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![Kees Cook](/assets/img/avatar_default.png)
When running on a system with >512MB RAM with a 32-bit kernel built with:
CONFIG_DEBUG_VIRTUAL=y
CONFIG_HIGHMEM=y
CONFIG_HARDENED_USERCOPY=y
all execve()s will fail due to argv copying into kmap()ed pages, and on
usercopy checking the calls ultimately of virt_to_page() will be looking
for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:
------------[ cut here ]------------
kernel BUG at ../arch/x86/mm/physaddr.c:83!
invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.3.0-rc8 #6
Hardware name: Dell Inc. Inspiron 1318/0C236D, BIOS A04 01/15/2009
EIP: __phys_addr+0xaf/0x100
...
Call Trace:
__check_object_size+0xaf/0x3c0
? __might_sleep+0x80/0xa0
copy_strings+0x1c2/0x370
copy_strings_kernel+0x2b/0x40
__do_execve_file+0x4ca/0x810
? kmem_cache_alloc+0x1c7/0x370
do_execve+0x1b/0x20
...
The check is from arch/x86/mm/physaddr.c:
VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);
Due to the kmap() in fs/exec.c:
kaddr = kmap(kmapped_page);
...
if (copy_from_user(kaddr+offset, str, bytes_to_copy)) ...
Now we can fetch the correct page to avoid the pfn check. In both cases,
hardened usercopy will need to walk the page-span checker (if enabled)
to do sanity checking.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: f5509cc18d
("mm: Hardened usercopy")
Cc: Matthew Wilcox <willy@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/201909171056.7F2FFD17@keescook
309 lines
9.5 KiB
C
309 lines
9.5 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* This implements the various checks for CONFIG_HARDENED_USERCOPY*,
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* which are designed to protect kernel memory from needless exposure
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* and overwrite under many unintended conditions. This code is based
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* on PAX_USERCOPY, which is:
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*
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* Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
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* Security Inc.
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*/
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#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
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#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>
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#include <linux/sched/task_stack.h>
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#include <linux/thread_info.h>
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#include <linux/atomic.h>
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#include <linux/jump_label.h>
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#include <asm/sections.h>
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/*
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* Checks if a given pointer and length is contained by the current
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* stack frame (if possible).
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*
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* Returns:
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* NOT_STACK: not at all on the stack
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* GOOD_FRAME: fully within a valid stack frame
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* GOOD_STACK: fully on the stack (when can't do frame-checking)
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* BAD_STACK: error condition (invalid stack position or bad stack frame)
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*/
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static noinline int check_stack_object(const void *obj, unsigned long len)
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{
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const void * const stack = task_stack_page(current);
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const void * const stackend = stack + THREAD_SIZE;
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int ret;
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/* Object is not on the stack at all. */
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if (obj + len <= stack || stackend <= obj)
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return NOT_STACK;
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/*
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* Reject: object partially overlaps the stack (passing the
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* the check above means at least one end is within the stack,
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* so if this check fails, the other end is outside the stack).
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*/
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if (obj < stack || stackend < obj + len)
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return BAD_STACK;
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/* Check if object is safely within a valid frame. */
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ret = arch_within_stack_frames(stack, stackend, obj, len);
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if (ret)
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return ret;
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return GOOD_STACK;
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}
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/*
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* If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
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* 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
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* __check_object_size() function. Normal stack buffer usage should never
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* 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
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* bytes for a given cache is being accessed (via the cache's usersize and
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* useroffset fields). To adjust a cache whitelist, use the usercopy-aware
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* kmem_cache_create_usercopy() function to create the cache (and
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* 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,
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unsigned long offset, unsigned long len)
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{
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WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
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to_user ? "exposure" : "overwrite",
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to_user ? "from" : "to",
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name ? : "unknown?!",
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detail ? " '" : "", detail ? : "", detail ? "'" : "",
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offset, len);
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}
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void __noreturn usercopy_abort(const char *name, const char *detail,
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bool to_user, unsigned long offset,
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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)!\n",
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to_user ? "exposure" : "overwrite",
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to_user ? "from" : "to",
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name ? : "unknown?!",
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detail ? " '" : "", detail ? : "", detail ? "'" : "",
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offset, len);
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/*
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* For greater effect, it would be nice to do do_group_exit(),
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* but BUG() actually hooks all the lock-breaking and per-arch
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* Oops code, so that is used here instead.
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*/
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BUG();
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}
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/* 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,
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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;
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/* 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;
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return true;
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}
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/* Is this address range in the kernel text area? */
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static inline void check_kernel_text_object(const unsigned long ptr,
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unsigned long n, bool to_user)
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{
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unsigned long textlow = (unsigned long)_stext;
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unsigned long texthigh = (unsigned long)_etext;
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unsigned long textlow_linear, texthigh_linear;
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if (overlaps(ptr, n, textlow, texthigh))
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usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
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/*
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* Some architectures have virtual memory mappings with a secondary
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* mapping of the kernel text, i.e. there is more than one virtual
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* kernel address that points to the kernel image. It is usually
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* when there is a separate linear physical memory mapping, in that
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* __pa() is not just the reverse of __va(). This can be detected
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* and checked:
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*/
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textlow_linear = (unsigned long)lm_alias(textlow);
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/* No different mapping: we're done. */
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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,
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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,
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bool to_user)
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{
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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. */
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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,
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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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/*
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* Sometimes the kernel data regions are not marked Reserved (see
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* check below). And sometimes [_sdata,_edata) does not cover
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* rodata and/or bss, so check each range explicitly.
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*/
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/* Allow reads of kernel rodata region (if not marked as Reserved). */
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if (ptr >= (const void *)__start_rodata &&
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end <= (const void *)__end_rodata) {
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if (!to_user)
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usercopy_abort("rodata", NULL, to_user, 0, n);
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return;
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}
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/* Allow kernel data region (if not marked as Reserved). */
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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). */
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if (ptr >= (const void *)__bss_start &&
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end <= (const void *)__bss_stop)
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return;
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/* Is the object wholly within one base page? */
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if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
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((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);
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if (likely(endpage == page))
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return;
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/*
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* Reject if range is entirely either Reserved (i.e. special or
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* device memory), or CMA. Otherwise, reject since the object spans
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* several independently allocated pages.
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*/
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is_reserved = PageReserved(page);
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is_cma = is_migrate_cma_page(page);
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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) {
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page = virt_to_head_page(ptr);
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if (is_reserved && !PageReserved(page))
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usercopy_abort("spans Reserved and non-Reserved pages",
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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,
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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,
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bool to_user)
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{
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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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/*
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* When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
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* highmem page or fallback to virt_to_page(). The following
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* is effectively a highmem-aware virt_to_head_page().
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*/
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page = compound_head(kmap_to_page((void *)ptr));
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if (PageSlab(page)) {
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/* Check slab allocator for flags and size. */
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__check_heap_object(ptr, n, page, to_user);
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} else {
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/* Verify object does not incorrectly span multiple pages. */
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check_page_span(ptr, n, page, to_user);
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}
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}
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static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
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/*
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* Validates that the given object is:
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* - not bogus address
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* - fully contained by stack (or stack frame, when available)
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* - fully within SLAB object (or object whitelist area, when available)
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* - not in kernel text
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*/
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void __check_object_size(const void *ptr, unsigned long n, bool to_user)
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{
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if (static_branch_unlikely(&bypass_usercopy_checks))
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return;
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/* Skip all tests if size is zero. */
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if (!n)
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return;
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/* 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. */
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switch (check_stack_object(ptr, n)) {
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case NOT_STACK:
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/* Object is not touching the current process stack. */
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break;
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case GOOD_FRAME:
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case GOOD_STACK:
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/*
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* Object is either in the correct frame (when it
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* is possible to check) or just generally on the
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* process stack (when frame checking not available).
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*/
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return;
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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. */
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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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}
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EXPORT_SYMBOL(__check_object_size);
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static bool enable_checks __initdata = true;
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static int __init parse_hardened_usercopy(char *str)
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{
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return strtobool(str, &enable_checks);
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}
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__setup("hardened_usercopy=", parse_hardened_usercopy);
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static int __init set_hardened_usercopy(void)
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{
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if (enable_checks == false)
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static_branch_enable(&bypass_usercopy_checks);
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return 1;
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}
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late_initcall(set_hardened_usercopy);
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