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

From: Marco Elver
Date: Fri Mar 04 2022 - 13:15:06 EST

On Thu, 3 Mar 2022 at 04:15, Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> wrote:
>
> KFENCE aims at production environments, but it does not allow enabling
> after system startup because kfence_pool only alloc pages from memblock.
> Consider the following production scene:
> At first, for performance considerations, production machines do not
> enable KFENCE.
> However, after running for a while, the kernel is suspected to have
> memory errors. (e.g., a sibling machine crashed.)
> So other production machines need to enable KFENCE, but it's hard for
> them to reboot.

I think having this flexibility isn't bad, but your usecase just
doesn't make sense (to us at least, based on our experience).

So I would simply remove the above as it will give folks the wrong
impression. The below paragraph can be improved a little, but should
be enough.

> Allow enabling KFENCE by alloc pages after system startup, even if
> KFENCE is not enabled during booting.

The above doesn't parse very well -- my suggestion:
"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 patch is similar to what the KFENCE(early version) do on ARM64.
> Instead of alloc_pages(), we'd prefer alloc_contig_pages() to get exact
> number of pages.
> I'm not sure about the impact of breaking __ro_after_init. I've tested
> with hackbench, and it seems no performance regression.
> Or any problem about security?

Performance would be the main consideration. However, I think
__read_mostly should be as good as __ro_after_init in terms of
performance.

> ---
> mm/kfence/core.c | 96 ++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 76 insertions(+), 20 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 19eb123c0bba..ae69b2a113a4 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -93,7 +93,7 @@ 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. */
>
> /*
> @@ -534,17 +534,18 @@ static void rcu_guarded_free(struct rcu_head *h)
> kfence_guarded_free((void *)meta->addr, meta, false);
> }
>
> -static bool __init kfence_init_pool(void)
> +/*
> + * The main part of init kfence pool.

"Initialization of the KFENCE pool after its allocation."

> + * Return 0 if succeed. Otherwise return the address where error occurs.

"Return 0 on success; otherwise returns the address up to which
partial initialization succeeded."

> + */
> +static unsigned long __kfence_init_pool(void)

Keep this function simply named 'kfence_init_pool()' - it's a static
function, and we can be more descriptive with the other function
names.

> {
> 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);
>
> @@ -562,7 +563,7 @@ 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]);
> }
> @@ -575,7 +576,7 @@ 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;
> }
> @@ -592,7 +593,7 @@ 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;
> }
> @@ -605,9 +606,21 @@ static bool __init kfence_init_pool(void)
> */
> kmemleak_free(__kfence_pool);
>
> - return true;
> + return 0;
> +}
> +
> +static bool __init kfence_init_pool(void)

Just call this kfence_init_pool_early().

> +{
> + 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
> @@ -620,6 +633,22 @@ static bool __init kfence_init_pool(void)
> return false;
> }
>
> +static bool kfence_init_pool_late(void)
> +{
> + unsigned long addr, free_pages;
> +
> + addr = __kfence_init_pool();
> +
> + if (!addr)
> + return true;
> +
> + /* Same as above. */
> + free_pages = (KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)) / PAGE_SIZE;
> + free_contig_range(page_to_pfn(virt_to_page(addr)), free_pages);
> + __kfence_pool = NULL;
> + return false;
> +}
> +
> /* === DebugFS Interface ==================================================== */
>
> static int stats_show(struct seq_file *seq, void *v)
> @@ -768,31 +797,58 @@ void __init kfence_alloc_pool(void)
> pr_err("failed to allocate pool\n");
> }
>
> +static inline void __kfence_init(void)

Don't make this 'inline', I see no reason for it. If the compiler
thinks it's really worth inlining, it'll do it anyway.

Also, just call it 'kfence_init_enable()' (sprinkling '__' everywhere
really doesn't improve readability if we can avoid it).

> +{
> + 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()) {
> 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();
> +}
> +
> +static int kfence_init_late(void)
> +{
> + struct page *pages;
> + const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;

Order 'nr_pages' above 'pages' (reverse xmas-tree).


> + pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
> +
> + if (!pages)
> + return -ENOMEM;
> +
> + __kfence_pool = page_to_virt(pages);
> +
> + if (!kfence_init_pool_late()) {
> + pr_err("%s failed\n", __func__);
> + return -EBUSY;
> + }
> +
> + __kfence_init();
> + 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
>
> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@xxxxxxxxxxxxxxxx.
> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20220303031505.28495-3-dtcccc%40linux.alibaba.com.