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Date: Tue, 28 Mar 2023 21:32:28 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Marco Elver <elver@...gle.com>
Cc: Muchun Song <songmuchun@...edance.com>, glider@...gle.com,
dvyukov@...gle.com, akpm@...ux-foundation.org, jannh@...gle.com,
sjpark@...zon.de, kasan-dev@...glegroups.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] mm: kfence: change kfence pool page layout
> On Mar 28, 2023, at 20:59, Marco Elver <elver@...gle.com> wrote:
>
> On Tue, 28 Mar 2023 at 11:58, 'Muchun Song' via kasan-dev
> <kasan-dev@...glegroups.com> wrote:
>>
>> The original kfence pool layout (Given a layout with 2 objects):
>>
>> +------------+------------+------------+------------+------------+------------+
>> | guard page | guard page | object | guard page | object | guard page |
>> +------------+------------+------------+------------+------------+------------+
>> | | |
>> +----kfence_metadata[0]---+----kfence_metadata[1]---+
>>
>> The comment says "the additional page in the beginning gives us an even
>> number of pages, which simplifies the mapping of address to metadata index".
>>
>> However, removing the additional page does not complicate any mapping
>> calculations. So changing it to the new layout to save a page. And remmove
>> the KFENCE_ERROR_INVALID test since we cannot test this case easily.
>>
>> The new kfence pool layout (Given a layout with 2 objects):
>>
>> +------------+------------+------------+------------+------------+
>> | guard page | object | guard page | object | guard page |
>> +------------+------------+------------+------------+------------+
>> | | |
>> +----kfence_metadata[0]---+----kfence_metadata[1]---+
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> ---
>> include/linux/kfence.h | 8 ++------
>> mm/kfence/core.c | 40 ++++++++--------------------------------
>> mm/kfence/kfence.h | 2 +-
>> mm/kfence/kfence_test.c | 14 --------------
>> 4 files changed, 11 insertions(+), 53 deletions(-)
>>
>> diff --git a/include/linux/kfence.h b/include/linux/kfence.h
>> index 726857a4b680..25b13a892717 100644
>> --- a/include/linux/kfence.h
>> +++ b/include/linux/kfence.h
>> @@ -19,12 +19,8 @@
>>
>> extern unsigned long kfence_sample_interval;
>>
>> -/*
>> - * We allocate an even number of pages, as it simplifies calculations to map
>> - * address to metadata indices; effectively, the very first page serves as an
>> - * extended guard page, but otherwise has no special purpose.
>> - */
>> -#define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
>> +/* The last page serves as an extended guard page. */
>
> The last page is just a normal guard page? I.e. the last 2 pages are:
> <object page> | <guard page>
Right.
The new kfence pool layout (Given a layout with 2 objects):
+------------+------------+------------+------------+------------+
| guard page | object | guard page | object | guard page |
+------------+------------+------------+------------+------------+
| | | ^
+----kfence_metadata[0]---+----kfence_metadata[1]---+ |
|
|
the last page
>
> Or did I misunderstand?
>
>> +#define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS * 2 + 1) * PAGE_SIZE)
>> extern char *__kfence_pool;
>>
>> DECLARE_STATIC_KEY_FALSE(kfence_allocation_key);
>> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
>> index 41befcb3b069..f205b860f460 100644
>> --- a/mm/kfence/core.c
>> +++ b/mm/kfence/core.c
>> @@ -240,24 +240,7 @@ static inline void kfence_unprotect(unsigned long addr)
>>
>> static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
>> {
>> - unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
>> - unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
>> -
>> - /* The checks do not affect performance; only called from slow-paths. */
>> -
>> - /* Only call with a pointer into kfence_metadata. */
>> - if (KFENCE_WARN_ON(meta < kfence_metadata ||
>> - meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
>> - return 0;
>
> Could we retain this WARN_ON? Or just get rid of
> metadata_to_pageaddr() altogether, because there's only 1 use left and
> the function would now just be a simple ALIGN_DOWN() anyway.
I'll inline this function to its caller since the warning is unlikely.
>
>> - /*
>> - * This metadata object only ever maps to 1 page; verify that the stored
>> - * address is in the expected range.
>> - */
>> - if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
>> - return 0;
>> -
>> - return pageaddr;
>> + return ALIGN_DOWN(meta->addr, PAGE_SIZE);
>> }
>>
>> /*
>> @@ -535,34 +518,27 @@ static void kfence_init_pool(void)
>> unsigned long addr = (unsigned long)__kfence_pool;
>> int i;
>>
>> - /*
>> - * Protect the first 2 pages. The first page is mostly unnecessary, and
>> - * merely serves as an extended guard page. However, adding one
>> - * additional page in the beginning gives us an even number of pages,
>> - * which simplifies the mapping of address to metadata index.
>> - */
>> - for (i = 0; i < 2; i++, addr += PAGE_SIZE)
>> - kfence_protect(addr);
>> -
>> for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++, addr += 2 * PAGE_SIZE) {
>> struct kfence_metadata *meta = &kfence_metadata[i];
>> - struct slab *slab = page_slab(virt_to_page(addr));
>> + struct slab *slab = page_slab(virt_to_page(addr + PAGE_SIZE));
>>
>> /* Initialize metadata. */
>> INIT_LIST_HEAD(&meta->list);
>> raw_spin_lock_init(&meta->lock);
>> meta->state = KFENCE_OBJECT_UNUSED;
>> - meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
>> + meta->addr = addr + PAGE_SIZE;
>> list_add_tail(&meta->list, &kfence_freelist);
>>
>> - /* Protect the right redzone. */
>> - kfence_protect(addr + PAGE_SIZE);
>> + /* Protect the left redzone. */
>> + kfence_protect(addr);
>>
>> __folio_set_slab(slab_folio(slab));
>> #ifdef CONFIG_MEMCG
>> slab->memcg_data = (unsigned long)&meta->objcg | MEMCG_DATA_OBJCGS;
>> #endif
>> }
>> +
>> + kfence_protect(addr);
>> }
>>
>> static bool __init kfence_init_pool_early(void)
>> @@ -1043,7 +1019,7 @@ bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs
>>
>> atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
>>
>> - if (page_index % 2) {
>> + if (page_index % 2 == 0) {
>> /* This is a redzone, report a buffer overflow. */
>> struct kfence_metadata *meta;
>> int distance = 0;
>> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
>> index 600f2e2431d6..249d420100a7 100644
>> --- a/mm/kfence/kfence.h
>> +++ b/mm/kfence/kfence.h
>> @@ -110,7 +110,7 @@ static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
>> * __kfence_pool, in which case we would report an "invalid access"
>> * error.
>> */
>> - index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
>> + index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2);
>> if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
>> return NULL;
>
> Assume there is a right OOB that hit the last guard page. In this case
>
> addr >= __kfence_pool + (NUM_OBJECTS * 2 * PAGE_SIZE) && addr <
> __kfence_pool + POOL_SIZE
>
> therefore
>
> index >= (NUM_OBJECTS * 2 * PAGE_SIZE) / (PAGE_SIZE * 2) && index <
> POOL_SIZE / (PAGE_SIZE * 2)
> index == NUM_OBJECTS
>
> And according to the above comparison, this will return NULL and
> report KFENCE_ERROR_INVALID, which is wrong.
Look at kfence_handle_page_fault(), which first look up "addr - PAGE_SIZE" (passed
to addr_to_metadata()) and then look up "addr + PAGE_SIZE", the former will not
return NULL, the latter will return NULL. So kfence will report KFENCE_ERROR_OOB
in this case, right? Or what I missed here?
>
>> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
>> index b5d66a69200d..d479f9c8afb1 100644
>> --- a/mm/kfence/kfence_test.c
>> +++ b/mm/kfence/kfence_test.c
>> @@ -637,19 +637,6 @@ static void test_gfpzero(struct kunit *test)
>> KUNIT_EXPECT_FALSE(test, report_available());
>> }
>>
>> -static void test_invalid_access(struct kunit *test)
>> -{
>> - const struct expect_report expect = {
>> - .type = KFENCE_ERROR_INVALID,
>> - .fn = test_invalid_access,
>> - .addr = &__kfence_pool[10],
>> - .is_write = false,
>> - };
>> -
>> - READ_ONCE(__kfence_pool[10]);
>> - KUNIT_EXPECT_TRUE(test, report_matches(&expect));
>> -}
>> -
>> /* Test SLAB_TYPESAFE_BY_RCU works. */
>> static void test_memcache_typesafe_by_rcu(struct kunit *test)
>> {
>> @@ -787,7 +774,6 @@ static struct kunit_case kfence_test_cases[] = {
>> KUNIT_CASE(test_kmalloc_aligned_oob_write),
>> KUNIT_CASE(test_shrink_memcache),
>> KUNIT_CASE(test_memcache_ctor),
>> - KUNIT_CASE(test_invalid_access),
>
> The test can be retained by doing an access to a guard page in between
> 2 unallocated objects. But it's probably not that easy to reliably set
> that up (could try to allocate 2 objects and see if they're next to
> each other, then free them).
Yes, it's not easy to trigger it 100%. So I removed the test.
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