Re: [PATCH] kfence: shorten critical sections of alloc/free

[Alexander Potapenko]

On Thu, Sep 30, 2021 at 5:37 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Initializing memory and setting/checking the canary bytes is relatively
> expensive, and doing so in the meta->lock critical sections extends the
> duration with preemption and interrupts disabled unnecessarily.
>
> Any reads to meta->addr and meta->size in kfence_guarded_alloc() and
> kfence_guarded_free() don't require locking meta->lock as long as the
> object is removed from the freelist: only kfence_guarded_alloc() sets
> meta->addr and meta->size after removing it from the freelist,  which
> requires a preceding kfence_guarded_free() returning it to the list or
> the initial state.
>
> Therefore move reads to meta->addr and meta->size, including expensive
> memory initialization using them, out of meta->lock critical sections.
>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>

> ---
>  mm/kfence/core.c | 38 +++++++++++++++++++++-----------------
>  1 file changed, 21 insertions(+), 17 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index b61ef93d9f98..802905b1c89b 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -309,12 +309,19 @@ static inline bool set_canary_byte(u8 *addr)
>  /* Check canary byte at @addr. */
>  static inline bool check_canary_byte(u8 *addr)
>  {
> +       struct kfence_metadata *meta;
> +       unsigned long flags;
> +
>         if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
>                 return true;
>
>         atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> -       kfence_report_error((unsigned long)addr, false, NULL, addr_to_metadata((unsigned long)addr),
> -                           KFENCE_ERROR_CORRUPTION);
> +
> +       meta = addr_to_metadata((unsigned long)addr);
> +       raw_spin_lock_irqsave(&meta->lock, flags);
> +       kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
>         return false;
>  }
>
> @@ -324,8 +331,6 @@ static __always_inline void for_each_canary(const struct kfence_metadata *meta,
>         const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
>         unsigned long addr;
>
> -       lockdep_assert_held(&meta->lock);
> -
>         /*
>          * We'll iterate over each canary byte per-side until fn() returns
>          * false. However, we'll still iterate over the canary bytes to the
> @@ -414,8 +419,9 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
>         WRITE_ONCE(meta->cache, cache);
>         meta->size = size;
>         meta->alloc_stack_hash = alloc_stack_hash;
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
>
> -       for_each_canary(meta, set_canary_byte);
> +       alloc_covered_add(alloc_stack_hash, 1);
>
>         /* Set required struct page fields. */
>         page = virt_to_page(meta->addr);
> @@ -425,11 +431,8 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
>         if (IS_ENABLED(CONFIG_SLAB))
>                 page->s_mem = addr;
>
> -       raw_spin_unlock_irqrestore(&meta->lock, flags);
> -
> -       alloc_covered_add(alloc_stack_hash, 1);
> -
>         /* Memory initialization. */
> +       for_each_canary(meta, set_canary_byte);
>
>         /*
>          * We check slab_want_init_on_alloc() ourselves, rather than letting
> @@ -454,6 +457,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>  {
>         struct kcsan_scoped_access assert_page_exclusive;
>         unsigned long flags;
> +       bool init;
>
>         raw_spin_lock_irqsave(&meta->lock, flags);
>
> @@ -481,6 +485,13 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>                 meta->unprotected_page = 0;
>         }
>
> +       /* Mark the object as freed. */
> +       metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
> +       init = slab_want_init_on_free(meta->cache);
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> +       alloc_covered_add(meta->alloc_stack_hash, -1);
> +
>         /* Check canary bytes for memory corruption. */
>         for_each_canary(meta, check_canary_byte);
>
> @@ -489,16 +500,9 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>          * data is still there, and after a use-after-free is detected, we
>          * unprotect the page, so the data is still accessible.
>          */
> -       if (!zombie && unlikely(slab_want_init_on_free(meta->cache)))
> +       if (!zombie && unlikely(init))
>                 memzero_explicit(addr, meta->size);
>
> -       /* Mark the object as freed. */
> -       metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
> -
> -       raw_spin_unlock_irqrestore(&meta->lock, flags);
> -
> -       alloc_covered_add(meta->alloc_stack_hash, -1);
> -
>         /* Protect to detect use-after-frees. */
>         kfence_protect((unsigned long)addr);
>
> --
> 2.33.0.685.g46640cef36-goog
>


-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
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