Re: [PATCH v7 1/3] mm: Shuffle initial free memory to improve memory-side-cache utilization

[Dan Williams]

On Mon, Jan 7, 2019 at 4:19 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Mon, Jan 7, 2019 at 3:33 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > Randomization of the page allocator improves the average utilization of
> > a direct-mapped memory-side-cache. Memory side caching is a platform
> > capability that Linux has been previously exposed to in HPC
> > (high-performance computing) environments on specialty platforms. In
> > that instance it was a smaller pool of high-bandwidth-memory relative to
> > higher-capacity / lower-bandwidth DRAM. Now, this capability is going to
> > be found on general purpose server platforms where DRAM is a cache in
> > front of higher latency persistent memory [1].
> >
> > Robert offered an explanation of the state of the art of Linux
> > interactions with memory-side-caches [2], and I copy it here:
> >
> >     It's been a problem in the HPC space:
> >     http://www.nersc.gov/research-and-development/knl-cache-mode-performance-coe/
> >
> >     A kernel module called zonesort is available to try to help:
> >     https://software.intel.com/en-us/articles/xeon-phi-software
> >
> >     and this abandoned patch series proposed that for the kernel:
> >     https://lkml.org/lkml/2017/8/23/195
> >
> >     Dan's patch series doesn't attempt to ensure buffers won't conflict, but
> >     also reduces the chance that the buffers will. This will make performance
> >     more consistent, albeit slower than "optimal" (which is near impossible
> >     to attain in a general-purpose kernel).  That's better than forcing
> >     users to deploy remedies like:
> >         "To eliminate this gradual degradation, we have added a Stream
> >          measurement to the Node Health Check that follows each job;
> >          nodes are rebooted whenever their measured memory bandwidth
> >          falls below 300 GB/s."
> >
> > A replacement for zonesort was merged upstream in commit cc9aec03e58f
> > "x86/numa_emulation: Introduce uniform split capability". With this
> > numa_emulation capability, memory can be split into cache sized
> > ("near-memory" sized) numa nodes. A bind operation to such a node, and
> > disabling workloads on other nodes, enables full cache performance.
> > However, once the workload exceeds the cache size then cache conflicts
> > are unavoidable. While HPC environments might be able to tolerate
> > time-scheduling of cache sized workloads, for general purpose server
> > platforms, the oversubscribed cache case will be the common case.
> >
> > The worst case scenario is that a server system owner benchmarks a
> > workload at boot with an un-contended cache only to see that performance
> > degrade over time, even below the average cache performance due to
> > excessive conflicts. Randomization clips the peaks and fills in the
> > valleys of cache utilization to yield steady average performance.
> >
> > Here are some performance impact details of the patches:
> >
> > 1/ An Intel internal synthetic memory bandwidth measurement tool, saw a
> > 3X speedup in a contrived case that tries to force cache conflicts. The
> > contrived cased used the numa_emulation capability to force an instance
> > of the benchmark to be run in two of the near-memory sized numa nodes.
> > If both instances were placed on the same emulated they would fit and
> > cause zero conflicts.  While on separate emulated nodes without
> > randomization they underutilized the cache and conflicted unnecessarily
> > due to the in-order allocation per node.
> >
> > 2/ A well known Java server application benchmark was run with a heap
> > size that exceeded cache size by 3X. The cache conflict rate was 8% for
> > the first run and degraded to 21% after page allocator aging. With
> > randomization enabled the rate levelled out at 11%.
> >
> > 3/ A MongoDB workload did not observe measurable difference in
> > cache-conflict rates, but the overall throughput dropped by 7% with
> > randomization in one case.
> >
> > 4/ Mel Gorman ran his suite of performance workloads with randomization
> > enabled on platforms without a memory-side-cache and saw a mix of some
> > improvements and some losses [3].
> >
> > While there is potentially significant improvement for applications that
> > depend on low latency access across a wide working-set, the performance
> > may be negligible to negative for other workloads. For this reason the
> > shuffle capability defaults to off unless a direct-mapped
> > memory-side-cache is detected. Even then, the page_alloc.shuffle=0
> > parameter can be specified to disable the randomization on those
> > systems.
> >
> > Outside of memory-side-cache utilization concerns there is potentially
> > security benefit from randomization. Some data exfiltration and
> > return-oriented-programming attacks rely on the ability to infer the
> > location of sensitive data objects. The kernel page allocator,
> > especially early in system boot, has predictable first-in-first out
> > behavior for physical pages. Pages are freed in physical address order
> > when first onlined.
> >
> > Quoting Kees:
> >     "While we already have a base-address randomization
> >      (CONFIG_RANDOMIZE_MEMORY), attacks against the same hardware and
> >      memory layouts would certainly be using the predictability of
> >      allocation ordering (i.e. for attacks where the base address isn't
> >      important: only the relative positions between allocated memory).
> >      This is common in lots of heap-style attacks. They try to gain
> >      control over ordering by spraying allocations, etc.
> >
> >      I'd really like to see this because it gives us something similar
> >      to CONFIG_SLAB_FREELIST_RANDOM but for the page allocator."
> >
> > While SLAB_FREELIST_RANDOM reduces the predictability of some local slab
> > caches it leaves vast bulk of memory to be predictably in order
> > allocated.  However, it should be noted, the concrete security benefits
> > are hard to quantify, and no known CVE is mitigated by this
> > randomization.
> >
> > Introduce shuffle_free_memory(), and its helper shuffle_zone(), to
> > perform a Fisher-Yates shuffle of the page allocator 'free_area' lists
> > when they are initially populated with free memory at boot and at
> > hotplug time. Do this based on either the presence of a
> > page_alloc.shuffle=Y command line parameter, or autodetection of a
> > memory-side-cache (to be added in a follow-on patch).
> >
> > The shuffling is done in terms of CONFIG_SHUFFLE_PAGE_ORDER sized free
> > pages where the default CONFIG_SHUFFLE_PAGE_ORDER is MAX_ORDER-1 i.e.
> > 10, 4MB this trades off randomization granularity for time spent
> > shuffling.  MAX_ORDER-1 was chosen to be minimally invasive to the page
> > allocator while still showing memory-side cache behavior improvements,
> > and the expectation that the security implications of finer granularity
> > randomization is mitigated by CONFIG_SLAB_FREELIST_RANDOM.
> >
> > The performance impact of the shuffling appears to be in the noise
> > compared to other memory initialization work. Also the bulk of the work
> > is done in the background as a part of deferred_init_memmap().
> >
> > This initial randomization can be undone over time so a follow-on patch
> > is introduced to inject entropy on page free decisions. It is reasonable
> > to ask if the page free entropy is sufficient, but it is not enough due
> > to the in-order initial freeing of pages. At the start of that process
> > putting page1 in front or behind page0 still keeps them close together,
> > page2 is still near page1 and has a high chance of being adjacent. As
> > more pages are added ordering diversity improves, but there is still
> > high page locality for the low address pages and this leads to no
> > significant impact to the cache conflict rate.
> >
> > [1]: https://itpeernetwork.intel.com/intel-optane-dc-persistent-memory-operating-modes/
> > [2]: https://lkml.org/lkml/2018/9/22/54
> > [3]: https://lkml.org/lkml/2018/10/12/309
> >
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks.

> With some comments below...
[..]
> > diff --git a/init/Kconfig b/init/Kconfig
> > index d47cb77a220e..db7758476e7a 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1714,6 +1714,42 @@ config SLAB_FREELIST_HARDENED
> >           sacrifies to harden the kernel slab allocator against common
> >           freelist exploit methods.
> >
> > +config SHUFFLE_PAGE_ALLOCATOR
> > +       bool "Page allocator randomization"
> > +       depends on ACPI_NUMA
>
> Why does this need ACPI_NUMA? (e.g. why can't I use this on a non-ACPI
> arm64 system?)

I was thinking this would be expanded for each platform-type that will
implement the auto-detect capability. However, there really is no
direct dependency and if you wanted to just use the command line
switch that should be allowed on any platform.

I'll delete this dependency for v8, but I'll hold off on that posting
awaiting feedback from mm folks.

>
> > +       default SLAB_FREELIST_RANDOM
> > +       help
> > +         Randomization of the page allocator improves the average
> > +         utilization of a direct-mapped memory-side-cache. See section
> > +         5.2.27 Heterogeneous Memory Attribute Table (HMAT) in the ACPI
> > +         6.2a specification for an example of how a platform advertises
> > +         the presence of a memory-side-cache. There are also incidental
> > +         security benefits as it reduces the predictability of page
> > +         allocations to compliment SLAB_FREELIST_RANDOM, but the
> > +         default granularity of shuffling on 4MB (MAX_ORDER) pages is
> > +         selected based on cache utilization benefits.
> > +
> > +         While the randomization improves cache utilization it may
> > +         negatively impact workloads on platforms without a cache. For
> > +         this reason, by default, the randomization is enabled only
> > +         after runtime detection of a direct-mapped memory-side-cache.
> > +         Otherwise, the randomization may be force enabled with the
> > +         'page_alloc.shuffle' kernel command line parameter.
> > +
> > +         Say Y if unsure.
> > +
> > +config SHUFFLE_PAGE_ORDER
> > +       depends on SHUFFLE_PAGE_ALLOCATOR
> > +       int "Page allocator shuffle order"
> > +       range 0 10
> > +       default 10
> > +       help
> > +         Specify the granularity at which shuffling (randomization) is
> > +         performed. By default this is set to MAX_ORDER-1 to minimize
> > +         runtime impact of randomization and with the expectation that
> > +         SLAB_FREELIST_RANDOM mitigates heap attacks on smaller
> > +         object granularities.
> > +
> >  config SLUB_CPU_PARTIAL
> >         default y
> >         depends on SLUB && SMP
> > diff --git a/mm/Makefile b/mm/Makefile
> > index d210cc9d6f80..ac5e5ba78874 100644
> > --- a/mm/Makefile
> > +++ b/mm/Makefile
> > @@ -33,7 +33,7 @@ mmu-$(CONFIG_MMU)     += process_vm_access.o
> >  endif
> >
> >  obj-y                  := filemap.o mempool.o oom_kill.o fadvise.o \
> > -                          maccess.o page_alloc.o page-writeback.o \
> > +                          maccess.o page-writeback.o \
> >                            readahead.o swap.o truncate.o vmscan.o shmem.o \
> >                            util.o mmzone.o vmstat.o backing-dev.o \
> >                            mm_init.o mmu_context.o percpu.o slab_common.o \
> > @@ -41,6 +41,11 @@ obj-y                        := filemap.o mempool.o oom_kill.o fadvise.o \
> >                            interval_tree.o list_lru.o workingset.o \
> >                            debug.o $(mmu-y)
> >
> > +# Give 'page_alloc' its own module-parameter namespace
> > +page-alloc-y := page_alloc.o
> > +page-alloc-$(CONFIG_SHUFFLE_PAGE_ALLOCATOR) += shuffle.o
> > +
> > +obj-y += page-alloc.o
>
> I'll get over it, but having both page-alloc.o and page_alloc.o hurts me. :)

It's a cheeky hack, if it doesn't survive review I won't lose any
sleep over it. I was just tempted by the siren call of the
infrastructure built-up around module_param_call().

>
> >  obj-y += init-mm.o
> >  obj-y += memblock.o
> >
> > diff --git a/mm/memblock.c b/mm/memblock.c
> > index 022d4cbb3618..3602f7a2eab4 100644
> > --- a/mm/memblock.c
> > +++ b/mm/memblock.c
> > @@ -17,6 +17,7 @@
> >  #include <linux/poison.h>
> >  #include <linux/pfn.h>
> >  #include <linux/debugfs.h>
> > +#include <linux/shuffle.h>
> >  #include <linux/kmemleak.h>
> >  #include <linux/seq_file.h>
> >  #include <linux/memblock.h>
> > @@ -1929,9 +1930,16 @@ static unsigned long __init free_low_memory_core_early(void)
> >          *  low ram will be on Node1
> >          */
> >         for_each_free_mem_range(i, NUMA_NO_NODE, MEMBLOCK_NONE, &start, &end,
> > -                               NULL)
> > +                               NULL) {
> > +               pg_data_t *pgdat;
> > +
> >                 count += __free_memory_core(start, end);
> >
> > +               for_each_online_pgdat(pgdat)
> > +                       shuffle_free_memory(pgdat, PHYS_PFN(start),
> > +                                       PHYS_PFN(end));
> > +       }
> > +
> >         return count;
> >  }
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index b9a667d36c55..7caffb9a91ab 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -23,6 +23,7 @@
> >  #include <linux/highmem.h>
> >  #include <linux/vmalloc.h>
> >  #include <linux/ioport.h>
> > +#include <linux/shuffle.h>
> >  #include <linux/delay.h>
> >  #include <linux/migrate.h>
> >  #include <linux/page-isolation.h>
> > @@ -895,6 +896,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
> >         zone->zone_pgdat->node_present_pages += onlined_pages;
> >         pgdat_resize_unlock(zone->zone_pgdat, &flags);
> >
> > +       shuffle_zone(zone, pfn, zone_end_pfn(zone));
> > +
> >         if (onlined_pages) {
> >                 node_states_set_node(nid, &arg);
> >                 if (need_zonelists_rebuild)
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index cde5dac6229a..2adcd6da8a07 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -61,6 +61,7 @@
> >  #include <linux/sched/rt.h>
> >  #include <linux/sched/mm.h>
> >  #include <linux/page_owner.h>
> > +#include <linux/shuffle.h>
> >  #include <linux/kthread.h>
> >  #include <linux/memcontrol.h>
> >  #include <linux/ftrace.h>
> > @@ -1634,6 +1635,8 @@ static int __init deferred_init_memmap(void *data)
> >         }
> >         pgdat_resize_unlock(pgdat, &flags);
> >
> > +       shuffle_zone(zone, first_init_pfn, zone_end_pfn(zone));
> > +
> >         /* Sanity check that the next zone really is unpopulated */
> >         WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
> >
> > diff --git a/mm/shuffle.c b/mm/shuffle.c
> > new file mode 100644
> > index 000000000000..07961ff41a03
> > --- /dev/null
> > +++ b/mm/shuffle.c
> > @@ -0,0 +1,215 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// Copyright(c) 2018 Intel Corporation. All rights reserved.
> > +
> > +#include <linux/mm.h>
> > +#include <linux/init.h>
> > +#include <linux/mmzone.h>
> > +#include <linux/random.h>
> > +#include <linux/shuffle.h>
> > +#include <linux/moduleparam.h>
> > +#include "internal.h"
> > +
> > +DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
> > +static unsigned long shuffle_state;
> > +
> > +/*
> > + * Depending on the architecture, module parameter parsing may run
> > + * before, or after the cache detection. SHUFFLE_FORCE_DISABLE prevents,
> > + * or reverts the enabling of the shuffle implementation. SHUFFLE_ENABLE
> > + * attempts to turn on the implementation, but aborts if it finds
> > + * SHUFFLE_FORCE_DISABLE already set.
> > + */
> > +void page_alloc_shuffle(enum mm_shuffle_ctl ctl)
> > +{
> > +       if (ctl == SHUFFLE_FORCE_DISABLE)
> > +               set_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state);
> > +
> > +       if (test_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state)) {
> > +               if (test_and_clear_bit(SHUFFLE_ENABLE, &shuffle_state))
> > +                       static_branch_disable(&page_alloc_shuffle_key);
> > +       } else if (ctl == SHUFFLE_ENABLE
> > +                       && !test_and_set_bit(SHUFFLE_ENABLE, &shuffle_state))
> > +               static_branch_enable(&page_alloc_shuffle_key);
> > +}
> > +
> > +static bool shuffle_param;
> > +extern int shuffle_show(char *buffer, const struct kernel_param *kp)
> > +{
> > +       return sprintf(buffer, "%c\n", test_bit(SHUFFLE_ENABLE, &shuffle_state)
> > +                       ? 'Y' : 'N');
> > +}
> > +static int shuffle_store(const char *val, const struct kernel_param *kp)
> > +{
> > +       int rc = param_set_bool(val, kp);
> > +
> > +       if (rc < 0)
> > +               return rc;
> > +       if (shuffle_param)
> > +               page_alloc_shuffle(SHUFFLE_ENABLE);
> > +       else
> > +               page_alloc_shuffle(SHUFFLE_FORCE_DISABLE);
> > +       return 0;
> > +}
> > +module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
>
> If this is 0400, you don't intend it to be changed after boot. If it's
> supposed to be immutable, why not make these __init calls?

It's not changeable after boot, but it's still readable after boot.
This is there to allow interrogation of whether shuffling is in-effect
at runtime.

> > + * For two pages to be swapped in the shuffle, they must be free (on a
> > + * 'free_area' lru), have the same order, and have the same migratetype.
> > + */
> > +static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order)
> > +{
> > +       struct page *page;
> > +
> > +       /*
> > +        * Given we're dealing with randomly selected pfns in a zone we
> > +        * need to ask questions like...
> > +        */
> > +
> > +       /* ...is the pfn even in the memmap? */
> > +       if (!pfn_valid_within(pfn))
> > +               return NULL;
> > +
> > +       /* ...is the pfn in a present section or a hole? */
> > +       if (!pfn_present(pfn))
> > +               return NULL;
> > +
> > +       /* ...is the page free and currently on a free_area list? */
> > +       page = pfn_to_page(pfn);
> > +       if (!PageBuddy(page))
> > +               return NULL;
> > +
> > +       /*
> > +        * ...is the page on the same list as the page we will
> > +        * shuffle it with?
> > +        */
> > +       if (page_order(page) != order)
> > +               return NULL;
> > +
> > +       return page;
> > +}
> > +
> > +/*
> > + * Fisher-Yates shuffle the freelist which prescribes iterating through
> > + * an array, pfns in this case, and randomly swapping each entry with
> > + * another in the span, end_pfn - start_pfn.
> > + *
> > + * To keep the implementation simple it does not attempt to correct for
> > + * sources of bias in the distribution, like modulo bias or
> > + * pseudo-random number generator bias. I.e. the expectation is that
> > + * this shuffling raises the bar for attacks that exploit the
> > + * predictability of page allocations, but need not be a perfect
> > + * shuffle.
> > + *
> > + * Note that we don't use @z->zone_start_pfn and zone_end_pfn(@z)
> > + * directly since the caller may be aware of holes in the zone and can
> > + * improve the accuracy of the random pfn selection.
> > + */
> > +#define SHUFFLE_RETRY 10
> > +static void __meminit shuffle_zone_order(struct zone *z, unsigned long start_pfn,
> > +               unsigned long end_pfn, const int order)
> > +{
> > +       unsigned long i, flags;
> > +       const int order_pages = 1 << order;
> > +
> > +       if (start_pfn < z->zone_start_pfn)
> > +               start_pfn = z->zone_start_pfn;
> > +       if (end_pfn > zone_end_pfn(z))
> > +               end_pfn = zone_end_pfn(z);
> > +
> > +       /* probably means that start/end were outside the zone */
> > +       if (end_pfn <= start_pfn)
> > +               return;
> > +       spin_lock_irqsave(&z->lock, flags);
> > +       start_pfn = ALIGN(start_pfn, order_pages);
> > +       for (i = start_pfn; i < end_pfn; i += order_pages) {
> > +               unsigned long j;
> > +               int migratetype, retry;
> > +               struct page *page_i, *page_j;
> > +
> > +               /*
> > +                * We expect page_i, in the sub-range of a zone being
> > +                * added (@start_pfn to @end_pfn), to more likely be
> > +                * valid compared to page_j randomly selected in the
> > +                * span @zone_start_pfn to @spanned_pages.
> > +                */
> > +               page_i = shuffle_valid_page(i, order);
> > +               if (!page_i)
> > +                       continue;
> > +
> > +               for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
> > +                       /*
> > +                        * Pick a random order aligned page from the
> > +                        * start of the zone. Use the *whole* zone here
> > +                        * so that if it is freed in tiny pieces that we
> > +                        * randomize in the whole zone, not just within
> > +                        * those fragments.
> > +                        *
> > +                        * Since page_j comes from a potentially sparse
> > +                        * address range we want to try a bit harder to
> > +                        * find a shuffle point for page_i.
> > +                        */
> > +                       j = z->zone_start_pfn +
> > +                               ALIGN_DOWN(get_random_long() % z->spanned_pages,
> > +                                               order_pages);
>
> How late in the boot process does this happen, btw?

This happens early at mem_init() before the software rng is initialized.

> Do we get warnings
> from the RNG about early usage?

Yes, it would trigger on some platforms. It does not on my test system
because I'm running on an arch_get_random_long() enabled system.