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mm/usercopy.c
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// SPDX-License-Identifier: GPL-2.0-only |
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/* * 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. |
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*/ #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt #include <linux/mm.h> |
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#include <linux/highmem.h> |
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#include <linux/slab.h> |
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#include <linux/sched.h> |
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#include <linux/sched/task.h> #include <linux/sched/task_stack.h> |
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#include <linux/thread_info.h> |
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#include <linux/atomic.h> #include <linux/jump_label.h> |
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#include <asm/sections.h> |
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/* * 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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/* |
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* If these functions are reached, then CONFIG_HARDENED_USERCOPY has found * an unexpected state during a copy_from_user() or copy_to_user() call. |
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* 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. |
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* 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). |
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*/ |
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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)! ", to_user ? "exposure" : "overwrite", to_user ? "from" : "to", name ? : "unknown?!", detail ? " '" : "", detail ? : "", detail ? "'" : "", offset, len); } |
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void __noreturn usercopy_abort(const char *name, const char *detail, bool to_user, unsigned long offset, unsigned long len) |
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{ |
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pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)! ", to_user ? "exposure" : "overwrite", to_user ? "from" : "to", name ? : "unknown?!", detail ? " '" : "", detail ? : "", detail ? "'" : "", offset, len); |
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/* * 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). */ |
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static bool overlaps(const unsigned long ptr, unsigned long n, unsigned long low, unsigned long high) |
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{ |
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const unsigned long check_low = ptr; |
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unsigned long check_high = check_low + n; /* Does not overlap if entirely above or entirely below. */ |
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if (check_low >= high || check_high <= low) |
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return false; return true; } /* Is this address range in the kernel text area? */ |
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static inline void check_kernel_text_object(const unsigned long ptr, unsigned long n, bool to_user) |
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{ unsigned long textlow = (unsigned long)_stext; unsigned long texthigh = (unsigned long)_etext; unsigned long textlow_linear, texthigh_linear; if (overlaps(ptr, n, textlow, texthigh)) |
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usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n); |
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/* * 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: */ |
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textlow_linear = (unsigned long)lm_alias(textlow); |
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/* No different mapping: we're done. */ if (textlow_linear == textlow) |
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return; |
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/* Check the secondary mapping... */ |
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texthigh_linear = (unsigned long)lm_alias(texthigh); |
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if (overlaps(ptr, n, textlow_linear, texthigh_linear)) |
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usercopy_abort("linear kernel text", NULL, to_user, ptr - textlow_linear, n); |
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} |
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static inline void check_bogus_address(const unsigned long ptr, unsigned long n, bool to_user) |
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{ /* Reject if object wraps past end of memory. */ |
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if (ptr + (n - 1) < ptr) |
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usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); |
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/* Reject if NULL or ZERO-allocation. */ if (ZERO_OR_NULL_PTR(ptr)) |
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usercopy_abort("null address", NULL, to_user, ptr, n); |
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} |
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/* Checks for allocs that are marked in some way as spanning multiple pages. */ |
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static inline void check_page_span(const void *ptr, unsigned long n, struct page *page, bool to_user) |
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{ |
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#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN |
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const void *end = ptr + n - 1; |
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struct page *endpage; |
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bool is_reserved, is_cma; /* |
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* 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) |
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usercopy_abort("rodata", NULL, to_user, 0, n); return; |
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} /* Allow kernel data region (if not marked as Reserved). */ if (ptr >= (const void *)_sdata && end <= (const void *)_edata) |
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return; |
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/* Allow kernel bss region (if not marked as Reserved). */ if (ptr >= (const void *)__bss_start && end <= (const void *)__bss_stop) |
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return; |
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/* Is the object wholly within one base page? */ if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == ((unsigned long)end & (unsigned long)PAGE_MASK))) |
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return; |
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/* Allow if fully inside the same compound (__GFP_COMP) page. */ |
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endpage = virt_to_head_page(end); if (likely(endpage == page)) |
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return; |
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/* * 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) |
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usercopy_abort("spans multiple pages", NULL, to_user, 0, n); |
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for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) { page = virt_to_head_page(ptr); if (is_reserved && !PageReserved(page)) |
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usercopy_abort("spans Reserved and non-Reserved pages", NULL, to_user, 0, n); |
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if (is_cma && !is_migrate_cma_page(page)) |
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usercopy_abort("spans CMA and non-CMA pages", NULL, to_user, 0, n); |
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} |
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#endif |
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} |
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static inline void check_heap_object(const void *ptr, unsigned long n, bool to_user) |
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{ struct page *page; |
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if (!virt_addr_valid(ptr)) |
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return; |
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/* * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the * highmem page or fallback to virt_to_page(). The following * is effectively a highmem-aware virt_to_head_page(). */ page = compound_head(kmap_to_page((void *)ptr)); |
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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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} |
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static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks); |
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/* * Validates that the given object is: * - not bogus address |
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* - fully contained by stack (or stack frame, when available) * - fully within SLAB object (or object whitelist area, when available) |
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* - not in kernel text */ void __check_object_size(const void *ptr, unsigned long n, bool to_user) { |
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if (static_branch_unlikely(&bypass_usercopy_checks)) return; |
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/* Skip all tests if size is zero. */ if (!n) return; /* Check for invalid addresses. */ |
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check_bogus_address((const unsigned long)ptr, n, to_user); |
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/* 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: |
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usercopy_abort("process stack", NULL, to_user, 0, n); |
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} |
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/* Check for bad heap object. */ check_heap_object(ptr, n, to_user); |
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/* Check for object in kernel to avoid text exposure. */ |
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check_kernel_text_object((const unsigned long)ptr, n, to_user); |
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} EXPORT_SYMBOL(__check_object_size); |
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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); |