cregit-Linux how code gets into the kernel

Release 4.8 mm/usercopy.c

Directory: mm
 * 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 <asm/sections.h>

enum {

 * 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; }


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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(); }


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/* 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; }


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josh poimboeufjosh poimboeuf11.75%150.00%

/* 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)__va(__pa(textlow)); /* No different mapping: we're done. */ if (textlow_linear == textlow) return NULL; /* Check the secondary mapping... */ texthigh_linear = (unsigned long)__va(__pa(texthigh)); if (overlaps(ptr, n, textlow_linear, texthigh_linear)) return "<linear kernel text>"; return NULL; }


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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; }


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/* 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; }


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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); }


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/* * 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); }


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Directory: mm
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