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Message-ID: <62d96d1c-cca6-6837-cb66-3fc79990ce40@bytedance.com>
Date:   Tue, 18 Jul 2023 21:38:15 +0800
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Subject: Re: [PATCH v3] mm: kfence: allocate kfence_metadata at runtime
To:     Marco Elver <elver@google.com>, akpm@linux-foundation.org
Cc:     glider@google.com, dvyukov@google.com, kasan-dev@googlegroups.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        muchun.song@linux.dev, Kefeng Wang <wangkefeng.wang@huawei.com>,
        Peng Zhang <zhangpeng.00@bytedance.com>
References: <20230718073019.52513-1-zhangpeng.00@bytedance.com>
 <CANpmjNNUr17dKfBYumm54aqB9J-FaeWOW-az9cpkwMS6sd6+3A@mail.gmail.com>
From:   Peng Zhang <zhangpeng.00@bytedance.com>
In-Reply-To: <CANpmjNNUr17dKfBYumm54aqB9J-FaeWOW-az9cpkwMS6sd6+3A@mail.gmail.com>
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在 2023/7/18 20:39, Marco Elver 写道:
> On Tue, 18 Jul 2023 at 09:30, Peng Zhang <zhangpeng.00@bytedance.com> 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@bytedance.com>
> 
> This looks good (minor nit below).
Andrew, if there is no need to update, can you help to add the
deleted blank line below? Thanks.
> 
> Reviewed-by: Marco Elver <elver@google.com>
Marco, Thank you for your review!
> 
> 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@bytedance.com/
>> v2: https://lore.kernel.org/lkml/20230712081616.45177-1-zhangpeng.00@bytedance.com/
>>
>>   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
>>