/*
 * 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 <linux/thread_info.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;
}

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;

        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);