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

From: Marco Elver
Date: Mon Mar 07 2022 - 09:09:04 EST

On Mon, 7 Mar 2022 at 08:45, Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>

This looks good, thanks!

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
Tested-by: Marco Elver <elver@xxxxxxxxxx>


> ---
> 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);
> --
> 2.27.0
>