Re: [PATCH v3 2/2] kfence: Alloc kfence_pool after system startup

["liupeng (DM)" <liupeng256@huawei.com>]

On 2022/3/7 15:45, Tianchen Ding wrote:
> Allow enabling KFENCE after system startup by allocating its pool via the
> page allocator. This provides the flexibility to enable KFENCE even if it
> wasn't enabled at boot time.
>
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Tested-by: Peng Liu <liupeng256@huawei.com>
> ---
>   mm/kfence/core.c | 111 ++++++++++++++++++++++++++++++++++++++---------
>   1 file changed, 90 insertions(+), 21 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index caa4e84c8b79..f126b53b9b85 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -96,7 +96,7 @@ static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
>   module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
>   
>   /* The pool of pages used for guard pages and objects. */
> -char *__kfence_pool __ro_after_init;
> +char *__kfence_pool __read_mostly;
>   EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
>   
>   /*
> @@ -537,17 +537,19 @@ static void rcu_guarded_free(struct rcu_head *h)
>   	kfence_guarded_free((void *)meta->addr, meta, false);
>   }
>   
> -static bool __init kfence_init_pool(void)
> +/*
> + * Initialization of the KFENCE pool after its allocation.
> + * Returns 0 on success; otherwise returns the address up to
> + * which partial initialization succeeded.
> + */
> +static unsigned long kfence_init_pool(void)
>   {
>   	unsigned long addr = (unsigned long)__kfence_pool;
>   	struct page *pages;
>   	int i;
>   
> -	if (!__kfence_pool)
> -		return false;
> -
>   	if (!arch_kfence_init_pool())
> -		goto err;
> +		return addr;
>   
>   	pages = virt_to_page(addr);
>   
> @@ -565,7 +567,7 @@ static bool __init kfence_init_pool(void)
>   
>   		/* Verify we do not have a compound head page. */
>   		if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
> -			goto err;
> +			return addr;
>   
>   		__SetPageSlab(&pages[i]);
>   	}
> @@ -578,7 +580,7 @@ static bool __init kfence_init_pool(void)
>   	 */
>   	for (i = 0; i < 2; i++) {
>   		if (unlikely(!kfence_protect(addr)))
> -			goto err;
> +			return addr;
>   
>   		addr += PAGE_SIZE;
>   	}
> @@ -595,7 +597,7 @@ static bool __init kfence_init_pool(void)
>   
>   		/* Protect the right redzone. */
>   		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
> -			goto err;
> +			return addr;
>   
>   		addr += 2 * PAGE_SIZE;
>   	}
> @@ -608,9 +610,21 @@ static bool __init kfence_init_pool(void)
>   	 */
>   	kmemleak_free(__kfence_pool);
>   
> -	return true;
> +	return 0;
> +}
> +
> +static bool __init kfence_init_pool_early(void)
> +{
> +	unsigned long addr;
> +
> +	if (!__kfence_pool)
> +		return false;
> +
> +	addr = kfence_init_pool();
> +
> +	if (!addr)
> +		return true;
>   
> -err:
>   	/*
>   	 * Only release unprotected pages, and do not try to go back and change
>   	 * page attributes due to risk of failing to do so as well. If changing
> @@ -623,6 +637,26 @@ static bool __init kfence_init_pool(void)
>   	return false;
>   }
>   
> +static bool kfence_init_pool_late(void)
> +{
> +	unsigned long addr, free_size;
> +
> +	addr = kfence_init_pool();
> +
> +	if (!addr)
> +		return true;
> +
> +	/* 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(addr)), free_size / PAGE_SIZE);
> +#else
> +	free_pages_exact((void *)addr, free_size);
> +#endif
> +	__kfence_pool = NULL;
> +	return false;
> +}
> +
>   /* === DebugFS Interface ==================================================== */
>   
>   static int stats_show(struct seq_file *seq, void *v)
> @@ -771,31 +805,66 @@ void __init kfence_alloc_pool(void)
>   		pr_err("failed to allocate pool\n");
>   }
>   
> +static void kfence_init_enable(void)
> +{
> +	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
> +		static_branch_enable(&kfence_allocation_key);
> +	WRITE_ONCE(kfence_enabled, true);
> +	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
> +	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
> +		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
> +		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
> +}
> +
>   void __init kfence_init(void)
>   {
> +	stack_hash_seed = (u32)random_get_entropy();
> +
>   	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
>   	if (!kfence_sample_interval)
>   		return;
>   
> -	stack_hash_seed = (u32)random_get_entropy();
> -	if (!kfence_init_pool()) {
> +	if (!kfence_init_pool_early()) {
>   		pr_err("%s failed\n", __func__);
>   		return;
>   	}
>   
> -	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
> -		static_branch_enable(&kfence_allocation_key);
> -	WRITE_ONCE(kfence_enabled, true);
> -	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
> -	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
> -		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
> -		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
> +	kfence_init_enable();
> +}
> +
> +static int kfence_init_late(void)
> +{
> +	const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
> +#ifdef CONFIG_CONTIG_ALLOC
> +	struct page *pages;
> +
> +	pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
> +	if (!pages)
> +		return -ENOMEM;
> +	__kfence_pool = page_to_virt(pages);
> +#else
> +	if (nr_pages > 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;
> +#endif
> +
> +	if (!kfence_init_pool_late()) {
> +		pr_err("%s failed\n", __func__);
> +		return -EBUSY;
> +	}
> +
> +	kfence_init_enable();
> +	return 0;
>   }
>   
>   static int kfence_enable_late(void)
>   {
>   	if (!__kfence_pool)
> -		return -EINVAL;
> +		return kfence_init_late();
>   
>   	WRITE_ONCE(kfence_enabled, true);
>   	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);