khwasan: kernel hardware assisted address sanitizer

This patchset adds a new mode to KASAN [1], which is called KHWASAN (Kernel HardWare assisted Address SANitizer). There's still some work to do and there are a few TODOs in the code, so I'm publishing this as an RFC to collect some initial feedback.

The plan is to implement HWASan [2] for the kernel with the incentive, that it's going to have comparable to KASAN performance, but in the same time consume much less memory, trading that off for somewhat imprecise bug detection and being supported only for arm64.

The overall idea of the approach used by KHWASAN is the following:

1. By using the Top Byte Ignore arm64 CPU feature, we can store pointer tags in the top byte of each kernel pointer.

2. Using shadow memory, we can store memory tags for each chunk of kernel memory.

3. On each memory allocation, we can generate a random tag, embed it into the returned pointer and set the memory tags that correspond to this chunk of memory to the same value.

4. By using compiler instrumentation, before each memory access we can add a check that the pointer tag matches the tag of the memory that is being accessed.

5. On a tag mismatch we report an error.

[1] https://www.kernel.org/doc/html/latest/dev-tools/kasan.html

[2] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html

====== Technical details

KHWASAN is implemented in a very similar way to KASAN. This patchset essentially does the following:

1. TCR_TBI1 is set to enable Top Byte Ignore.

2. Shadow memory is used (with a different scale, 1:16, so each shadow byte corresponds to 16 bytes of kernel memory) to store memory tags.

3. All slab objects are aligned to shadow scale, which is 16 bytes.

4. All pointers returned from the slab allocator are tagged with a random tag and the corresponding shadow memory is poisoned with the same value.

5. Compiler instrumentation is used to insert tag checks. Either by calling callbacks or by inlining them (CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE flags are reused).

6. When a tag mismatch is detected in callback instrumentation mode KHWASAN simply prints a bug report. In case of inline instrumentation, clang inserts a brk instruction, and KHWASAN has it's own brk handler, which reports the bug.

7. The memory in between slab objects is marked with a reserved tag, and acts as a redzone.

8. When a slab object is freed it's marked with a reserved tag.

Bug detection is imprecise for two reasons:

1. We won't catch some small out-of-bounds accesses, that fall into the same shadow cell, as the last byte of a slab object.

2. We only have 1 byte to store tags, which means we have a 1/256 probability of a tag match for an incorrect access (actually even slightly less due to reserved tag values).

Despite that there's a particular type of bugs that KHWASAN can detect compared to KASAN: use-after-free after the object has been allocated by someone else.

====== Benchmarks

The following numbers were collected on Odroid C2 board. Both KASAN and KHWASAN were used in inline instrumentation mode.

Boot time [1]:
* ~1.7 sec for clean kernel
* ~5.0 sec for KASAN
* ~5.0 sec for KHWASAN

Slab memory usage after boot [2]:
* ~40 kb for clean kernel
* ~105 kb + 1/8th shadow ~= 118 kb for KASAN
* ~47 kb + 1/16th shadow ~= 50 kb for KHWASAN

Network performance [3]:
* 8.33 Gbits/sec for clean kernel
* 3.17 Gbits/sec for KASAN
* 2.85 Gbits/sec for KHWASAN

Note, that KHWASAN (compared to KASAN) doesn't require quarantine.

[1] Time before the ext4 driver is initialized.
[2] Measured as cat /proc/meminfo | grep Slab.
[3] Measured as iperf -s & iperf -c 127.0.0.1 -t 30.

====== Some notes

A few notes:

1. The patchset can be found here:
https://github.com/xairy/kasan-prototype/tree/khwasan

2. Building requires a recent LLVM version (r330044 or later).

3. Stack instrumentation is not supported yet and will be added later.

====== Changes

Changes in v1:
- Rebased onto 4.17-rc4.
- Updated benchmarking stats.
- Documented compiler version requirements, memory usage and slowdown.
- Dropped kvm patches, as clang + arm64 + kvm is completely broken [1].

Changes in RFC v3:
- Renamed CONFIG_KASAN_CLASSIC and CONFIG_KASAN_TAGS to CONFIG_KASAN_GENERIC and CONFIG_KASAN_HW respectively.
- Switch to -fsanitize=kernel-hwaddress instead of -fsanitize=hwaddress.
- Removed unnecessary excessive shadow initialization.
- Removed khwasan_enabled flag (it’s not needed since KHWASAN is initialized before any slab caches are used).
- Split out kasan_report.c and khwasan_report.c from report.c.
- Moved more common KASAN and KHWASAN functions to common.c.
- Added tagging to pagealloc.
- Rebased onto 4.17-rc1.
- Temporarily dropped patch that adds kvm support (arm64 + kvm + clang combo is broken right now [1]).

Changes in RFC v2:
- Removed explicit casts to u8 * for kasan_mem_to_shadow() calls.
- Introduced KASAN_TCR_FLAGS for setting the TCR_TBI1 flag.
- Added a comment regarding the non-atomic RMW sequence in khwasan_random_tag().
- Made all tag related functions accept const void *.
- Untagged pointers in __kimg_to_phys, which is used by virt_to_phys.
- Untagged pointers in show_ptr in fault handling logic.
- Untagged pointers passed to KVM.
- Added two reserved tag values: 0xFF and 0xFE.
- Used the reserved tag 0xFF to disable validity checking (to resolve the issue with pointer tag being lost after page_address + kmap usage).
- Used the reserved tag 0xFE to mark redzones and freed objects.
- Added mnemonics for esr manipulation in KHWASAN brk handler.
- Added a comment about the -recover flag.
- Some minor cleanups and fixes.
- Rebased onto 3215b9d5 (4.16-rc6+).
- Tested on real hardware (Odroid C2 board).
- Added better benchmarks.

[1] https://lkml.org/lkml/2018/4/19/775

Andrey Konovalov (16):
khwasan, mm: change kasan hooks signatures
khwasan: move common kasan and khwasan code to common.c
khwasan: add CONFIG_KASAN_GENERIC and CONFIG_KASAN_HW khwasan, arm64: adjust shadow size for CONFIG_KASAN_HW
khwasan: initialize shadow to 0xff
khwasan, arm64: untag virt address in __kimg_to_phys khwasan, arm64: fix up fault handling logic
khwasan: add tag related helper functions
khwasan, arm64: enable top byte ignore for the kernel khwasan, mm: perform untagged pointers comparison in krealloc
khwasan: split out kasan_report.c from report.c
khwasan: add bug reporting routines
khwasan: add hooks implementation
khwasan, arm64: add brk handler for inline instrumentation khwasan, mm, arm64: tag non slab memory allocated via pagealloc
khwasan: update kasan documentation

 Documentation/dev-tools/kasan.rst      | 213 +++++----
 arch/arm64/Kconfig                     |   1 +
 arch/arm64/Makefile                    |   2 +-
 arch/arm64/include/asm/brk-imm.h       |   2 +
 arch/arm64/include/asm/memory.h        |  35 +-
 arch/arm64/include/asm/pgtable-hwdef.h |   1 +
 arch/arm64/kernel/traps.c              |  69 ++-
 arch/arm64/mm/fault.c                  |   3 +
 arch/arm64/mm/kasan_init.c             |  18 +-
 arch/arm64/mm/proc.S                   |   8 +-
 include/linux/compiler-clang.h         |   5 +-
 include/linux/compiler-gcc.h           |   4 +
 include/linux/compiler.h               |   3 +-
 include/linux/kasan.h                  |  84 +++-
 include/linux/mm.h                     |  29 ++
 include/linux/page-flags-layout.h      |  10 +
 lib/Kconfig.kasan                      |  76 +++-
 mm/cma.c                               |   1 +
 mm/kasan/Makefile                      |   9 +-
 mm/kasan/common.c                      | 577 +++++++++++++++++++++++++
 mm/kasan/kasan.c                       | 489 +--------------------
 mm/kasan/kasan.h                       |  94 +++-
 mm/kasan/kasan_report.c                | 155 +++++++
 mm/kasan/khwasan.c                     | 163 +++++++
 mm/kasan/khwasan_report.c              |  60 +++
 mm/kasan/report.c                      | 271 +++---------
 mm/page_alloc.c                        |   1 +
 mm/slab.c                              |  12 +-
 mm/slab.h                              |   2 +-
 mm/slab_common.c                       |   6 +-
 mm/slub.c                              |  18 +-
 scripts/Makefile.kasan                 |  27 +-
 32 files changed, 1606 insertions(+), 842 deletions(-)
 create mode 100644 mm/kasan/common.c
 create mode 100644 mm/kasan/kasan_report.c
 create mode 100644 mm/kasan/khwasan.c
 create mode 100644 mm/kasan/khwasan_report.c

2.17.0.441.gb46fe60e1d-goog