designed to protect kernel memory from needless exposure
* and overwrite under many unintended conditions. This code is based
* on PAX_USERCOPY, which is:
* Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
* Security Inc.
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/mm.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/sched/task_stack.h>
#include <linux/thread_info.h>
#include <linux/atomic.h>
#include <linux/jump_label.h>
#include <asm/sections.h>
* Checks if a given pointer and length is contained by the current
* stack frame (if possible).
* Returns:
* NOT_STACK: not at all on the stack
* GOOD_FRAME: fully within a valid stack frame
* GOOD_STACK: fully on the stack (when can't do frame-checking)
* BAD_STACK: error condition (invalid stack position or bad stack frame)
static noinline int check_stack_object(const void *obj, unsigned long len)
const void * const stack = task_stack_page(current);
const void * const stackend = stack + THREAD_SIZE;
int ret;
/* Object is not on the stack at all. */
if (obj + len <= stack || stackend <= obj)
return NOT_STACK;
* Reject: object partially overlaps the stack (passing the
* the check above means at least one end is within the stack,
* so if this check fails, the other end is outside the stack).
if (obj < stack || stackend < obj + len)
return BAD_STACK;
/* Check if object is safely within a valid frame. */
ret = arch_within_stack_frames(stack, stackend, obj, len);
if (ret)
return ret;
return GOOD_STACK;
* If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
* an unexpected state during a copy_from_user() or copy_to_user() call.
* There are several checks being performed on the buffer by the
* __check_object_size() function. Normal stack buffer usage should never
* trip the checks, and kernel text addressing will always trip the check.
* For cache objects, it is checking that only the whitelisted range of
* bytes for a given cache is being accessed (via the cache's usersize and
* useroffset fields). To adjust a cache whitelist, use the usercopy-aware
* kmem_cache_create_usercopy() function to create the cache (and
* carefully audit the whitelist range).
void usercopy_warn(const char *name, const char *detail, bool to_user,
unsigned long offset, unsigned long len)
WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
to_user ? "exposure" : "overwrite",
to_user ? "from" : "to",
name ? : "unknown?!",
detail ? " '" : "", detail ? : "", detail ? "'" : "",
offset, len);
void __noreturn usercopy_abort(const char *name, const char *detail,
bool to_user, unsigned long offset,
unsigned long len)
pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
to_user ? "exposure" : "overwrite",
to_user ? "from" : "to",
name ? : "unknown?!",
detail ? " '" : "", detail ? : "", detail ? "'" : "",
offset, len);
* For greater effect, it would be nice to do do_group_exit(),
* but BUG() actually hooks all the lock-breaking and per-arch
* Oops code, so that is used here instead.
/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
static bool overlaps(const unsigned long ptr, unsigned long n,
unsigned long low, unsigned long high)
const unsigned long check_low = ptr;
unsigned long check_high = check_low + n;
/* Does not overlap if entirely above or entirely below. */
if (check_low >= high || check_high <= low)
return false;
return true;
/* Is this address range in the kernel text area? */
static inline void check_kernel_text_object(const unsigned long ptr,
unsigned long n, bool to_user)
unsigned long textlow = (unsigned long)_stext;
unsigned long texthigh = (unsigned long)_etext;
unsigned long textlow_linear, texthigh_linear;
if (overlaps(ptr, n, textlow, texthigh))
usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
* Some architectures have virtual memory mappings with a secondary
* mapping of the kernel text, i.e. there is more than one virtual
* kernel address that points to the kernel image. It is usually
* when there is a separate linear physical memory mapping, in that
* __pa() is not just the reverse of __va(). This can be detected
* and checked:
textlow_linear = (unsigned long)lm_alias(textlow);
/* No different mapping: we're done. */
if (textlow_linear == textlow)
/* Check the secondary mapping... */
texthigh_linear = (unsigned long)lm_alias(texthigh);
if (overlaps(ptr, n, textlow_linear, texthigh_linear))
usercopy_abort("linear kernel text", NULL, to_user,
ptr - textlow_linear, n);
static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
bool to_user)
/* Reject if object wraps past end of memory. */
if (ptr + (n - 1) < ptr)
usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
/* Reject if NULL or ZERO-allocation. */
if (ZERO_OR_NULL_PTR(ptr))
usercopy_abort("null address", NULL, to_user, ptr, n);
/* Checks for allocs that are marked in some way as spanning multiple pages. */
static inline void check_page_span(const void *ptr, unsigned long n,
struct page *page, bool to_user)
const void *end = ptr + n - 1;
struct page *endpage;
bool is_reserved, is_cma;
* Sometimes the kernel data regions are not marked Reserved (see
* check below). And sometimes [_sdata,_edata) does not cover
* rodata and/or bss, so check each range explicitly.
/* Allow reads of kernel rodata region (if not marked as Reserved). */
if (ptr >= (const void *)__start_rodata &&
end <= (const void *)__end_rodata) {
if (!to_user)
usercopy_abort("rodata", NULL, to_user, 0, n);
/* Allow kernel data region (if not marked as Reserved). */
if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
/* Allow kernel bss region (if not marked as Reserved). */
if (ptr >= (const void *)__bss_start &&
end <= (const void *)__bss_stop)
/* Is the object wholly within one base page? */
if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
((unsigned long)end & (unsigned long)PAGE_MASK)))
/* Allow if fully inside the same compound (__GFP_COMP) page. */
endpage = virt_to_head_page(end);
if (likely(endpage == page))
* Reject if range is entirely either Reserved (i.e. special or
* device memory), or CMA. Otherwise, reject since the object spans
* several independently allocated pages.
is_reserved = PageReserved(page);
is_cma = is_migrate_cma_page(page);
if (!is_reserved && !is_cma)
usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
page = virt_to_head_page(ptr);
if (is_reserved && !PageReserved(page))
usercopy_abort("spans Reserved and non-Reserved pages",
NULL, to_user, 0, n);
if (is_cma && !is_migrate_cma_page(page))
usercopy_abort("spans CMA and non-CMA pages", NULL,
to_user, 0, n);
static inline void check_heap_object(const void *ptr, unsigned long n,
bool to_user)
struct page *page;
if (!virt_addr_valid(ptr))
* When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
* highmem page or fallback to virt_to_page(). The following
* is effectively a highmem-aware virt_to_head_page().
page = compound_head(kmap_to_page((void *)ptr));
if (PageSlab(page)) {
/* Check slab allocator for flags and size. */
__check_heap_object(ptr, n, page, to_user);
} else {
/* Verify object does not incorrectly span multiple pages. */
check_page_span(ptr, n, page, to_user);
static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
* Validates that the given object is:
* - not bogus address
* - fully contained by stack (or stack frame, when available)
* - fully within SLAB object (or object whitelist area, when available)
* - not in kernel text
void __check_object_size(const void *ptr, unsigned long n, bool to_user)
if (static_branch_unlikely(&bypass_usercopy_checks))
/* Skip all tests if size is zero. */
if (!n)
/* Check for invalid addresses. */
check_bogus_address((const unsigned long)ptr, n, to_user);
/* Check for bad stack object. */
switch (check_stack_object(ptr, n)) {
/* Object is not touching the current process stack. */
* Object is either in the correct frame (when it
* is possible to check) or just generally on the
* process stack (when frame checking not available).
usercopy_abort("process stack", NULL, to_user, 0, n);
/* Check for bad heap object. */
check_heap_object(ptr, n, to_user);
/* Check for object in kernel to avoid text exposure. */
check_kernel_text_object((const unsigned long)ptr, n, to_user);
static bool enable_checks __initdata = true;
static int __init parse_hardened_usercopy(char *str)
return strtobool(str, &enable_checks);
__setup("hardened_usercopy=", parse_hardened_usercopy);
static int __init set_hardened_usercopy(void)
if (enable_checks == false)
return 1;