cregit-Linux how code gets into the kernel

Release 4.16 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 <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; }

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PersonTokensPropCommitsCommitProp
Kees Cook96100.00%1100.00%
Total96100.00%1100.00%

/* * 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); }

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Kees Cook71100.00%1100.00%
Total71100.00%1100.00%


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(); }

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Kees Cook74100.00%2100.00%
Total74100.00%2100.00%

/* 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; }

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PersonTokensPropCommitsCommitProp
Kees Cook5498.18%266.67%
Josh Poimboeuf11.82%133.33%
Total55100.00%3100.00%

/* 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); }

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Kees Cook13198.50%266.67%
Laura Abbott21.50%133.33%
Total133100.00%3100.00%


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); }

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PersonTokensPropCommitsCommitProp
Kees Cook64100.00%2100.00%
Total64100.00%2100.00%

/* 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 }

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Kees Cook295100.00%3100.00%
Total295100.00%3100.00%


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); } }

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PersonTokensPropCommitsCommitProp
Kees Cook76100.00%3100.00%
Total76100.00%3100.00%

/* * 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) { /* 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); }

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PersonTokensPropCommitsCommitProp
Kees Cook103100.00%3100.00%
Total103100.00%3100.00%

EXPORT_SYMBOL(__check_object_size);

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Kees Cook99198.41%550.00%
Ingo Molnar90.89%220.00%
Keun-o Park40.40%110.00%
Laura Abbott20.20%110.00%
Josh Poimboeuf10.10%110.00%
Total1007100.00%10100.00%
Directory: mm
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