/*
* 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.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*
*/
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/sched/task_stack.h>
#include <asm/sections.h>
enum {
BAD_STACK = -1,
NOT_STACK = 0,
GOOD_FRAME,
GOOD_STACK,
};
/*
* 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;
}
static void report_usercopy(const void *ptr, unsigned long len,
bool to_user, const char *type)
{
pr_emerg("kernel memory %s attempt detected %s %p (%s) (%lu bytes)\n",
to_user ? "exposure" : "overwrite",
to_user ? "from" : "to", ptr, type ? : "unknown", 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.
*/
BUG();
}
/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
static bool overlaps(const void *ptr, unsigned long n, unsigned long low,
unsigned long high)
{
unsigned long check_low = (uintptr_t)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 const char *check_kernel_text_object(const void *ptr,
unsigned long n)
{
unsigned long textlow = (unsigned long)_stext;
unsigned long texthigh = (unsigned long)_etext;
unsigned long textlow_linear, texthigh_linear;
if (overlaps(ptr, n, textlow, texthigh))
return "<kernel text>";
/*
* 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)
return NULL;
/* Check the secondary mapping... */
texthigh_linear = (unsigned long)lm_alias(texthigh);
if (overlaps(ptr, n, textlow_linear, texthigh_linear))
return "<linear kernel text>";
return NULL;
}
static inline const char *check_bogus_address(const void *ptr, unsigned long n)
{
/* Reject if object wraps past end of memory. */
if ((unsigned long)ptr + n < (unsigned long)ptr)
return "<wrapped address>";
/* Reject if NULL or ZERO-allocation. */
if (ZERO_OR_NULL_PTR(ptr))
return "<null>";
return NULL;
}
/* Checks for allocs that are marked in some way as spanning multiple pages. */
static inline const char *check_page_span(const void *ptr, unsigned long n,
struct page *page, bool to_user)
{
#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
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)
return "<rodata>";
return NULL;
}
/* Allow kernel data region (if not marked as Reserved). */
if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
return NULL;
/* Allow kernel bss region (if not marked as Reserved). */
if (ptr >= (const void *)__bss_start &&
end <= (const void *)__bss_stop)
return NULL;
/* Is the object wholly within one base page? */
if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
((unsigned long)end & (unsigned long)PAGE_MASK)))
return NULL;
/* Allow if fully inside the same compound (__GFP_COMP) page. */
endpage = virt_to_head_page(end);
if (likely(endpage == page))
return NULL;
/*
* 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)
return "<spans multiple pages>";
for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
page = virt_to_head_page(ptr);
if (is_reserved && !PageReserved(page))
return "<spans Reserved and non-Reserved pages>";
if (is_cma && !is_migrate_cma_page(page))
return "<spans CMA and non-CMA pages>";
}
#endif
return NULL;
}
static inline const char *check_heap_object(const void *ptr, unsigned long n,
bool to_user)
{
struct page *page;
/*
* Some architectures (arm64) return true for virt_addr_valid() on
* vmalloced addresses. Work around this by checking for vmalloc
* first.
*
* We also need to check for module addresses explicitly since we
* may copy static data from modules to userspace
*/
if (is_vmalloc_or_module_addr(ptr))
return NULL;
if (!virt_addr_valid(ptr))
return NULL;
page = virt_to_head_page(ptr);
/* Check slab allocator for flags and size. */
if (PageSlab(page))
return __check_heap_object(ptr, n, page);
/* Verify object does not incorrectly span multiple pages. */
return check_page_span(ptr, n, page, to_user);
}
/*
* Validates that the given object is:
* - not bogus address
* - known-safe heap or stack object
* - not in kernel text
*/
void __check_object_size(const void *ptr, unsigned long n, bool to_user)
{
const char *err;
/* Skip all tests if size is zero. */
if (!n)
return;
/* Check for invalid addresses. */
err = check_bogus_address(ptr, n);
if (err)
goto report;
/* Check for bad heap object. */
err = check_heap_object(ptr, n, to_user);
if (err)
goto report;
/* Check for bad stack object. */
switch (check_stack_object(ptr, n)) {
case NOT_STACK:
/* Object is not touching the current process stack. */
break;
case GOOD_FRAME:
case GOOD_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).
*/
return;
default:
err = "<process stack>";
goto report;
}
/* Check for object in kernel to avoid text exposure. */
err = check_kernel_text_object(ptr, n);
if (!err)
return;
report:
report_usercopy(ptr, n, to_user, err);
}
EXPORT_SYMBOL(__check_object_size);
