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

[Mark Rutland]

On Thu, Aug 15, 2019 at 10:16:33AM +1000, Daniel Axtens wrote:
> 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.

I'm happy to send patches for arm64 once we've settled some conflicting
rework going on for 52-bit VA support.

> 
> 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.

FWIW, I haven't spotted such memory exhaustion after a week of Syzkaller
fuzzing with the last patchset, across 3 machines, so that sounds fine
to me.

Otherwise, this looks good to me now! For the x86 and fork patch, feel
free to add:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> 
> v1: https://lore.kernel.org/linux-mm/20190725055503.19507-1-dja@xxxxxxxxxx/
> v2: https://lore.kernel.org/linux-mm/20190729142108.23343-1-dja@xxxxxxxxxx/
>  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@xxxxxxxxxx/
>  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
>