/*
 * 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 <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.
         */
        BUG();
}

/* 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)
                return;

        /* 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 < 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)
{
#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)
                        usercopy_abort("rodata", NULL, to_user, 0, n);
                return;
        }

        /* Allow kernel data region (if not marked as Reserved). */
        if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
                return;

        /* Allow kernel bss region (if not marked as Reserved). */
        if (ptr >= (const void *)__bss_start &&
            end <= (const void *)__bss_stop)
                return;

        /* 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;

        /* Allow if fully inside the same compound (__GFP_COMP) page. */
        endpage = virt_to_head_page(end);
        if (likely(endpage == page))
                return;

        /*
         * 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);
        }
#endif
}

static inline void check_heap_object(const void *ptr, unsigned long n,
                                     bool to_user)
{
        struct page *page;

        if (!virt_addr_valid(ptr))
                return;

        page = virt_to_head_page(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
 * - known-safe heap or stack object
 * - not in kernel text
 */
void __check_object_size(const void *ptr, unsigned long n, bool to_user)
{
        if (static_branch_unlikely(&bypass_usercopy_checks))
                return;

        /* Skip all tests if size is zero. */
        if (!n)
                return;

        /* Check for invalid addresses. */
        check_bogus_address((const unsigned long)ptr, n, to_user);

        /* Check for bad heap object. */
        check_heap_object(ptr, n, to_user);

        /* 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:
                usercopy_abort("process stack", NULL, to_user, 0, n);
        }

        /* Check for object in kernel to avoid text exposure. */
        check_kernel_text_object((const unsigned long)ptr, n, to_user);
}
EXPORT_SYMBOL(__check_object_size);

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)
                static_branch_enable(&bypass_usercopy_checks);
        return 1;
}

late_initcall(set_hardened_usercopy);