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36be5cba99
Where the compiler instruments meminstrinsics by generating calls to
__asan/__hwasan_ prefixed functions, let the compiler consider
memintrinsics as builtin again.
To do so, never override memset/memmove/memcpy if the compiler does the
correct instrumentation - even on !GENERIC_ENTRY architectures.
[elver@google.com: powerpc: don't rename memintrinsics if compiler adds prefixes]
Link: https://lore.kernel.org/all/20230224085942.1791837-1-elver@google.com/ [1]
Link: https://lkml.kernel.org/r/20230227094726.3833247-1-elver@google.com
Link: https://lkml.kernel.org/r/20230224085942.1791837-2-elver@google.com
Fixes: 69d4c0d321
("entry, kasan, x86: Disallow overriding mem*() functions")
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
651 lines
18 KiB
C
651 lines
18 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* This file contains KASAN runtime code that manages shadow memory for
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* generic and software tag-based KASAN modes.
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*
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* Copyright (c) 2014 Samsung Electronics Co., Ltd.
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* Author: Andrey Ryabinin <ryabinin.a.a@gmail.com>
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*
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* Some code borrowed from https://github.com/xairy/kasan-prototype by
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* Andrey Konovalov <andreyknvl@gmail.com>
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*/
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#include <linux/init.h>
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#include <linux/kasan.h>
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#include <linux/kernel.h>
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#include <linux/kfence.h>
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#include <linux/kmemleak.h>
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#include <linux/memory.h>
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#include <linux/mm.h>
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#include <linux/string.h>
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#include <linux/types.h>
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#include <linux/vmalloc.h>
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#include <asm/cacheflush.h>
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#include <asm/tlbflush.h>
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#include "kasan.h"
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bool __kasan_check_read(const volatile void *p, unsigned int size)
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{
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return kasan_check_range((unsigned long)p, size, false, _RET_IP_);
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}
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EXPORT_SYMBOL(__kasan_check_read);
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bool __kasan_check_write(const volatile void *p, unsigned int size)
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{
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return kasan_check_range((unsigned long)p, size, true, _RET_IP_);
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}
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EXPORT_SYMBOL(__kasan_check_write);
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#if !defined(CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX) && !defined(CONFIG_GENERIC_ENTRY)
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/*
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* CONFIG_GENERIC_ENTRY relies on compiler emitted mem*() calls to not be
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* instrumented. KASAN enabled toolchains should emit __asan_mem*() functions
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* for the sites they want to instrument.
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*
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* If we have a compiler that can instrument meminstrinsics, never override
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* these, so that non-instrumented files can safely consider them as builtins.
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*/
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#undef memset
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void *memset(void *addr, int c, size_t len)
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{
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if (!kasan_check_range((unsigned long)addr, len, true, _RET_IP_))
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return NULL;
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return __memset(addr, c, len);
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}
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#ifdef __HAVE_ARCH_MEMMOVE
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#undef memmove
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void *memmove(void *dest, const void *src, size_t len)
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{
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if (!kasan_check_range((unsigned long)src, len, false, _RET_IP_) ||
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!kasan_check_range((unsigned long)dest, len, true, _RET_IP_))
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return NULL;
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return __memmove(dest, src, len);
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}
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#endif
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#undef memcpy
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void *memcpy(void *dest, const void *src, size_t len)
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{
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if (!kasan_check_range((unsigned long)src, len, false, _RET_IP_) ||
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!kasan_check_range((unsigned long)dest, len, true, _RET_IP_))
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return NULL;
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return __memcpy(dest, src, len);
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}
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#endif
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void *__asan_memset(void *addr, int c, size_t len)
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{
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if (!kasan_check_range((unsigned long)addr, len, true, _RET_IP_))
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return NULL;
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return __memset(addr, c, len);
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}
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EXPORT_SYMBOL(__asan_memset);
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#ifdef __HAVE_ARCH_MEMMOVE
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void *__asan_memmove(void *dest, const void *src, size_t len)
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{
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if (!kasan_check_range((unsigned long)src, len, false, _RET_IP_) ||
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!kasan_check_range((unsigned long)dest, len, true, _RET_IP_))
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return NULL;
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return __memmove(dest, src, len);
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}
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EXPORT_SYMBOL(__asan_memmove);
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#endif
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void *__asan_memcpy(void *dest, const void *src, size_t len)
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{
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if (!kasan_check_range((unsigned long)src, len, false, _RET_IP_) ||
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!kasan_check_range((unsigned long)dest, len, true, _RET_IP_))
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return NULL;
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return __memcpy(dest, src, len);
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}
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EXPORT_SYMBOL(__asan_memcpy);
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#ifdef CONFIG_KASAN_SW_TAGS
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void *__hwasan_memset(void *addr, int c, size_t len) __alias(__asan_memset);
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EXPORT_SYMBOL(__hwasan_memset);
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#ifdef __HAVE_ARCH_MEMMOVE
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void *__hwasan_memmove(void *dest, const void *src, size_t len) __alias(__asan_memmove);
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EXPORT_SYMBOL(__hwasan_memmove);
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#endif
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void *__hwasan_memcpy(void *dest, const void *src, size_t len) __alias(__asan_memcpy);
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EXPORT_SYMBOL(__hwasan_memcpy);
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#endif
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void kasan_poison(const void *addr, size_t size, u8 value, bool init)
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{
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void *shadow_start, *shadow_end;
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if (!kasan_arch_is_ready())
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return;
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/*
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* Perform shadow offset calculation based on untagged address, as
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* some of the callers (e.g. kasan_poison_object_data) pass tagged
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* addresses to this function.
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*/
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addr = kasan_reset_tag(addr);
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/* Skip KFENCE memory if called explicitly outside of sl*b. */
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if (is_kfence_address(addr))
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return;
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if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
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return;
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if (WARN_ON(size & KASAN_GRANULE_MASK))
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return;
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shadow_start = kasan_mem_to_shadow(addr);
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shadow_end = kasan_mem_to_shadow(addr + size);
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__memset(shadow_start, value, shadow_end - shadow_start);
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}
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EXPORT_SYMBOL(kasan_poison);
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#ifdef CONFIG_KASAN_GENERIC
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void kasan_poison_last_granule(const void *addr, size_t size)
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{
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if (!kasan_arch_is_ready())
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return;
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if (size & KASAN_GRANULE_MASK) {
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u8 *shadow = (u8 *)kasan_mem_to_shadow(addr + size);
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*shadow = size & KASAN_GRANULE_MASK;
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}
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}
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#endif
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void kasan_unpoison(const void *addr, size_t size, bool init)
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{
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u8 tag = get_tag(addr);
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/*
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* Perform shadow offset calculation based on untagged address, as
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* some of the callers (e.g. kasan_unpoison_object_data) pass tagged
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* addresses to this function.
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*/
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addr = kasan_reset_tag(addr);
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/*
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* Skip KFENCE memory if called explicitly outside of sl*b. Also note
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* that calls to ksize(), where size is not a multiple of machine-word
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* size, would otherwise poison the invalid portion of the word.
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*/
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if (is_kfence_address(addr))
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return;
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if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
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return;
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/* Unpoison all granules that cover the object. */
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kasan_poison(addr, round_up(size, KASAN_GRANULE_SIZE), tag, false);
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/* Partially poison the last granule for the generic mode. */
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if (IS_ENABLED(CONFIG_KASAN_GENERIC))
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kasan_poison_last_granule(addr, size);
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}
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#ifdef CONFIG_MEMORY_HOTPLUG
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static bool shadow_mapped(unsigned long addr)
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{
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pgd_t *pgd = pgd_offset_k(addr);
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p4d_t *p4d;
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pud_t *pud;
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pmd_t *pmd;
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pte_t *pte;
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if (pgd_none(*pgd))
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return false;
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p4d = p4d_offset(pgd, addr);
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if (p4d_none(*p4d))
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return false;
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pud = pud_offset(p4d, addr);
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if (pud_none(*pud))
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return false;
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/*
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* We can't use pud_large() or pud_huge(), the first one is
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* arch-specific, the last one depends on HUGETLB_PAGE. So let's abuse
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* pud_bad(), if pud is bad then it's bad because it's huge.
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*/
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if (pud_bad(*pud))
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return true;
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pmd = pmd_offset(pud, addr);
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if (pmd_none(*pmd))
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return false;
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if (pmd_bad(*pmd))
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return true;
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pte = pte_offset_kernel(pmd, addr);
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return !pte_none(*pte);
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}
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static int __meminit kasan_mem_notifier(struct notifier_block *nb,
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unsigned long action, void *data)
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{
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struct memory_notify *mem_data = data;
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unsigned long nr_shadow_pages, start_kaddr, shadow_start;
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unsigned long shadow_end, shadow_size;
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nr_shadow_pages = mem_data->nr_pages >> KASAN_SHADOW_SCALE_SHIFT;
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start_kaddr = (unsigned long)pfn_to_kaddr(mem_data->start_pfn);
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shadow_start = (unsigned long)kasan_mem_to_shadow((void *)start_kaddr);
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shadow_size = nr_shadow_pages << PAGE_SHIFT;
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shadow_end = shadow_start + shadow_size;
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if (WARN_ON(mem_data->nr_pages % KASAN_GRANULE_SIZE) ||
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WARN_ON(start_kaddr % KASAN_MEMORY_PER_SHADOW_PAGE))
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return NOTIFY_BAD;
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switch (action) {
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case MEM_GOING_ONLINE: {
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void *ret;
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/*
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* If shadow is mapped already than it must have been mapped
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* during the boot. This could happen if we onlining previously
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* offlined memory.
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*/
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if (shadow_mapped(shadow_start))
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return NOTIFY_OK;
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ret = __vmalloc_node_range(shadow_size, PAGE_SIZE, shadow_start,
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shadow_end, GFP_KERNEL,
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PAGE_KERNEL, VM_NO_GUARD,
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pfn_to_nid(mem_data->start_pfn),
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__builtin_return_address(0));
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if (!ret)
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return NOTIFY_BAD;
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kmemleak_ignore(ret);
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return NOTIFY_OK;
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}
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case MEM_CANCEL_ONLINE:
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case MEM_OFFLINE: {
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struct vm_struct *vm;
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/*
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* shadow_start was either mapped during boot by kasan_init()
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* or during memory online by __vmalloc_node_range().
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* In the latter case we can use vfree() to free shadow.
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* Non-NULL result of the find_vm_area() will tell us if
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* that was the second case.
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*
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* Currently it's not possible to free shadow mapped
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* during boot by kasan_init(). It's because the code
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* to do that hasn't been written yet. So we'll just
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* leak the memory.
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*/
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vm = find_vm_area((void *)shadow_start);
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if (vm)
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vfree((void *)shadow_start);
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}
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}
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return NOTIFY_OK;
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}
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static int __init kasan_memhotplug_init(void)
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{
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hotplug_memory_notifier(kasan_mem_notifier, DEFAULT_CALLBACK_PRI);
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return 0;
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}
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core_initcall(kasan_memhotplug_init);
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#endif
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#ifdef CONFIG_KASAN_VMALLOC
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void __init __weak kasan_populate_early_vm_area_shadow(void *start,
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unsigned long size)
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{
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}
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static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
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void *unused)
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{
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unsigned long page;
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pte_t pte;
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if (likely(!pte_none(*ptep)))
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return 0;
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page = __get_free_page(GFP_KERNEL);
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if (!page)
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return -ENOMEM;
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memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
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pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
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spin_lock(&init_mm.page_table_lock);
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if (likely(pte_none(*ptep))) {
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set_pte_at(&init_mm, addr, ptep, pte);
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page = 0;
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}
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spin_unlock(&init_mm.page_table_lock);
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if (page)
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free_page(page);
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return 0;
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}
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int kasan_populate_vmalloc(unsigned long addr, unsigned long size)
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{
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unsigned long shadow_start, shadow_end;
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int ret;
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if (!kasan_arch_is_ready())
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return 0;
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if (!is_vmalloc_or_module_addr((void *)addr))
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return 0;
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shadow_start = (unsigned long)kasan_mem_to_shadow((void *)addr);
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shadow_end = (unsigned long)kasan_mem_to_shadow((void *)addr + size);
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/*
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* User Mode Linux maps enough shadow memory for all of virtual memory
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* at boot, so doesn't need to allocate more on vmalloc, just clear it.
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*
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* The remaining CONFIG_UML checks in this file exist for the same
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* reason.
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*/
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if (IS_ENABLED(CONFIG_UML)) {
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__memset((void *)shadow_start, KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
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return 0;
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}
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shadow_start = PAGE_ALIGN_DOWN(shadow_start);
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shadow_end = PAGE_ALIGN(shadow_end);
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ret = apply_to_page_range(&init_mm, shadow_start,
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shadow_end - shadow_start,
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kasan_populate_vmalloc_pte, NULL);
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if (ret)
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return ret;
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flush_cache_vmap(shadow_start, shadow_end);
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/*
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* We need to be careful about inter-cpu effects here. Consider:
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*
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* CPU#0 CPU#1
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* WRITE_ONCE(p, vmalloc(100)); while (x = READ_ONCE(p)) ;
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* p[99] = 1;
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*
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* With compiler instrumentation, that ends up looking like this:
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*
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* CPU#0 CPU#1
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* // vmalloc() allocates memory
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* // let a = area->addr
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* // we reach kasan_populate_vmalloc
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* // and call kasan_unpoison:
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* STORE shadow(a), unpoison_val
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* ...
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* STORE shadow(a+99), unpoison_val x = LOAD p
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* // rest of vmalloc process <data dependency>
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* STORE p, a LOAD shadow(x+99)
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*
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* If there is no barrier between the end of unpoisoning the shadow
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* and the store of the result to p, the stores could be committed
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* in a different order by CPU#0, and CPU#1 could erroneously observe
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* poison in the shadow.
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*
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* We need some sort of barrier between the stores.
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*
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* In the vmalloc() case, this is provided by a smp_wmb() in
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* clear_vm_uninitialized_flag(). In the per-cpu allocator and in
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* get_vm_area() and friends, the caller gets shadow allocated but
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* doesn't have any pages mapped into the virtual address space that
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* has been reserved. Mapping those pages in will involve taking and
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* releasing a page-table lock, which will provide the barrier.
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*/
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return 0;
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}
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static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
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void *unused)
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{
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unsigned long page;
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page = (unsigned long)__va(pte_pfn(*ptep) << PAGE_SHIFT);
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spin_lock(&init_mm.page_table_lock);
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if (likely(!pte_none(*ptep))) {
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pte_clear(&init_mm, addr, ptep);
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free_page(page);
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}
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spin_unlock(&init_mm.page_table_lock);
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return 0;
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}
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/*
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* Release the backing for the vmalloc region [start, end), which
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* lies within the free region [free_region_start, free_region_end).
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*
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* This can be run lazily, long after the region was freed. It runs
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* under vmap_area_lock, so it's not safe to interact with the vmalloc/vmap
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* infrastructure.
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*
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* How does this work?
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* -------------------
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*
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* We have a region that is page aligned, labeled as A.
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* That might not map onto the shadow in a way that is page-aligned:
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*
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* start end
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* v v
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* |????????|????????|AAAAAAAA|AA....AA|AAAAAAAA|????????| < vmalloc
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* -------- -------- -------- -------- --------
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* | | | | |
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* | | | /-------/ |
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* \-------\|/------/ |/---------------/
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* ||| ||
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* |??AAAAAA|AAAAAAAA|AA??????| < shadow
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* (1) (2) (3)
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*
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* First we align the start upwards and the end downwards, so that the
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* shadow of the region aligns with shadow page boundaries. In the
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* example, this gives us the shadow page (2). This is the shadow entirely
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* covered by this allocation.
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*
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* Then we have the tricky bits. We want to know if we can free the
|
|
* partially covered shadow pages - (1) and (3) in the example. For this,
|
|
* we are given the start and end of the free region that contains this
|
|
* allocation. Extending our previous example, we could have:
|
|
*
|
|
* free_region_start free_region_end
|
|
* | start end |
|
|
* v v v v
|
|
* |FFFFFFFF|FFFFFFFF|AAAAAAAA|AA....AA|AAAAAAAA|FFFFFFFF| < vmalloc
|
|
* -------- -------- -------- -------- --------
|
|
* | | | | |
|
|
* | | | /-------/ |
|
|
* \-------\|/------/ |/---------------/
|
|
* ||| ||
|
|
* |FFAAAAAA|AAAAAAAA|AAF?????| < shadow
|
|
* (1) (2) (3)
|
|
*
|
|
* Once again, we align the start of the free region up, and the end of
|
|
* the free region down so that the shadow is page aligned. So we can free
|
|
* page (1) - we know no allocation currently uses anything in that page,
|
|
* because all of it is in the vmalloc free region. But we cannot free
|
|
* page (3), because we can't be sure that the rest of it is unused.
|
|
*
|
|
* We only consider pages that contain part of the original region for
|
|
* freeing: we don't try to free other pages from the free region or we'd
|
|
* end up trying to free huge chunks of virtual address space.
|
|
*
|
|
* Concurrency
|
|
* -----------
|
|
*
|
|
* How do we know that we're not freeing a page that is simultaneously
|
|
* being used for a fresh allocation in kasan_populate_vmalloc(_pte)?
|
|
*
|
|
* We _can_ have kasan_release_vmalloc and kasan_populate_vmalloc running
|
|
* at the same time. While we run under free_vmap_area_lock, the population
|
|
* code does not.
|
|
*
|
|
* free_vmap_area_lock instead operates to ensure that the larger range
|
|
* [free_region_start, free_region_end) is safe: because __alloc_vmap_area and
|
|
* the per-cpu region-finding algorithm both run under free_vmap_area_lock,
|
|
* no space identified as free will become used while we are running. This
|
|
* means that so long as we are careful with alignment and only free shadow
|
|
* pages entirely covered by the free region, we will not run in to any
|
|
* trouble - any simultaneous allocations will be for disjoint regions.
|
|
*/
|
|
void kasan_release_vmalloc(unsigned long start, unsigned long end,
|
|
unsigned long free_region_start,
|
|
unsigned long free_region_end)
|
|
{
|
|
void *shadow_start, *shadow_end;
|
|
unsigned long region_start, region_end;
|
|
unsigned long size;
|
|
|
|
if (!kasan_arch_is_ready())
|
|
return;
|
|
|
|
region_start = ALIGN(start, KASAN_MEMORY_PER_SHADOW_PAGE);
|
|
region_end = ALIGN_DOWN(end, KASAN_MEMORY_PER_SHADOW_PAGE);
|
|
|
|
free_region_start = ALIGN(free_region_start, KASAN_MEMORY_PER_SHADOW_PAGE);
|
|
|
|
if (start != region_start &&
|
|
free_region_start < region_start)
|
|
region_start -= KASAN_MEMORY_PER_SHADOW_PAGE;
|
|
|
|
free_region_end = ALIGN_DOWN(free_region_end, KASAN_MEMORY_PER_SHADOW_PAGE);
|
|
|
|
if (end != region_end &&
|
|
free_region_end > region_end)
|
|
region_end += KASAN_MEMORY_PER_SHADOW_PAGE;
|
|
|
|
shadow_start = kasan_mem_to_shadow((void *)region_start);
|
|
shadow_end = kasan_mem_to_shadow((void *)region_end);
|
|
|
|
if (shadow_end > shadow_start) {
|
|
size = shadow_end - shadow_start;
|
|
if (IS_ENABLED(CONFIG_UML)) {
|
|
__memset(shadow_start, KASAN_SHADOW_INIT, shadow_end - shadow_start);
|
|
return;
|
|
}
|
|
apply_to_existing_page_range(&init_mm,
|
|
(unsigned long)shadow_start,
|
|
size, kasan_depopulate_vmalloc_pte,
|
|
NULL);
|
|
flush_tlb_kernel_range((unsigned long)shadow_start,
|
|
(unsigned long)shadow_end);
|
|
}
|
|
}
|
|
|
|
void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
|
|
kasan_vmalloc_flags_t flags)
|
|
{
|
|
/*
|
|
* Software KASAN modes unpoison both VM_ALLOC and non-VM_ALLOC
|
|
* mappings, so the KASAN_VMALLOC_VM_ALLOC flag is ignored.
|
|
* Software KASAN modes can't optimize zeroing memory by combining it
|
|
* with setting memory tags, so the KASAN_VMALLOC_INIT flag is ignored.
|
|
*/
|
|
|
|
if (!kasan_arch_is_ready())
|
|
return (void *)start;
|
|
|
|
if (!is_vmalloc_or_module_addr(start))
|
|
return (void *)start;
|
|
|
|
/*
|
|
* Don't tag executable memory with the tag-based mode.
|
|
* The kernel doesn't tolerate having the PC register tagged.
|
|
*/
|
|
if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) &&
|
|
!(flags & KASAN_VMALLOC_PROT_NORMAL))
|
|
return (void *)start;
|
|
|
|
start = set_tag(start, kasan_random_tag());
|
|
kasan_unpoison(start, size, false);
|
|
return (void *)start;
|
|
}
|
|
|
|
/*
|
|
* Poison the shadow for a vmalloc region. Called as part of the
|
|
* freeing process at the time the region is freed.
|
|
*/
|
|
void __kasan_poison_vmalloc(const void *start, unsigned long size)
|
|
{
|
|
if (!kasan_arch_is_ready())
|
|
return;
|
|
|
|
if (!is_vmalloc_or_module_addr(start))
|
|
return;
|
|
|
|
size = round_up(size, KASAN_GRANULE_SIZE);
|
|
kasan_poison(start, size, KASAN_VMALLOC_INVALID, false);
|
|
}
|
|
|
|
#else /* CONFIG_KASAN_VMALLOC */
|
|
|
|
int kasan_alloc_module_shadow(void *addr, size_t size, gfp_t gfp_mask)
|
|
{
|
|
void *ret;
|
|
size_t scaled_size;
|
|
size_t shadow_size;
|
|
unsigned long shadow_start;
|
|
|
|
shadow_start = (unsigned long)kasan_mem_to_shadow(addr);
|
|
scaled_size = (size + KASAN_GRANULE_SIZE - 1) >>
|
|
KASAN_SHADOW_SCALE_SHIFT;
|
|
shadow_size = round_up(scaled_size, PAGE_SIZE);
|
|
|
|
if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
|
|
return -EINVAL;
|
|
|
|
if (IS_ENABLED(CONFIG_UML)) {
|
|
__memset((void *)shadow_start, KASAN_SHADOW_INIT, shadow_size);
|
|
return 0;
|
|
}
|
|
|
|
ret = __vmalloc_node_range(shadow_size, 1, shadow_start,
|
|
shadow_start + shadow_size,
|
|
GFP_KERNEL,
|
|
PAGE_KERNEL, VM_NO_GUARD, NUMA_NO_NODE,
|
|
__builtin_return_address(0));
|
|
|
|
if (ret) {
|
|
struct vm_struct *vm = find_vm_area(addr);
|
|
__memset(ret, KASAN_SHADOW_INIT, shadow_size);
|
|
vm->flags |= VM_KASAN;
|
|
kmemleak_ignore(ret);
|
|
|
|
if (vm->flags & VM_DEFER_KMEMLEAK)
|
|
kmemleak_vmalloc(vm, size, gfp_mask);
|
|
|
|
return 0;
|
|
}
|
|
|
|
return -ENOMEM;
|
|
}
|
|
|
|
void kasan_free_module_shadow(const struct vm_struct *vm)
|
|
{
|
|
if (IS_ENABLED(CONFIG_UML))
|
|
return;
|
|
|
|
if (vm->flags & VM_KASAN)
|
|
vfree(kasan_mem_to_shadow(vm->addr));
|
|
}
|
|
|
|
#endif
|