Re: [PATCH v3] mm: kfence: allocate kfence_metadata at runtime

From: Marco Elver
Date: Tue Jul 18 2023 - 08:40:24 EST

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On Tue, 18 Jul 2023 at 09:30, Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> wrote:
>
> kfence_metadata is currently a static array. For the purpose of allocating
> scalable __kfence_pool, we first change it to runtime allocation of
> metadata. Since the size of an object of kfence_metadata is 1160 bytes, we
> can save at least 72 pages (with default 256 objects) without enabling
> kfence.
>
> Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>

This looks good (minor nit below).

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

Thanks!

> ---
> Changes since v2:
> - Fix missing renaming of kfence_alloc_pool.
> - Add __read_mostly for kfence_metadata and kfence_metadata_init.
> - Use smp_store_release() and smp_load_acquire() to access kfence_metadata.
> - Some tweaks to comments and git log.
>
> v1: https://lore.kernel.org/lkml/20230710032714.26200-1-zhangpeng.00@xxxxxxxxxxxxx/
> v2: https://lore.kernel.org/lkml/20230712081616.45177-1-zhangpeng.00@xxxxxxxxxxxxx/
>
> include/linux/kfence.h | 11 ++--
> mm/kfence/core.c | 124 ++++++++++++++++++++++++++++-------------
> mm/kfence/kfence.h | 5 +-
> mm/mm_init.c | 2 +-
> 4 files changed, 97 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/kfence.h b/include/linux/kfence.h
> index 726857a4b680..401af4757514 100644
> --- a/include/linux/kfence.h
> +++ b/include/linux/kfence.h
> @@ -59,15 +59,16 @@ static __always_inline bool is_kfence_address(const void *addr)
> }
>
> /**
> - * kfence_alloc_pool() - allocate the KFENCE pool via memblock
> + * kfence_alloc_pool_and_metadata() - allocate the KFENCE pool and KFENCE
> + * metadata via memblock
> */
> -void __init kfence_alloc_pool(void);
> +void __init kfence_alloc_pool_and_metadata(void);
>
> /**
> * kfence_init() - perform KFENCE initialization at boot time
> *
> - * Requires that kfence_alloc_pool() was called before. This sets up the
> - * allocation gate timer, and requires that workqueues are available.
> + * Requires that kfence_alloc_pool_and_metadata() was called before. This sets
> + * up the allocation gate timer, and requires that workqueues are available.
> */
> void __init kfence_init(void);
>
> @@ -223,7 +224,7 @@ bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *sla
> #else /* CONFIG_KFENCE */
>
> static inline bool is_kfence_address(const void *addr) { return false; }
> -static inline void kfence_alloc_pool(void) { }
> +static inline void kfence_alloc_pool_and_metadata(void) { }
> static inline void kfence_init(void) { }
> static inline void kfence_shutdown_cache(struct kmem_cache *s) { }
> static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index dad3c0eb70a0..6b526435886c 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -116,7 +116,15 @@ EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
> * backing pages (in __kfence_pool).
> */
> static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
> -struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +struct kfence_metadata *kfence_metadata __read_mostly;
> +
> +/*
> + * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
> + * So introduce kfence_metadata_init to initialize metadata, and then make
> + * kfence_metadata visible after initialization is successful. This prevents
> + * potential UAF or access to uninitialized metadata.
> + */
> +static struct kfence_metadata *kfence_metadata_init __read_mostly;
>
> /* Freelist with available objects. */
> static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
> @@ -591,7 +599,7 @@ static unsigned long kfence_init_pool(void)
>
> __folio_set_slab(slab_folio(slab));
> #ifdef CONFIG_MEMCG
> - slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
> + slab->memcg_data = (unsigned long)&kfence_metadata_init[i / 2 - 1].objcg |
> MEMCG_DATA_OBJCGS;
> #endif
> }
> @@ -610,7 +618,7 @@ static unsigned long kfence_init_pool(void)
> }
>
> for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> - struct kfence_metadata *meta = &kfence_metadata[i];
> + struct kfence_metadata *meta = &kfence_metadata_init[i];
>
> /* Initialize metadata. */
> INIT_LIST_HEAD(&meta->list);
> @@ -626,6 +634,12 @@ static unsigned long kfence_init_pool(void)
> addr += 2 * PAGE_SIZE;
> }
>
> + /*
> + * Make kfence_metadata visible only when initialization is successful.
> + * Otherwise, if the initialization fails and kfence_metadata is freed,
> + * it may cause UAF in kfence_shutdown_cache().
> + */
> + smp_store_release(&kfence_metadata, kfence_metadata_init);
> return 0;
>
> reset_slab:
> @@ -672,26 +686,10 @@ static bool __init kfence_init_pool_early(void)
> */
> memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
> __kfence_pool = NULL;
> - return false;
> -}
> -
> -static bool kfence_init_pool_late(void)
> -{
> - unsigned long addr, free_size;
>
> - addr = kfence_init_pool();
> -
> - if (!addr)
> - return true;
> + memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
> + kfence_metadata_init = NULL;
>
> - /* Same as above. */
> - free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
> -#ifdef CONFIG_CONTIG_ALLOC
> - free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE);
> -#else
> - free_pages_exact((void *)addr, free_size);
> -#endif
> - __kfence_pool = NULL;
> return false;
> }
>
> @@ -841,19 +839,30 @@ static void toggle_allocation_gate(struct work_struct *work)
>
> /* === Public interface ===================================================== */
>
> -void __init kfence_alloc_pool(void)
> +void __init kfence_alloc_pool_and_metadata(void)
> {
> if (!kfence_sample_interval)
> return;
>
> - /* if the pool has already been initialized by arch, skip the below. */
> - if (__kfence_pool)
> - return;
> -
> - __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
> -
> + /*
> + * If the pool has already been initialized by arch, there is no need to
> + * re-allocate the memory pool.
> + */
> if (!__kfence_pool)
> + __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
> +
> + if (!__kfence_pool) {
> pr_err("failed to allocate pool\n");
> + return;
> + }
> +
> + /* The memory allocated by memblock has been zeroed out. */
> + kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
> + if (!kfence_metadata_init) {
> + pr_err("failed to allocate metadata\n");
> + memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
> + __kfence_pool = NULL;
> + }
> }
>
> static void kfence_init_enable(void)
> @@ -895,33 +904,68 @@ void __init kfence_init(void)
>
> static int kfence_init_late(void)
> {
> - const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
> + const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
> + const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
> + unsigned long addr = (unsigned long)__kfence_pool;
> + unsigned long free_size = KFENCE_POOL_SIZE;
> + int err = -ENOMEM;
> +
> #ifdef CONFIG_CONTIG_ALLOC
> struct page *pages;
> -

Unnecessary blank line removal (it looks worse now).

> - pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
> + pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
> + NULL);
> if (!pages)
> return -ENOMEM;
> +
> __kfence_pool = page_to_virt(pages);
> + pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
> + NULL);
> + if (pages)
> + kfence_metadata_init = page_to_virt(pages);
> #else
> - if (nr_pages > MAX_ORDER_NR_PAGES) {
> + if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
> + nr_pages_meta > MAX_ORDER_NR_PAGES) {
> pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
> return -EINVAL;
> }
> +
> __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
> if (!__kfence_pool)
> return -ENOMEM;
> +
> + kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
> #endif
>
> - if (!kfence_init_pool_late()) {
> - pr_err("%s failed\n", __func__);
> - return -EBUSY;
> + if (!kfence_metadata_init)
> + goto free_pool;
> +
> + memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
> + addr = kfence_init_pool();
> + if (!addr) {
> + kfence_init_enable();
> + kfence_debugfs_init();
> + return 0;
> }
>
> - kfence_init_enable();
> - kfence_debugfs_init();
> + pr_err("%s failed\n", __func__);
> + free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
> + err = -EBUSY;
>
> - return 0;
> +#ifdef CONFIG_CONTIG_ALLOC
> + free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
> + nr_pages_meta);
> +free_pool:
> + free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
> + free_size / PAGE_SIZE);
> +#else
> + free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
> +free_pool:
> + free_pages_exact((void *)addr, free_size);
> +#endif
> +
> + kfence_metadata_init = NULL;
> + __kfence_pool = NULL;
> + return err;
> }
>
> static int kfence_enable_late(void)
> @@ -941,6 +985,10 @@ void kfence_shutdown_cache(struct kmem_cache *s)
> struct kfence_metadata *meta;
> int i;
>
> + /* Pairs with release in kfence_init_pool(). */
> + if (!smp_load_acquire(&kfence_metadata))
> + return;
> +
> for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> bool in_use;
>
> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
> index 392fb273e7bd..f46fbb03062b 100644
> --- a/mm/kfence/kfence.h
> +++ b/mm/kfence/kfence.h
> @@ -102,7 +102,10 @@ struct kfence_metadata {
> #endif
> };
>
> -extern struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +#define KFENCE_METADATA_SIZE PAGE_ALIGN(sizeof(struct kfence_metadata) * \
> + CONFIG_KFENCE_NUM_OBJECTS)
> +
> +extern struct kfence_metadata *kfence_metadata;
>
> static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
> {
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 7f7f9c677854..3d0a63c75829 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2721,7 +2721,7 @@ void __init mm_core_init(void)
> */
> page_ext_init_flatmem();
> mem_debugging_and_hardening_init();
> - kfence_alloc_pool();
> + kfence_alloc_pool_and_metadata();
> report_meminit();
> kmsan_init_shadow();
> stack_depot_early_init();
> --
> 2.20.1
>

Next message: Vincent Whitchurch: "[PATCH] ubifs: Fix memory leak of bud->log_hash"
Previous message: zelong dong: "[PATCH v2 2/3] reset: reset-meson: add support for the Meson-C3 SoC Reset Controller"
In reply to: Peng Zhang: "[PATCH v3] mm: kfence: allocate kfence_metadata at runtime"
Next in thread: Peng Zhang: "Re: [PATCH v3] mm: kfence: allocate kfence_metadata at runtime"
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