2022-03-05 18:55:26

by Tianchen Ding

[permalink] [raw]
Subject: [PATCH v2 2/2] kfence: Alloc kfence_pool after system startup

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 <[email protected]>
---
mm/kfence/core.c | 99 ++++++++++++++++++++++++++++++++++++++----------
1 file changed, 78 insertions(+), 21 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index caa4e84c8b79..f46d63dd7676 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 +637,22 @@ 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)
@@ -771,31 +801,58 @@ 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;
+ 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);
+
+ 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


2022-03-07 05:45:02

by Marco Elver

[permalink] [raw]
Subject: Re: [PATCH v2 2/2] kfence: Alloc kfence_pool after system startup

On Sat, 5 Mar 2022 at 15:49, Tianchen Ding <[email protected]> wrote:
[...]
> +static int kfence_init_late(void)
> +{
> + const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
> + struct page *pages;
> +
> + pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);

> mm/kfence/core.c:836:17: error: implicit declaration of function ‘alloc_contig_pages’ [-Werror=implicit-function-declaration]

This doesn't build without CMA. See ifdef CONFIG_CONTIG_ALLOC in
gfp.h, which declares alloc_contig_pages.

Will alloc_pages() work as you expect? If so, perhaps only use
alloc_contig_pages() #ifdef CONFIG_CONTIG_ALLOC.

Thanks,
-- Marco

2022-03-07 09:43:37

by Tianchen Ding

[permalink] [raw]
Subject: Re: [PATCH v2 2/2] kfence: Alloc kfence_pool after system startup

On 2022/3/7 10:23, Tianchen Ding wrote:
> On 2022/3/7 07:52, Marco Elver wrote:
>> On Sat, 5 Mar 2022 at 15:49, Tianchen Ding <[email protected]>
>> wrote:
>> [...]
>>> +static int kfence_init_late(void)
>>> +{
>>> +       const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
>>> +       struct page *pages;
>>> +
>>> +       pages = alloc_contig_pages(nr_pages, GFP_KERNEL,
>>> first_online_node, NULL);
>>
>>> mm/kfence/core.c:836:17: error: implicit declaration of function
>>> ‘alloc_contig_pages’ [-Werror=implicit-function-declaration]
>>
>> This doesn't build without CMA. See ifdef CONFIG_CONTIG_ALLOC in
>> gfp.h, which declares alloc_contig_pages.
>>
>> Will alloc_pages() work as you expect? If so, perhaps only use
>> alloc_contig_pages() #ifdef CONFIG_CONTIG_ALLOC.
>>
>
> alloc_pages() will be fine. We could free "tail" pages after inited.
> Will send v3 soon.
>

Oh, I remember why we use alloc_contig_pages()...
alloc_pages() (or alloc_pages_exact()) only support pages less than
MAX_ORDER (default 11). The alloc would fail when KFENCE_NUM_OBJECTS >= 512.

So the design would be:
ifndef CONFIG_CONTIG_ALLOC and KFENCE_NUM_OBJECTS exceeds MAX_ORDER, we
do not support alloc KFENCE pool after system startup.

>> Thanks,
>> -- Marco
>

2022-03-07 09:52:04

by Tianchen Ding

[permalink] [raw]
Subject: Re: [PATCH v2 2/2] kfence: Alloc kfence_pool after system startup

On 2022/3/7 07:52, Marco Elver wrote:
> On Sat, 5 Mar 2022 at 15:49, Tianchen Ding <[email protected]> wrote:
> [...]
>> +static int kfence_init_late(void)
>> +{
>> + const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
>> + struct page *pages;
>> +
>> + pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
>
>> mm/kfence/core.c:836:17: error: implicit declaration of function ‘alloc_contig_pages’ [-Werror=implicit-function-declaration]
>
> This doesn't build without CMA. See ifdef CONFIG_CONTIG_ALLOC in
> gfp.h, which declares alloc_contig_pages.
>
> Will alloc_pages() work as you expect? If so, perhaps only use
> alloc_contig_pages() #ifdef CONFIG_CONTIG_ALLOC.
>

alloc_pages() will be fine. We could free "tail" pages after inited.
Will send v3 soon.

> Thanks,
> -- Marco