Message-Id: <20191031093909.9228-1-dja@axtens.net>
Date:   Thu, 31 Oct 2019 20:39:05 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     kasan-dev@...glegroups.com, linux-mm@...ck.org, x86@...nel.org,
        aryabinin@...tuozzo.com, glider@...gle.com, luto@...nel.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com,
        dvyukov@...gle.com, christophe.leroy@....fr
Cc:     linuxppc-dev@...ts.ozlabs.org, gor@...ux.ibm.com,
        Daniel Axtens <dja@...ens.net>
Subject: [PATCH v11 0/4] kasan: support backing vmalloc space with real shadow memory

Currently, vmalloc space is backed by the early shadow page. This
means that kasan is incompatible with VMAP_STACK.

This series provides a mechanism to back vmalloc space with real,
dynamically allocated memory. I have only wired up x86, because that's
the only currently supported arch I can work with easily, but it's
very easy to wire up other architectures, and it appears that there is
some work-in-progress code to do this on arm64 and s390.

This has been discussed before in the context of VMAP_STACK:
 - https://bugzilla.kernel.org/show_bug.cgi?id=202009
 - https://lkml.org/lkml/2018/7/22/198
 - https://lkml.org/lkml/2019/7/19/822

In terms of implementation details:

Most mappings in vmalloc space are small, requiring less than a full
page of shadow space. Allocating a full shadow page per mapping would
therefore be wasteful. Furthermore, to ensure that different mappings
use different shadow pages, mappings would have to be aligned to
KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.

Instead, share backing space across multiple mappings. Allocate a
backing page when a mapping in vmalloc space uses a particular page of
the shadow region. This page can be shared by other vmalloc mappings
later on.

We hook in to the vmap infrastructure to lazily clean up unused shadow
memory.

Testing with test_vmalloc.sh on an x86 VM with 2 vCPUs shows that:

 - Turning on KASAN, inline instrumentation, without vmalloc, introuduces
   a 4.1x-4.2x slowdown in vmalloc operations.

 - Turning this on introduces the following slowdowns over KASAN:
     * ~1.76x slower single-threaded (test_vmalloc.sh performance)
     * ~2.18x slower when both cpus are performing operations
       simultaneously (test_vmalloc.sh sequential_test_order=1)

This is unfortunate but given that this is a debug feature only, not
the end of the world. The benchmarks are also a stress-test for the
vmalloc subsystem: they're not indicative of an overall 2x slowdown!


v1: https://lore.kernel.org/linux-mm/20190725055503.19507-1-dja@axtens.net/
v2: https://lore.kernel.org/linux-mm/20190729142108.23343-1-dja@axtens.net/
 Address review comments:
 - Patch 1: use kasan_unpoison_shadow's built-in handling of
            ranges that do not align to a full shadow byte
 - Patch 3: prepopulate pgds rather than faulting things in
v3: https://lore.kernel.org/linux-mm/20190731071550.31814-1-dja@axtens.net/
 Address comments from Mark Rutland:
 - kasan_populate_vmalloc is a better name
 - handle concurrency correctly
 - various nits and cleanups
 - relax module alignment in KASAN_VMALLOC case
v4: https://lore.kernel.org/linux-mm/20190815001636.12235-1-dja@axtens.net/
 Changes to patch 1 only:
 - Integrate Mark's rework, thanks Mark!
 - handle the case where kasan_populate_shadow might fail
 - poision shadow on free, allowing the alloc path to just
     unpoision memory that it uses
v5: https://lore.kernel.org/linux-mm/20190830003821.10737-1-dja@axtens.net/
 Address comments from Christophe Leroy:
 - Fix some issues with my descriptions in commit messages and docs
 - Dynamically free unused shadow pages by hooking into the vmap book-keeping
 - Split out the test into a separate patch
 - Optional patch to track the number of pages allocated
 - minor checkpatch cleanups
v6: https://lore.kernel.org/linux-mm/20190902112028.23773-1-dja@axtens.net/
 Properly guard freeing pages in patch 1, drop debugging code.
v7: https://lore.kernel.org/linux-mm/20190903145536.3390-1-dja@axtens.net/
    Add a TLB flush on freeing, thanks Mark Rutland.
    Explain more clearly how I think freeing is concurrency-safe.
v8: https://lore.kernel.org/linux-mm/20191001065834.8880-1-dja@axtens.net/
    rename kasan_vmalloc/shadow_pages to kasan/vmalloc_shadow_pages
v9: https://lore.kernel.org/linux-mm/20191017012506.28503-1-dja@axtens.net/
    (attempt to) address a number of review comments for patch 1.
v10: https://lore.kernel.org/linux-mm/20191029042059.28541-1-dja@axtens.net/
     - rebase on linux-next, pulling in Vlad's new work on splitting the
       vmalloc locks.
     - after much discussion of barriers, document where I think they
       are needed and why. Thanks Mark and Andrey.
     - clean up some TLB flushing and checkpatch bits
v11: Address review comments from Andrey and Vlad, drop patch 5, add benchmarking
     results.

Daniel Axtens (4):
  kasan: support backing vmalloc space with real shadow memory
  kasan: add test for vmalloc
  fork: support VMAP_STACK with KASAN_VMALLOC
  x86/kasan: support KASAN_VMALLOC

 Documentation/dev-tools/kasan.rst |  63 ++++++++
 arch/Kconfig                      |   9 +-
 arch/x86/Kconfig                  |   1 +
 arch/x86/mm/kasan_init_64.c       |  61 ++++++++
 include/linux/kasan.h             |  31 ++++
 include/linux/moduleloader.h      |   2 +-
 include/linux/vmalloc.h           |  12 ++
 kernel/fork.c                     |   4 +
 lib/Kconfig.kasan                 |  16 +++
 lib/test_kasan.c                  |  26 ++++
 mm/kasan/common.c                 | 231 ++++++++++++++++++++++++++++++
 mm/kasan/generic_report.c         |   3 +
 mm/kasan/kasan.h                  |   1 +
 mm/vmalloc.c                      |  53 +++++--
 14 files changed, 500 insertions(+), 13 deletions(-)

-- 
2.20.1

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