[PATCH v4 0/3] kasan: support backing vmalloc space with real shadow memory

[Daniel Axtens]

Currently, vmalloc space is backed by the early shadow page. This
means that kasan is incompatible with VMAP_STACK, and it also provides
a hurdle for architectures that do not have a dedicated module space
(like powerpc64).

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.

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 the first time a mapping in vmalloc space uses a
particular page of the shadow region. Keep this page around
regardless of whether the mapping is later freed - in the mean time
the page could have become shared by another vmalloc mapping.

This can in theory lead to unbounded memory growth, but the vmalloc
allocator is pretty good at reusing addresses, so the practical memory
usage appears to grow at first but then stay fairly stable.

If we run into practical memory exhaustion issues, I'm happy to
consider hooking into the book-keeping that vmap does, but I am not
convinced that it will be an issue.

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: 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

Daniel Axtens (3):
  kasan: support backing vmalloc space with real shadow memory
  fork: support VMAP_STACK with KASAN_VMALLOC
  x86/kasan: support KASAN_VMALLOC

 Documentation/dev-tools/kasan.rst | 60 +++++++++++++++++++++++++++
 arch/Kconfig                      |  9 +++--
 arch/x86/Kconfig                  |  1 +
 arch/x86/mm/kasan_init_64.c       | 61 ++++++++++++++++++++++++++++
 include/linux/kasan.h             | 24 +++++++++++
 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                 | 67 +++++++++++++++++++++++++++++++
 mm/kasan/generic_report.c         |  3 ++
 mm/kasan/kasan.h                  |  1 +
 mm/vmalloc.c                      | 28 ++++++++++++-
 14 files changed, 308 insertions(+), 6 deletions(-)

-- 
2.20.1