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![Elliot Berman](/assets/img/avatar_default.png)
If rcu_read_lock_sched tracing is enabled, the tracing subsystem can
perform a jump which needs to be checked by CFI. For example, stm_ftrace
source is enabled as a module and hooks into enabled ftrace events. This
can cause an recursive loop where find_shadow_check_fn ->
rcu_read_lock_sched -> (call to stm_ftrace generates cfi slowpath) ->
find_shadow_check_fn -> rcu_read_lock_sched -> ...
To avoid the recursion, either the ftrace codes needs to be marked with
__no_cfi or CFI should not trace. Use the "_notrace" in CFI to avoid
tracing so that CFI can guard ftrace.
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Cc: stable@vger.kernel.org
Fixes: cf68fffb66
("add support for Clang CFI")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210811155914.19550-1-quic_eberman@quicinc.com
329 lines
8.1 KiB
C
329 lines
8.1 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Clang Control Flow Integrity (CFI) error and slowpath handling.
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*
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* Copyright (C) 2021 Google LLC
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*/
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#include <linux/hardirq.h>
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#include <linux/kallsyms.h>
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#include <linux/module.h>
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#include <linux/mutex.h>
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#include <linux/printk.h>
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#include <linux/ratelimit.h>
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#include <linux/rcupdate.h>
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#include <linux/vmalloc.h>
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#include <asm/cacheflush.h>
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#include <asm/set_memory.h>
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/* Compiler-defined handler names */
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#ifdef CONFIG_CFI_PERMISSIVE
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#define cfi_failure_handler __ubsan_handle_cfi_check_fail
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#else
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#define cfi_failure_handler __ubsan_handle_cfi_check_fail_abort
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#endif
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static inline void handle_cfi_failure(void *ptr)
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{
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if (IS_ENABLED(CONFIG_CFI_PERMISSIVE))
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WARN_RATELIMIT(1, "CFI failure (target: %pS):\n", ptr);
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else
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panic("CFI failure (target: %pS)\n", ptr);
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}
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#ifdef CONFIG_MODULES
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#ifdef CONFIG_CFI_CLANG_SHADOW
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/*
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* Index type. A 16-bit index can address at most (2^16)-2 pages (taking
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* into account SHADOW_INVALID), i.e. ~256M with 4k pages.
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*/
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typedef u16 shadow_t;
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#define SHADOW_INVALID ((shadow_t)~0UL)
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struct cfi_shadow {
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/* Page index for the beginning of the shadow */
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unsigned long base;
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/* An array of __cfi_check locations (as indices to the shadow) */
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shadow_t shadow[1];
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} __packed;
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/*
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* The shadow covers ~128M from the beginning of the module region. If
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* the region is larger, we fall back to __module_address for the rest.
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*/
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#define __SHADOW_RANGE (_UL(SZ_128M) >> PAGE_SHIFT)
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/* The in-memory size of struct cfi_shadow, always at least one page */
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#define __SHADOW_PAGES ((__SHADOW_RANGE * sizeof(shadow_t)) >> PAGE_SHIFT)
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#define SHADOW_PAGES max(1UL, __SHADOW_PAGES)
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#define SHADOW_SIZE (SHADOW_PAGES << PAGE_SHIFT)
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/* The actual size of the shadow array, minus metadata */
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#define SHADOW_ARR_SIZE (SHADOW_SIZE - offsetof(struct cfi_shadow, shadow))
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#define SHADOW_ARR_SLOTS (SHADOW_ARR_SIZE / sizeof(shadow_t))
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static DEFINE_MUTEX(shadow_update_lock);
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static struct cfi_shadow __rcu *cfi_shadow __read_mostly;
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/* Returns the index in the shadow for the given address */
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static inline int ptr_to_shadow(const struct cfi_shadow *s, unsigned long ptr)
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{
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unsigned long index;
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unsigned long page = ptr >> PAGE_SHIFT;
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if (unlikely(page < s->base))
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return -1; /* Outside of module area */
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index = page - s->base;
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if (index >= SHADOW_ARR_SLOTS)
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return -1; /* Cannot be addressed with shadow */
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return (int)index;
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}
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/* Returns the page address for an index in the shadow */
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static inline unsigned long shadow_to_ptr(const struct cfi_shadow *s,
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int index)
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{
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if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
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return 0;
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return (s->base + index) << PAGE_SHIFT;
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}
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/* Returns the __cfi_check function address for the given shadow location */
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static inline unsigned long shadow_to_check_fn(const struct cfi_shadow *s,
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int index)
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{
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if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
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return 0;
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if (unlikely(s->shadow[index] == SHADOW_INVALID))
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return 0;
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/* __cfi_check is always page aligned */
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return (s->base + s->shadow[index]) << PAGE_SHIFT;
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}
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static void prepare_next_shadow(const struct cfi_shadow __rcu *prev,
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struct cfi_shadow *next)
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{
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int i, index, check;
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/* Mark everything invalid */
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memset(next->shadow, 0xFF, SHADOW_ARR_SIZE);
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if (!prev)
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return; /* No previous shadow */
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/* If the base address didn't change, an update is not needed */
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if (prev->base == next->base) {
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memcpy(next->shadow, prev->shadow, SHADOW_ARR_SIZE);
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return;
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}
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/* Convert the previous shadow to the new address range */
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for (i = 0; i < SHADOW_ARR_SLOTS; ++i) {
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if (prev->shadow[i] == SHADOW_INVALID)
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continue;
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index = ptr_to_shadow(next, shadow_to_ptr(prev, i));
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if (index < 0)
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continue;
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check = ptr_to_shadow(next,
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shadow_to_check_fn(prev, prev->shadow[i]));
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if (check < 0)
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continue;
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next->shadow[index] = (shadow_t)check;
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}
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}
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static void add_module_to_shadow(struct cfi_shadow *s, struct module *mod,
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unsigned long min_addr, unsigned long max_addr)
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{
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int check_index;
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unsigned long check = (unsigned long)mod->cfi_check;
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unsigned long ptr;
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if (unlikely(!PAGE_ALIGNED(check))) {
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pr_warn("cfi: not using shadow for module %s\n", mod->name);
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return;
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}
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check_index = ptr_to_shadow(s, check);
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if (check_index < 0)
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return; /* Module not addressable with shadow */
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/* For each page, store the check function index in the shadow */
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for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
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int index = ptr_to_shadow(s, ptr);
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if (index >= 0) {
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/* Each page must only contain one module */
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WARN_ON_ONCE(s->shadow[index] != SHADOW_INVALID);
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s->shadow[index] = (shadow_t)check_index;
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}
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}
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}
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static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod,
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unsigned long min_addr, unsigned long max_addr)
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{
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unsigned long ptr;
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for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
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int index = ptr_to_shadow(s, ptr);
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if (index >= 0)
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s->shadow[index] = SHADOW_INVALID;
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}
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}
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typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *,
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unsigned long min_addr, unsigned long max_addr);
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static void update_shadow(struct module *mod, unsigned long base_addr,
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update_shadow_fn fn)
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{
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struct cfi_shadow *prev;
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struct cfi_shadow *next;
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unsigned long min_addr, max_addr;
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next = vmalloc(SHADOW_SIZE);
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mutex_lock(&shadow_update_lock);
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prev = rcu_dereference_protected(cfi_shadow,
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mutex_is_locked(&shadow_update_lock));
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if (next) {
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next->base = base_addr >> PAGE_SHIFT;
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prepare_next_shadow(prev, next);
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min_addr = (unsigned long)mod->core_layout.base;
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max_addr = min_addr + mod->core_layout.text_size;
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fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);
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set_memory_ro((unsigned long)next, SHADOW_PAGES);
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}
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rcu_assign_pointer(cfi_shadow, next);
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mutex_unlock(&shadow_update_lock);
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synchronize_rcu();
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if (prev) {
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set_memory_rw((unsigned long)prev, SHADOW_PAGES);
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vfree(prev);
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}
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}
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void cfi_module_add(struct module *mod, unsigned long base_addr)
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{
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update_shadow(mod, base_addr, add_module_to_shadow);
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}
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void cfi_module_remove(struct module *mod, unsigned long base_addr)
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{
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update_shadow(mod, base_addr, remove_module_from_shadow);
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}
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static inline cfi_check_fn ptr_to_check_fn(const struct cfi_shadow __rcu *s,
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unsigned long ptr)
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{
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int index;
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if (unlikely(!s))
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return NULL; /* No shadow available */
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index = ptr_to_shadow(s, ptr);
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if (index < 0)
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return NULL; /* Cannot be addressed with shadow */
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return (cfi_check_fn)shadow_to_check_fn(s, index);
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}
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static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
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{
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cfi_check_fn fn;
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rcu_read_lock_sched_notrace();
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fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
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rcu_read_unlock_sched_notrace();
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return fn;
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}
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#else /* !CONFIG_CFI_CLANG_SHADOW */
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static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
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{
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return NULL;
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}
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#endif /* CONFIG_CFI_CLANG_SHADOW */
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static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
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{
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cfi_check_fn fn = NULL;
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struct module *mod;
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rcu_read_lock_sched_notrace();
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mod = __module_address(ptr);
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if (mod)
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fn = mod->cfi_check;
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rcu_read_unlock_sched_notrace();
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return fn;
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}
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static inline cfi_check_fn find_check_fn(unsigned long ptr)
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{
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cfi_check_fn fn = NULL;
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if (is_kernel_text(ptr))
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return __cfi_check;
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/*
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* Indirect call checks can happen when RCU is not watching. Both
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* the shadow and __module_address use RCU, so we need to wake it
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* up if necessary.
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*/
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RCU_NONIDLE({
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if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
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fn = find_shadow_check_fn(ptr);
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if (!fn)
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fn = find_module_check_fn(ptr);
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});
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return fn;
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}
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void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
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{
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cfi_check_fn fn = find_check_fn((unsigned long)ptr);
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if (likely(fn))
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fn(id, ptr, diag);
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else /* Don't allow unchecked modules */
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handle_cfi_failure(ptr);
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}
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EXPORT_SYMBOL(__cfi_slowpath_diag);
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#else /* !CONFIG_MODULES */
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void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
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{
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handle_cfi_failure(ptr); /* No modules */
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}
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EXPORT_SYMBOL(__cfi_slowpath_diag);
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#endif /* CONFIG_MODULES */
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void cfi_failure_handler(void *data, void *ptr, void *vtable)
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{
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handle_cfi_failure(ptr);
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}
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EXPORT_SYMBOL(cfi_failure_handler);
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