[RFC PATCH v2 04/21] cfi: Remove CONFIG_CFI_CLANG_SHADOW

Sami Tolvanen
2022-05-13T20:22:11Z

In preparation to switching to -fsanitize=kcfi, remove support for the
CFI module shadow that will no longer be needed.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/Kconfig | 10 --
include/linux/cfi.h | 12 ---
kernel/cfi.c | 237 +-------------------------------------------
kernel/module.c | 15 ---
4 files changed, 1 insertion(+), 273 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 31c4fdc4a4ba..625db6376726 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -739,16 +739,6 @@ config CFI_CLANG

https://clang.llvm.org/docs/ControlFlowIntegrity.html

-config CFI_CLANG_SHADOW
- bool "Use CFI shadow to speed up cross-module checks"
- default y
- depends on CFI_CLANG && MODULES
- help
- If you select this option, the kernel builds a fast look-up table of
- CFI check functions in loaded modules to reduce performance overhead.
-
- If unsure, say Y.
-
config CFI_PERMISSIVE
bool "Use CFI in permissive mode"
depends on CFI_CLANG
diff --git a/include/linux/cfi.h b/include/linux/cfi.h
index c6dfc1ed0626..4ab51c067007 100644
--- a/include/linux/cfi.h
+++ b/include/linux/cfi.h
@@ -20,18 +20,6 @@ extern void __cfi_check(uint64_t id, void *ptr, void *diag);
#define __CFI_ADDRESSABLE(fn, __attr) \
const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn

-#ifdef CONFIG_CFI_CLANG_SHADOW
-
-extern void cfi_module_add(struct module *mod, unsigned long base_addr);
-extern void cfi_module_remove(struct module *mod, unsigned long base_addr);
-
-#else
-
-static inline void cfi_module_add(struct module *mod, unsigned long base_addr) {}
-static inline void cfi_module_remove(struct module *mod, unsigned long base_addr) {}
-
-#endif /* CONFIG_CFI_CLANG_SHADOW */
-
#else /* !CONFIG_CFI_CLANG */

#ifdef CONFIG_X86_KERNEL_IBT
diff --git a/kernel/cfi.c b/kernel/cfi.c
index 9594cfd1cf2c..2cc0d01ea980 100644
--- a/kernel/cfi.c
+++ b/kernel/cfi.c
@@ -32,237 +32,6 @@ static inline void handle_cfi_failure(void *ptr)
}

#ifdef CONFIG_MODULES
-#ifdef CONFIG_CFI_CLANG_SHADOW
-/*
- * Index type. A 16-bit index can address at most (2^16)-2 pages (taking
- * into account SHADOW_INVALID), i.e. ~256M with 4k pages.
- */
-typedef u16 shadow_t;
-#define SHADOW_INVALID ((shadow_t)~0UL)
-
-struct cfi_shadow {
- /* Page index for the beginning of the shadow */
- unsigned long base;
- /* An array of __cfi_check locations (as indices to the shadow) */
- shadow_t shadow[1];
-} __packed;
-
-/*
- * The shadow covers ~128M from the beginning of the module region. If
- * the region is larger, we fall back to __module_address for the rest.
- */
-#define __SHADOW_RANGE (_UL(SZ_128M) >> PAGE_SHIFT)
-
-/* The in-memory size of struct cfi_shadow, always at least one page */
-#define __SHADOW_PAGES ((__SHADOW_RANGE * sizeof(shadow_t)) >> PAGE_SHIFT)
-#define SHADOW_PAGES max(1UL, __SHADOW_PAGES)
-#define SHADOW_SIZE (SHADOW_PAGES << PAGE_SHIFT)
-
-/* The actual size of the shadow array, minus metadata */
-#define SHADOW_ARR_SIZE (SHADOW_SIZE - offsetof(struct cfi_shadow, shadow))
-#define SHADOW_ARR_SLOTS (SHADOW_ARR_SIZE / sizeof(shadow_t))
-
-static DEFINE_MUTEX(shadow_update_lock);
-static struct cfi_shadow __rcu *cfi_shadow __read_mostly;
-
-/* Returns the index in the shadow for the given address */
-static inline int ptr_to_shadow(const struct cfi_shadow *s, unsigned long ptr)
-{
- unsigned long index;
- unsigned long page = ptr >> PAGE_SHIFT;
-
- if (unlikely(page < s->base))
- return -1; /* Outside of module area */
-
- index = page - s->base;
-
- if (index >= SHADOW_ARR_SLOTS)
- return -1; /* Cannot be addressed with shadow */
-
- return (int)index;
-}
-
-/* Returns the page address for an index in the shadow */
-static inline unsigned long shadow_to_ptr(const struct cfi_shadow *s,
- int index)
-{
- if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
- return 0;
-
- return (s->base + index) << PAGE_SHIFT;
-}
-
-/* Returns the __cfi_check function address for the given shadow location */
-static inline unsigned long shadow_to_check_fn(const struct cfi_shadow *s,
- int index)
-{
- if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
- return 0;
-
- if (unlikely(s->shadow[index] == SHADOW_INVALID))
- return 0;
-
- /* __cfi_check is always page aligned */
- return (s->base + s->shadow[index]) << PAGE_SHIFT;
-}
-
-static void prepare_next_shadow(const struct cfi_shadow __rcu *prev,
- struct cfi_shadow *next)
-{
- int i, index, check;
-
- /* Mark everything invalid */
- memset(next->shadow, 0xFF, SHADOW_ARR_SIZE);
-
- if (!prev)
- return; /* No previous shadow */
-
- /* If the base address didn't change, an update is not needed */
- if (prev->base == next->base) {
- memcpy(next->shadow, prev->shadow, SHADOW_ARR_SIZE);
- return;
- }
-
- /* Convert the previous shadow to the new address range */
- for (i = 0; i < SHADOW_ARR_SLOTS; ++i) {
- if (prev->shadow[i] == SHADOW_INVALID)
- continue;
-
- index = ptr_to_shadow(next, shadow_to_ptr(prev, i));
- if (index < 0)
- continue;
-
- check = ptr_to_shadow(next,
- shadow_to_check_fn(prev, prev->shadow[i]));
- if (check < 0)
- continue;
-
- next->shadow[index] = (shadow_t)check;
- }
-}
-
-static void add_module_to_shadow(struct cfi_shadow *s, struct module *mod,
- unsigned long min_addr, unsigned long max_addr)
-{
- int check_index;
- unsigned long check = (unsigned long)mod->cfi_check;
- unsigned long ptr;
-
- if (unlikely(!PAGE_ALIGNED(check))) {
- pr_warn("cfi: not using shadow for module %s\n", mod->name);
- return;
- }
-
- check_index = ptr_to_shadow(s, check);
- if (check_index < 0)
- return; /* Module not addressable with shadow */
-
- /* For each page, store the check function index in the shadow */
- for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
- int index = ptr_to_shadow(s, ptr);
-
- if (index >= 0) {
- /* Each page must only contain one module */
- WARN_ON_ONCE(s->shadow[index] != SHADOW_INVALID);
- s->shadow[index] = (shadow_t)check_index;
- }
- }
-}
-
-static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod,
- unsigned long min_addr, unsigned long max_addr)
-{
- unsigned long ptr;
-
- for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
- int index = ptr_to_shadow(s, ptr);
-
- if (index >= 0)
- s->shadow[index] = SHADOW_INVALID;
- }
-}
-
-typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *,
- unsigned long min_addr, unsigned long max_addr);
-
-static void update_shadow(struct module *mod, unsigned long base_addr,
- update_shadow_fn fn)
-{
- struct cfi_shadow *prev;
- struct cfi_shadow *next;
- unsigned long min_addr, max_addr;
-
- next = vmalloc(SHADOW_SIZE);
-
- mutex_lock(&shadow_update_lock);
- prev = rcu_dereference_protected(cfi_shadow,
- mutex_is_locked(&shadow_update_lock));
-
- if (next) {
- next->base = base_addr >> PAGE_SHIFT;
- prepare_next_shadow(prev, next);
-
- min_addr = (unsigned long)mod->core_layout.base;
- max_addr = min_addr + mod->core_layout.text_size;
- fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);
-
- set_memory_ro((unsigned long)next, SHADOW_PAGES);
- }
-
- rcu_assign_pointer(cfi_shadow, next);
- mutex_unlock(&shadow_update_lock);
- synchronize_rcu();
-
- if (prev) {
- set_memory_rw((unsigned long)prev, SHADOW_PAGES);
- vfree(prev);
- }
-}
-
-void cfi_module_add(struct module *mod, unsigned long base_addr)
-{
- update_shadow(mod, base_addr, add_module_to_shadow);
-}
-
-void cfi_module_remove(struct module *mod, unsigned long base_addr)
-{
- update_shadow(mod, base_addr, remove_module_from_shadow);
-}
-
-static inline cfi_check_fn ptr_to_check_fn(const struct cfi_shadow __rcu *s,
- unsigned long ptr)
-{
- int index;
-
- if (unlikely(!s))
- return NULL; /* No shadow available */
-
- index = ptr_to_shadow(s, ptr);
- if (index < 0)
- return NULL; /* Cannot be addressed with shadow */
-
- return (cfi_check_fn)shadow_to_check_fn(s, index);
-}
-
-static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
-{
- cfi_check_fn fn;
-
- rcu_read_lock_sched_notrace();
- fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
- rcu_read_unlock_sched_notrace();
-
- return fn;
-}
-
-#else /* !CONFIG_CFI_CLANG_SHADOW */
-
-static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
-{
- return NULL;
-}
-
-#endif /* CONFIG_CFI_CLANG_SHADOW */

static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
{
@@ -291,11 +60,7 @@ static inline cfi_check_fn find_check_fn(unsigned long ptr)
* up if necessary.
*/
RCU_NONIDLE({
- if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
- fn = find_shadow_check_fn(ptr);
-
- if (!fn)
- fn = find_module_check_fn(ptr);
+ fn = find_module_check_fn(ptr);
});

return fn;
diff --git a/kernel/module.c b/kernel/module.c
index 6cea788fd965..296fe02323e9 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2151,8 +2151,6 @@ void __weak module_arch_freeing_init(struct module *mod)
{
}

-static void cfi_cleanup(struct module *mod);
-
/* Free a module, remove from lists, etc. */
static void free_module(struct module *mod)
{
@@ -2194,9 +2192,6 @@ static void free_module(struct module *mod)
synchronize_rcu();
mutex_unlock(&module_mutex);

- /* Clean up CFI for the module. */
- cfi_cleanup(mod);
-
/* This may be empty, but that's OK */
module_arch_freeing_init(mod);
module_memfree(mod->init_layout.base);
@@ -4141,7 +4136,6 @@ static int load_module(struct load_info *info, const char __user *uargs,
synchronize_rcu();
kfree(mod->args);
free_arch_cleanup:
- cfi_cleanup(mod);
module_arch_cleanup(mod);
free_modinfo:
free_modinfo(mod);
@@ -4530,15 +4524,6 @@ static void cfi_init(struct module *mod)
if (exit)
mod->exit = *exit;
#endif
-
- cfi_module_add(mod, module_addr_min);
-#endif
-}
-
-static void cfi_cleanup(struct module *mod)
-{
-#ifdef CONFIG_CFI_CLANG
- cfi_module_remove(mod, module_addr_min);
#endif
}

--
2.36.0.550.gb090851708-goog