Re: [PATCH v3 4/5] kfence: limit currently covered allocations when pool nearly full

[Alexander Potapenko]

On Thu, Sep 23, 2021 at 1:19 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Thu, 23 Sept 2021 at 12:48, Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > One of KFENCE's main design principles is that with increasing uptime,
> > allocation coverage increases sufficiently to detect previously
> > undetected bugs.
> >
> > We have observed that frequent long-lived allocations of the same
> > source (e.g. pagecache) tend to permanently fill up the KFENCE pool
> > with increasing system uptime, thus breaking the above requirement.
> > The workaround thus far had been increasing the sample interval and/or
> > increasing the KFENCE pool size, but is no reliable solution.
> >
> > To ensure diverse coverage of allocations, limit currently covered
> > allocations of the same source once pool utilization reaches 75%
> > (configurable via `kfence.skip_covered_thresh`) or above. The effect is
> > retaining reasonable allocation coverage when the pool is close to full.
> >
> > A side-effect is that this also limits frequent long-lived allocations
> > of the same source filling up the pool permanently.
> >
> > Uniqueness of an allocation for coverage purposes is based on its
> > (partial) allocation stack trace (the source). A Counting Bloom filter
> > is used to check if an allocation is covered; if the allocation is
> > currently covered, the allocation is skipped by KFENCE.
> >
> > Testing was done using:
> >
> >         (a) a synthetic workload that performs frequent long-lived
> >             allocations (default config values; sample_interval=1;
> >             num_objects=63), and
> >
> >         (b) normal desktop workloads on an otherwise idle machine where
> >             the problem was first reported after a few days of uptime
> >             (default config values).
> >
> > In both test cases the sampled allocation rate no longer drops to zero
> > at any point. In the case of (b) we observe (after 2 days uptime) 15%
> > unique allocations in the pool, 77% pool utilization, with 20% "skipped
> > allocations (covered)".
> >
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
>
> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>

>
> > ---
> > v3:
> > * Remove unneeded !alloc_stack_hash checks.
> > * Remove unneeded meta->alloc_stack_hash=0 in kfence_guarded_free().
> >
> > v2:
> > * Switch to counting bloom filter to guarantee currently covered
> >   allocations being skipped.
> > * Use a module param for skip_covered threshold.
> > * Use kfence pool address as hash entropy.
> > * Use filter_irq_stacks().
> > ---
> >  mm/kfence/core.c   | 103 ++++++++++++++++++++++++++++++++++++++++++++-
> >  mm/kfence/kfence.h |   2 +
> >  2 files changed, 103 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > index db01814f8ff0..58a0f6f1acc5 100644
> > --- a/mm/kfence/core.c
> > +++ b/mm/kfence/core.c
> > @@ -11,11 +11,13 @@
> >  #include <linux/bug.h>
> >  #include <linux/debugfs.h>
> >  #include <linux/irq_work.h>
> > +#include <linux/jhash.h>
> >  #include <linux/kcsan-checks.h>
> >  #include <linux/kfence.h>
> >  #include <linux/kmemleak.h>
> >  #include <linux/list.h>
> >  #include <linux/lockdep.h>
> > +#include <linux/log2.h>
> >  #include <linux/memblock.h>
> >  #include <linux/moduleparam.h>
> >  #include <linux/random.h>
> > @@ -82,6 +84,10 @@ static const struct kernel_param_ops sample_interval_param_ops = {
> >  };
> >  module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
> >
> > +/* Pool usage% threshold when currently covered allocations are skipped. */
> > +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;
> >  EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
> > @@ -105,6 +111,25 @@ DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
> >  /* Gates the allocation, ensuring only one succeeds in a given period. */
> >  atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
> >
> > +/*
> > + * A Counting Bloom filter of allocation coverage: limits currently covered
> > + * allocations of the same source filling up the pool.
> > + *
> > + * Assuming a range of 15%-85% unique allocations in the pool at any point in

Where do these 85% come from?

> > + * time, the below parameters provide a probablity of 0.02-0.33 for false
> > + * positive hits respectively:
> > + *
> > + *     P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
> > + */
> > +#define ALLOC_COVERED_HNUM     2
> > +#define ALLOC_COVERED_SIZE     (1 << (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2))
> > +#define ALLOC_COVERED_HNEXT(h) (1664525 * (h) + 1013904223)

Unless we are planning to change these primes, can you use
next_pseudo_random32() instead?


> > +#define ALLOC_COVERED_MASK     (ALLOC_COVERED_SIZE - 1)
> > +static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
> > +
> > +/* Stack depth used to determine uniqueness of an allocation. */
> > +#define UNIQUE_ALLOC_STACK_DEPTH 8UL
> > +
> >  /* Statistics counters for debugfs. */
> >  enum kfence_counter_id {
> >         KFENCE_COUNTER_ALLOCATED,
> > @@ -114,6 +139,7 @@ enum kfence_counter_id {
> >         KFENCE_COUNTER_BUGS,
> >         KFENCE_COUNTER_SKIP_INCOMPAT,
> >         KFENCE_COUNTER_SKIP_CAPACITY,
> > +       KFENCE_COUNTER_SKIP_COVERED,
> >         KFENCE_COUNTER_COUNT,
> >  };
> >  static atomic_long_t counters[KFENCE_COUNTER_COUNT];
> > @@ -125,11 +151,60 @@ static const char *const counter_names[] = {
> >         [KFENCE_COUNTER_BUGS]           = "total bugs",
> >         [KFENCE_COUNTER_SKIP_INCOMPAT]  = "skipped allocations (incompatible)",
> >         [KFENCE_COUNTER_SKIP_CAPACITY]  = "skipped allocations (capacity)",
> > +       [KFENCE_COUNTER_SKIP_COVERED]   = "skipped allocations (covered)",
> >  };
> >  static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
> >
> >  /* === Internals ============================================================ */
> >
> > +static inline bool should_skip_covered(void)
> > +{
> > +       unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
> > +
> > +       return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
> > +}
> > +
> > +static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
> > +{
> > +       /* Some randomness across reboots / different machines. */
> > +       u32 seed = (u32)((unsigned long)__kfence_pool >> (BITS_PER_LONG - 32));
> > +
> > +       num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
> > +       num_entries = filter_irq_stacks(stack_entries, num_entries);
> > +       return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), seed);
> > +}
> > +
> > +/*
> > + * Adds (or subtracts) count @val for allocation stack trace hash
> > + * @alloc_stack_hash from Counting Bloom filter.
> > + */
> > +static void alloc_covered_add(u32 alloc_stack_hash, int val)
> > +{
> > +       int i;
> > +
> > +       for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
> > +               atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
> > +               alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
> > +       }
> > +}
> > +
> > +/*
> > + * Returns true if the allocation stack trace hash @alloc_stack_hash is
> > + * currently contained (non-zero count) in Counting Bloom filter.
> > + */
> > +static bool alloc_covered_contains(u32 alloc_stack_hash)
> > +{
> > +       int i;
> > +
> > +       for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
> > +               if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
> > +                       return false;
> > +               alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
> > +       }
> > +
> > +       return true;
> > +}
> > +
> >  static bool kfence_protect(unsigned long addr)
> >  {
> >         return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
> > @@ -269,7 +344,8 @@ static __always_inline void for_each_canary(const struct kfence_metadata *meta,
> >  }
> >
> >  static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
> > -                                 unsigned long *stack_entries, size_t num_stack_entries)
> > +                                 unsigned long *stack_entries, size_t num_stack_entries,
> > +                                 u32 alloc_stack_hash)
> >  {
> >         struct kfence_metadata *meta = NULL;
> >         unsigned long flags;
> > @@ -332,6 +408,8 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
> >         /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
> >         WRITE_ONCE(meta->cache, cache);
> >         meta->size = size;
> > +       meta->alloc_stack_hash = alloc_stack_hash;
> > +
> >         for_each_canary(meta, set_canary_byte);
> >
> >         /* Set required struct page fields. */
> > @@ -344,6 +422,8 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
> >
> >         raw_spin_unlock_irqrestore(&meta->lock, flags);
> >
> > +       alloc_covered_add(alloc_stack_hash, 1);
> > +
> >         /* Memory initialization. */
> >
> >         /*
> > @@ -412,6 +492,8 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
> >
> >         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);
> >
> > @@ -752,6 +834,7 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> >  {
> >         unsigned long stack_entries[KFENCE_STACK_DEPTH];
> >         size_t num_stack_entries;
> > +       u32 alloc_stack_hash;
> >
> >         /*
> >          * Perform size check before switching kfence_allocation_gate, so that
> > @@ -799,7 +882,23 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> >
> >         num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
> >
> > -       return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries);
> > +       /*
> > +        * Do expensive check for coverage of allocation in slow-path after
> > +        * allocation_gate has already become non-zero, even though it might
> > +        * mean not making any allocation within a given sample interval.
> > +        *
> > +        * This ensures reasonable allocation coverage when the pool is almost
> > +        * full, including avoiding long-lived allocations of the same source
> > +        * filling up the pool (e.g. pagecache allocations).
> > +        */
> > +       alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
> > +       if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
> > +               atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
> > +               return NULL;
> > +       }
> > +
> > +       return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
> > +                                   alloc_stack_hash);
> >  }
> >
> >  size_t kfence_ksize(const void *addr)
> > diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
> > index c1f23c61e5f9..2a2d5de9d379 100644
> > --- a/mm/kfence/kfence.h
> > +++ b/mm/kfence/kfence.h
> > @@ -87,6 +87,8 @@ struct kfence_metadata {
> >         /* Allocation and free stack information. */
> >         struct kfence_track alloc_track;
> >         struct kfence_track free_track;
> > +       /* For updating alloc_covered on frees. */
> > +       u32 alloc_stack_hash;
> >  };
> >
> >  extern struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> > --
> > 2.33.0.464.g1972c5931b-goog
> >



--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

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