[PATCH 01/11] kasan: docs: clean up sections

[PATCH 01/11] kasan: docs: clean up sections

[Andrey Konovalov]

Update KASAN documentation:

- Give some sections clearer names.
- Remove unneeded subsections in the "Tests" section.
- Move the "For developers" section and split into subsections.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 50 +++++++++++++++----------------
 1 file changed, 25 insertions(+), 25 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index ddf4239a5890..c9484f34da2a 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -168,24 +168,6 @@ particular KASAN features.
   report or also panic the kernel (default: ``report``). Note, that tag
   checking gets disabled after the first reported bug.
 
-For developers
-~~~~~~~~~~~~~~
-
-Software KASAN modes use compiler instrumentation to insert validity checks.
-Such instrumentation might be incompatible with some part of the kernel, and
-therefore needs to be disabled. To disable instrumentation for specific files
-or directories, add a line similar to the following to the respective kernel
-Makefile:
-
-- For a single file (e.g. main.o)::
-
-    KASAN_SANITIZE_main.o := n
-
-- For all files in one directory::
-
-    KASAN_SANITIZE := n
-
-
 Implementation details
 ----------------------
 
@@ -299,8 +281,8 @@ support MTE (but supports TBI).
 Hardware tag-based KASAN only reports the first found bug. After that MTE tag
 checking gets disabled.
 
-What memory accesses are sanitised by KASAN?
---------------------------------------------
+Shadow memory
+-------------
 
 The kernel maps memory in a number of different parts of the address
 space. This poses something of a problem for KASAN, which requires
@@ -362,8 +344,29 @@ unmapped. This will require changes in arch-specific code.
 This allows ``VMAP_STACK`` support on x86, and can simplify support of
 architectures that do not have a fixed module region.
 
-CONFIG_KASAN_KUNIT_TEST and CONFIG_KASAN_MODULE_TEST
-----------------------------------------------------
+For developers
+--------------
+
+Ignoring accesses
+~~~~~~~~~~~~~~~~~
+
+Software KASAN modes use compiler instrumentation to insert validity checks.
+Such instrumentation might be incompatible with some part of the kernel, and
+therefore needs to be disabled. To disable instrumentation for specific files
+or directories, add a line similar to the following to the respective kernel
+Makefile:
+
+- For a single file (e.g. main.o)::
+
+    KASAN_SANITIZE_main.o := n
+
+- For all files in one directory::
+
+    KASAN_SANITIZE := n
+
+
+Tests
+~~~~~
 
 KASAN tests consist of two parts:
 
@@ -409,21 +412,18 @@ Or, if one of the tests failed::
 There are a few ways to run KUnit-compatible KASAN tests.
 
 1. Loadable module
-~~~~~~~~~~~~~~~~~~
 
 With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
 a loadable module and run on any architecture that supports KASAN by loading
 the module with insmod or modprobe. The module is called ``test_kasan``.
 
 2. Built-In
-~~~~~~~~~~~
 
 With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
 on any architecure that supports KASAN. These and any other KUnit tests enabled
 will run and print the results at boot as a late-init call.
 
 3. Using kunit_tool
-~~~~~~~~~~~~~~~~~~~
 
 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it's also
 possible use ``kunit_tool`` to see the results of these and other KUnit tests
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 02/11] kasan: docs: update overview section

[Andrey Konovalov]

Update the "Overview" section in KASAN documentation:

- Outline main use cases for each mode.
- Mention that HW_TAGS mode need compiler support too.
- Move the part about SLUB/SLAB support from "Usage" to "Overview".
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index c9484f34da2a..343a683d0520 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -11,17 +11,31 @@ designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
 2. software tag-based KASAN (similar to userspace HWASan),
 3. hardware tag-based KASAN (based on hardware memory tagging).
 
-Software KASAN modes (1 and 2) use compile-time instrumentation to insert
-validity checks before every memory access, and therefore require a compiler
+Generic KASAN is mainly used for debugging due to a large memory overhead.
+Software tag-based KASAN can be used for dogfood testing as it has a lower
+memory overhead that allows using it with real workloads. Hardware tag-based
+KASAN comes with low memory and performance overheads and, therefore, can be
+used in production. Either as an in-field memory bug detector or as a security
+mitigation.
+
+Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
+validity checks before every memory access and, therefore, require a compiler
 version that supports that.
 
-Generic KASAN is supported in both GCC and Clang. With GCC it requires version
+Generic KASAN is supported in GCC and Clang. With GCC, it requires version
 8.3.0 or later. Any supported Clang version is compatible, but detection of
 out-of-bounds accesses for global variables is only supported since Clang 11.
 
-Tag-based KASAN is only supported in Clang.
+Software tag-based KASAN mode is only supported in Clang.
 
-Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
+The hardware KASAN mode (#3) relies on hardware to perform the checks but
+still requires a compiler version that supports memory tagging instructions.
+This mode is supported in Clang 11+.
+
+Both software KASAN modes work with SLUB and SLAB memory allocators,
+while the hardware tag-based KASAN currently only supports SLUB.
+
+Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
 
 Usage
@@ -39,9 +53,6 @@ For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
 CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
 The former produces smaller binary while the latter is 1.1 - 2 times faster.
 
-Both software KASAN modes work with both SLUB and SLAB memory allocators,
-while the hardware tag-based KASAN currently only support SLUB.
-
 For better error reports that include stack traces, enable CONFIG_STACKTRACE.
 
 To augment reports with last allocation and freeing stack of the physical page,
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 03/11] kasan: docs: update usage section

[Andrey Konovalov]

Update the "Usage" section in KASAN documentation:

- Add inline code snippet markers.
- Reword the part about stack traces for clarity.
- Other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 343a683d0520..f21c0cbebcb3 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -41,22 +41,21 @@ and riscv architectures, and tag-based KASAN modes are supported only for arm64.
 Usage
 -----
 
-To enable KASAN configure kernel with::
+To enable KASAN, configure the kernel with::
 
-	  CONFIG_KASAN = y
+	  CONFIG_KASAN=y
 
-and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN),
-CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN), and
-CONFIG_KASAN_HW_TAGS (to enable hardware tag-based KASAN).
+and choose between ``CONFIG_KASAN_GENERIC`` (to enable generic KASAN),
+``CONFIG_KASAN_SW_TAGS`` (to enable software tag-based KASAN), and
+``CONFIG_KASAN_HW_TAGS`` (to enable hardware tag-based KASAN).
 
-For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
-CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
-The former produces smaller binary while the latter is 1.1 - 2 times faster.
+For software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
+``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
+The former produces a smaller binary while the latter is 1.1-2 times faster.
 
-For better error reports that include stack traces, enable CONFIG_STACKTRACE.
-
-To augment reports with last allocation and freeing stack of the physical page,
-it is recommended to enable also CONFIG_PAGE_OWNER and boot with page_owner=on.
+To include alloc and free stack traces of affected slab objects into reports,
+enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
+physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
 
 Error reports
 ~~~~~~~~~~~~~
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 04/11] kasan: docs: update error reports section

[Andrey Konovalov]

Update the "Error reports" section in KASAN documentation:

- Mention that bug titles are best-effort.
- Move and reword the part about auxiliary stacks from
  "Implementation details".
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 46 +++++++++++++++++--------------
 1 file changed, 26 insertions(+), 20 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index f21c0cbebcb3..5fe43489e94e 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -60,7 +60,7 @@ physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
 Error reports
 ~~~~~~~~~~~~~
 
-A typical out-of-bounds access generic KASAN report looks like this::
+A typical KASAN report looks like this::
 
     ==================================================================
     BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
@@ -133,33 +133,43 @@ A typical out-of-bounds access generic KASAN report looks like this::
      ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
     ==================================================================
 
-The header of the report provides a short summary of what kind of bug happened
-and what kind of access caused it. It's followed by a stack trace of the bad
-access, a stack trace of where the accessed memory was allocated (in case bad
-access happens on a slab object), and a stack trace of where the object was
-freed (in case of a use-after-free bug report). Next comes a description of
-the accessed slab object and information about the accessed memory page.
+The report header summarizes what kind of bug happened and what kind of access
+caused it. It is followed by a stack trace of the bad access, a stack trace of
+where the accessed memory was allocated (in case a slab object was accessed),
+and a stack trace of where the object was freed (in case of a use-after-free
+bug report). Next comes a description of the accessed slab object and the
+information about the accessed memory page.
 
-In the last section the report shows memory state around the accessed address.
-Internally KASAN tracks memory state separately for each memory granule, which
+In the end, the report shows the memory state around the accessed address.
+Internally, KASAN tracks memory state separately for each memory granule, which
 is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
 memory state section of the report shows the state of one of the memory
 granules that surround the accessed address.
 
-For generic KASAN the size of each memory granule is 8. The state of each
+For generic KASAN, the size of each memory granule is 8. The state of each
 granule is encoded in one shadow byte. Those 8 bytes can be accessible,
-partially accessible, freed or be a part of a redzone. KASAN uses the following
-encoding for each shadow byte: 0 means that all 8 bytes of the corresponding
+partially accessible, freed, or be a part of a redzone. KASAN uses the following
+encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
 memory region are accessible; number N (1 <= N <= 7) means that the first N
 bytes are accessible, and other (8 - N) bytes are not; any negative value
 indicates that the entire 8-byte word is inaccessible. KASAN uses different
 negative values to distinguish between different kinds of inaccessible memory
 like redzones or freed memory (see mm/kasan/kasan.h).
 
-In the report above the arrows point to the shadow byte 03, which means that
-the accessed address is partially accessible. For tag-based KASAN modes this
-last report section shows the memory tags around the accessed address
-(see the `Implementation details`_ section).
+In the report above, the arrow points to the shadow byte ``03``, which means
+that the accessed address is partially accessible.
+
+For tag-based KASAN modes, this last report section shows the memory tags around
+the accessed address (see the `Implementation details`_ section).
+
+Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
+are best-effort: KASAN prints the most probable bug type based on the limited
+information it has. The actual type of the bug might be different.
+
+Generic KASAN also reports up to two auxiliary call stack traces. These stack
+traces point to places in code that interacted with the object but that are not
+directly present in the bad access stack trace. Currently, this includes
+call_rcu() and workqueue queuing.
 
 Boot parameters
 ~~~~~~~~~~~~~~~
@@ -214,10 +224,6 @@ function calls GCC directly inserts the code to check the shadow memory.
 This option significantly enlarges kernel but it gives x1.1-x2 performance
 boost over outline instrumented kernel.
 
-Generic KASAN also reports the last 2 call stacks to creation of work that
-potentially has access to an object. Call stacks for the following are shown:
-call_rcu() and workqueue queuing.
-
 Generic KASAN is the only mode that delays the reuse of freed object via
 quarantine (see mm/kasan/quarantine.c for implementation).
 
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 05/11] kasan: docs: update boot parameters section

[Andrey Konovalov]

Update the "Boot parameters" section in KASAN documentation:

- Mention panic_on_warn.
- Mention kasan_multi_shot and its interaction with panic_on_warn.
- Clarify kasan.fault=panic interaction with panic_on_warn.
- A readability clean-up.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 5fe43489e94e..2f939241349d 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -174,10 +174,16 @@ call_rcu() and workqueue queuing.
 Boot parameters
 ~~~~~~~~~~~~~~~
 
+KASAN is affected by the generic ``panic_on_warn`` command line parameter.
+When it is enabled, KASAN panics the kernel after printing a bug report.
+
+By default, KASAN prints a bug report only for the first invalid memory access.
+With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
+effectively disables ``panic_on_warn`` for KASAN reports.
+
 Hardware tag-based KASAN mode (see the section about various modes below) is
 intended for use in production as a security mitigation. Therefore, it supports
-boot parameters that allow to disable KASAN competely or otherwise control
-particular KASAN features.
+boot parameters that allow disabling KASAN or controlling its features.
 
 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
 
@@ -185,8 +191,8 @@ particular KASAN features.
   traces collection (default: ``on``).
 
 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
-  report or also panic the kernel (default: ``report``). Note, that tag
-  checking gets disabled after the first reported bug.
+  report or also panic the kernel (default: ``report``). The panic happens even
+  if ``kasan_multi_shot`` is enabled.
 
 Implementation details
 ----------------------
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 06/11] kasan: docs: update GENERIC implementation details section

[Andrey Konovalov]

Update the "Implementation details" section for generic KASAN:

- Don't mention kmemcheck, it's not present in the kernel anymore.
- Don't mention GCC as the only supported compiler.
- Update kasan_mem_to_shadow() definition to match actual code.
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 2f939241349d..1fb4b715a3ce 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -200,12 +200,11 @@ Implementation details
 Generic KASAN
 ~~~~~~~~~~~~~
 
-From a high level perspective, KASAN's approach to memory error detection is
-similar to that of kmemcheck: use shadow memory to record whether each byte of
-memory is safe to access, and use compile-time instrumentation to insert checks
-of shadow memory on each memory access.
+Software KASAN modes use shadow memory to record whether each byte of memory is
+safe to access and use compile-time instrumentation to insert shadow memory
+checks before each memory access.
 
-Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
+Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
 translate a memory address to its corresponding shadow address.
 
@@ -214,23 +213,23 @@ address::
 
     static inline void *kasan_mem_to_shadow(const void *addr)
     {
-	return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
+	return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
 		+ KASAN_SHADOW_OFFSET;
     }
 
 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
 
 Compile-time instrumentation is used to insert memory access checks. Compiler
-inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
-memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
-access is valid or not by checking corresponding shadow memory.
+inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
+each memory access of size 1, 2, 4, 8, or 16. These functions check whether
+memory accesses are valid or not by checking corresponding shadow memory.
 
-GCC 5.0 has possibility to perform inline instrumentation. Instead of making
-function calls GCC directly inserts the code to check the shadow memory.
-This option significantly enlarges kernel but it gives x1.1-x2 performance
-boost over outline instrumented kernel.
+With inline instrumentation, instead of making function calls, the compiler
+directly inserts the code to check shadow memory. This option significantly
+enlarges the kernel, but it gives an x1.1-x2 performance boost over the
+outline-instrumented kernel.
 
-Generic KASAN is the only mode that delays the reuse of freed object via
+Generic KASAN is the only mode that delays the reuse of freed objects via
 quarantine (see mm/kasan/quarantine.c for implementation).
 
 Software tag-based KASAN
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 07/11] kasan: docs: update SW_TAGS implementation details section

[Andrey Konovalov]

Update the "Implementation details" section for SW_TAGS KASAN:

- Clarify the introduction sentence.
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 39 +++++++++++++++----------------
 1 file changed, 19 insertions(+), 20 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 1fb4b715a3ce..dff18e180120 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -235,38 +235,37 @@ quarantine (see mm/kasan/quarantine.c for implementation).
 Software tag-based KASAN
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-Software tag-based KASAN requires software memory tagging support in the form
-of HWASan-like compiler instrumentation (see HWASan documentation for details).
-
-Software tag-based KASAN is currently only implemented for arm64 architecture.
+Software tag-based KASAN uses a software memory tagging approach to checking
+access validity. It is currently only implemented for the arm64 architecture.
 
 Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
-to store a pointer tag in the top byte of kernel pointers. Like generic KASAN
-it uses shadow memory to store memory tags associated with each 16-byte memory
-cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
+to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
+to store memory tags associated with each 16-byte memory cell (therefore, it
+dedicates 1/16th of the kernel memory for shadow memory).
 
-On each memory allocation software tag-based KASAN generates a random tag, tags
-the allocated memory with this tag, and embeds this tag into the returned
+On each memory allocation, software tag-based KASAN generates a random tag, tags
+the allocated memory with this tag, and embeds the same tag into the returned
 pointer.
 
 Software tag-based KASAN uses compile-time instrumentation to insert checks
-before each memory access. These checks make sure that tag of the memory that
-is being accessed is equal to tag of the pointer that is used to access this
-memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
+before each memory access. These checks make sure that the tag of the memory
+that is being accessed is equal to the tag of the pointer that is used to access
+this memory. In case of a tag mismatch, software tag-based KASAN prints a bug
+report.
 
-Software tag-based KASAN also has two instrumentation modes (outline, that
-emits callbacks to check memory accesses; and inline, that performs the shadow
+Software tag-based KASAN also has two instrumentation modes (outline, which
+emits callbacks to check memory accesses; and inline, which performs the shadow
 memory checks inline). With outline instrumentation mode, a bug report is
-simply printed from the function that performs the access check. With inline
-instrumentation a brk instruction is emitted by the compiler, and a dedicated
-brk handler is used to print bug reports.
+printed from the function that performs the access check. With inline
+instrumentation, a ``brk`` instruction is emitted by the compiler, and a
+dedicated ``brk`` handler is used to print bug reports.
 
 Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
-pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
+pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
 reserved to tag freed memory regions.
 
-Software tag-based KASAN currently only supports tagging of
-kmem_cache_alloc/kmalloc and page_alloc memory.
+Software tag-based KASAN currently only supports tagging of slab and page_alloc
+memory.
 
 Hardware tag-based KASAN
 ~~~~~~~~~~~~~~~~~~~~~~~~
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 08/11] kasan: docs: update HW_TAGS implementation details section

[Andrey Konovalov]

Update the "Implementation details" section for HW_TAGS KASAN:

- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index dff18e180120..f5c746a475c1 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -270,35 +270,35 @@ memory.
 Hardware tag-based KASAN
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-Hardware tag-based KASAN is similar to the software mode in concept, but uses
+Hardware tag-based KASAN is similar to the software mode in concept but uses
 hardware memory tagging support instead of compiler instrumentation and
 shadow memory.
 
 Hardware tag-based KASAN is currently only implemented for arm64 architecture
 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
-Instruction Set Architecture, and Top Byte Ignore (TBI).
+Instruction Set Architecture and Top Byte Ignore (TBI).
 
 Special arm64 instructions are used to assign memory tags for each allocation.
 Same tags are assigned to pointers to those allocations. On every memory
-access, hardware makes sure that tag of the memory that is being accessed is
-equal to tag of the pointer that is used to access this memory. In case of a
-tag mismatch a fault is generated and a report is printed.
+access, hardware makes sure that the tag of the memory that is being accessed is
+equal to the tag of the pointer that is used to access this memory. In case of a
+tag mismatch, a fault is generated, and a report is printed.
 
 Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
-pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
+pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
 reserved to tag freed memory regions.
 
-Hardware tag-based KASAN currently only supports tagging of
-kmem_cache_alloc/kmalloc and page_alloc memory.
+Hardware tag-based KASAN currently only supports tagging of slab and page_alloc
+memory.
 
-If the hardware doesn't support MTE (pre ARMv8.5), hardware tag-based KASAN
-won't be enabled. In this case all boot parameters are ignored.
+If the hardware does not support MTE (pre ARMv8.5), hardware tag-based KASAN
+will not be enabled. In this case, all KASAN boot parameters are ignored.
 
-Note, that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
-enabled. Even when kasan.mode=off is provided, or when the hardware doesn't
+Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
+enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
 support MTE (but supports TBI).
 
-Hardware tag-based KASAN only reports the first found bug. After that MTE tag
+Hardware tag-based KASAN only reports the first found bug. After that, MTE tag
 checking gets disabled.
 
 Shadow memory
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 09/11] kasan: docs: update shadow memory section

[Andrey Konovalov]

Update the "Shadow memory" section in KASAN documentation:

- Rearrange the introduction paragraph do it doesn't give a
  "KASAN has an issue" impression.
- Update the list of architectures with vmalloc support.
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index f5c746a475c1..2b61d90e136f 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -304,14 +304,11 @@ checking gets disabled.
 Shadow memory
 -------------
 
-The kernel maps memory in a number of different parts of the address
-space. This poses something of a problem for KASAN, which requires
-that all addresses accessed by instrumented code have a valid shadow
-region.
-
-The range of kernel virtual addresses is large: there is not enough
-real memory to support a real shadow region for every address that
-could be accessed by the kernel.
+The kernel maps memory in several different parts of the address space.
+The range of kernel virtual addresses is large: there is not enough real
+memory to support a real shadow region for every address that could be
+accessed by the kernel. Therefore, KASAN only maps real shadow for certain
+parts of the address space.
 
 By default
 ~~~~~~~~~~
@@ -323,10 +320,9 @@ page is mapped over the shadow area. This read-only shadow page
 declares all memory accesses as permitted.
 
 This presents a problem for modules: they do not live in the linear
-mapping, but in a dedicated module space. By hooking in to the module
-allocator, KASAN can temporarily map real shadow memory to cover
-them. This allows detection of invalid accesses to module globals, for
-example.
+mapping but in a dedicated module space. By hooking into the module
+allocator, KASAN temporarily maps real shadow memory to cover them.
+This allows detection of invalid accesses to module globals, for example.
 
 This also creates an incompatibility with ``VMAP_STACK``: if the stack
 lives in vmalloc space, it will be shadowed by the read-only page, and
@@ -337,9 +333,10 @@ CONFIG_KASAN_VMALLOC
 ~~~~~~~~~~~~~~~~~~~~
 
 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
-cost of greater memory usage. Currently this is only supported on x86.
+cost of greater memory usage. Currently, this is supported on x86,
+riscv, s390, and powerpc.
 
-This works by hooking into vmalloc and vmap, and dynamically
+This works by hooking into vmalloc and vmap and dynamically
 allocating real shadow memory to back the mappings.
 
 Most mappings in vmalloc space are small, requiring less than a full
@@ -358,10 +355,10 @@ memory.
 
 To avoid the difficulties around swapping mappings around, KASAN expects
 that the part of the shadow region that covers the vmalloc space will
-not be covered by the early shadow page, but will be left
-unmapped. This will require changes in arch-specific code.
+not be covered by the early shadow page but will be left unmapped.
+This will require changes in arch-specific code.
 
-This allows ``VMAP_STACK`` support on x86, and can simplify support of
+This allows ``VMAP_STACK`` support on x86 and can simplify support of
 architectures that do not have a fixed module region.
 
 For developers
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 10/11] kasan: docs: update ignoring accesses section

[Andrey Konovalov]

Update the "Ignoring accesses" section in KASAN documentation:

- Mention kasan_disable/enable_current().
- Mention kasan_reset_tag()/page_kasan_tag_reset().
- A punctuation clean-up.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 2b61d90e136f..6628b133c9ad 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -368,7 +368,7 @@ Ignoring accesses
 ~~~~~~~~~~~~~~~~~
 
 Software KASAN modes use compiler instrumentation to insert validity checks.
-Such instrumentation might be incompatible with some part of the kernel, and
+Such instrumentation might be incompatible with some parts of the kernel, and
 therefore needs to be disabled. To disable instrumentation for specific files
 or directories, add a line similar to the following to the respective kernel
 Makefile:
@@ -381,6 +381,19 @@ Makefile:
 
     KASAN_SANITIZE := n
 
+Other parts of the kernel might access metadata for allocated objects. Normally,
+KASAN detects and reports such accesses, but in certain cases (e.g., in memory
+allocators) these accesses are valid. Disabling instrumentation for memory
+allocators files helps with accesses that happen directly in that code for
+software KASAN modes. But it does not help when the accesses happen indirectly
+(through generic function calls) or with the hardware tag-based mode that does
+not use compiler instrumentation.
+
+To disable KASAN reports in a certain part of the kernel code:
+
+- For software modes, add a
+  ``kasan_disable_current()``/``kasan_enable_current()`` critical section.
+- For tag-based modes, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``.
 
 Tests
 ~~~~~
-- 
2.31.0.rc2.261.g7f71774620-goog

[PATCH 11/11] kasan: docs: update tests section

[Andrey Konovalov]

Update the "Tests" section in KASAN documentation:

- Add an introductory sentence.
- Add proper indentation for the list of ways to run KUnit tests.
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kasan.rst | 33 +++++++++++++++----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 6628b133c9ad..c4a3c8a9fe71 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -398,19 +398,20 @@ To disable KASAN reports in a certain part of the kernel code:
 Tests
 ~~~~~
 
-KASAN tests consist of two parts:
+There are KASAN tests that allow verifying that KASAN works and can detect
+certain types of memory corruptions. The tests consist of two parts:
 
 1. Tests that are integrated with the KUnit Test Framework. Enabled with
 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
-automatically in a few different ways, see the instructions below.
+automatically in a few different ways; see the instructions below.
 
 2. Tests that are currently incompatible with KUnit. Enabled with
 ``CONFIG_KASAN_MODULE_TEST`` and can only be run as a module. These tests can
-only be verified manually, by loading the kernel module and inspecting the
+only be verified manually by loading the kernel module and inspecting the
 kernel log for KASAN reports.
 
-Each KUnit-compatible KASAN test prints a KASAN report if an error is detected.
-Then the test prints its number and status.
+Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
+error is detected. Then the test prints its number and status.
 
 When a test passes::
 
@@ -438,27 +439,25 @@ Or, if one of the tests failed::
 
         not ok 1 - kasan
 
-
 There are a few ways to run KUnit-compatible KASAN tests.
 
 1. Loadable module
 
-With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
-a loadable module and run on any architecture that supports KASAN by loading
-the module with insmod or modprobe. The module is called ``test_kasan``.
+   With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
+   module and run by loading the `test_kasan.ko`` with ``insmod`` or
+   ``modprobe``.
 
 2. Built-In
 
-With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
-on any architecure that supports KASAN. These and any other KUnit tests enabled
-will run and print the results at boot as a late-init call.
+   With ``CONFIG_KUNIT`` built-in, KASAN-KUnit tests can be built-in as well.
+   In this case, the tests will run at boot as a late-init call.
 
 3. Using kunit_tool
 
-With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it's also
-possible use ``kunit_tool`` to see the results of these and other KUnit tests
-in a more readable way. This will not print the KASAN reports of the tests that
-passed. Use `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
-for more up-to-date information on ``kunit_tool``.
+   With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
+   possible to use ``kunit_tool`` to see the results of KUnit tests in a more
+   readable way. This will not print the KASAN reports of the tests that passed.
+   See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
+   for more up-to-date information on ``kunit_tool``.
 
 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
-- 
2.31.0.rc2.261.g7f71774620-goog

Re: [PATCH 02/11] kasan: docs: update overview section

[Marco Elver]

On Thu, 11 Mar 2021 at 22:37, Andrey Konovalov <andreyknvl@google.com> wrote:
>
> Update the "Overview" section in KASAN documentation:
>
> - Outline main use cases for each mode.
> - Mention that HW_TAGS mode need compiler support too.
> - Move the part about SLUB/SLAB support from "Usage" to "Overview".
> - Punctuation, readability, and other minor clean-ups.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kasan.rst | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index c9484f34da2a..343a683d0520 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -11,17 +11,31 @@ designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
>  2. software tag-based KASAN (similar to userspace HWASan),
>  3. hardware tag-based KASAN (based on hardware memory tagging).
>
> -Software KASAN modes (1 and 2) use compile-time instrumentation to insert
> -validity checks before every memory access, and therefore require a compiler
> +Generic KASAN is mainly used for debugging due to a large memory overhead.
> +Software tag-based KASAN can be used for dogfood testing as it has a lower
> +memory overhead that allows using it with real workloads. Hardware tag-based
> +KASAN comes with low memory and performance overheads and, therefore, can be
> +used in production. Either as an in-field memory bug detector or as a security
> +mitigation.
> +
> +Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
> +validity checks before every memory access and, therefore, require a compiler
>  version that supports that.
>
> -Generic KASAN is supported in both GCC and Clang. With GCC it requires version
> +Generic KASAN is supported in GCC and Clang. With GCC, it requires version
>  8.3.0 or later. Any supported Clang version is compatible, but detection of
>  out-of-bounds accesses for global variables is only supported since Clang 11.
>
> -Tag-based KASAN is only supported in Clang.
> +Software tag-based KASAN mode is only supported in Clang.
>
> -Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
> +The hardware KASAN mode (#3) relies on hardware to perform the checks but
> +still requires a compiler version that supports memory tagging instructions.
> +This mode is supported in Clang 11+.

Doesn't HW_TAGS mode work with GCC as well? While the sentence doesn't
say "exclusively", the mention of Clang 11+ makes me think it's only
Clang.

> +Both software KASAN modes work with SLUB and SLAB memory allocators,
> +while the hardware tag-based KASAN currently only supports SLUB.
> +
> +Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
>  and riscv architectures, and tag-based KASAN modes are supported only for arm64.
>
>  Usage
> @@ -39,9 +53,6 @@ For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
>  CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
>  The former produces smaller binary while the latter is 1.1 - 2 times faster.
>
> -Both software KASAN modes work with both SLUB and SLAB memory allocators,
> -while the hardware tag-based KASAN currently only support SLUB.
> -
>  For better error reports that include stack traces, enable CONFIG_STACKTRACE.
>
>  To augment reports with last allocation and freeing stack of the physical page,
> --
> 2.31.0.rc2.261.g7f71774620-goog
>

Re: [PATCH 11/11] kasan: docs: update tests section

[Marco Elver]

On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
[...]
> -With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
> -a loadable module and run on any architecture that supports KASAN by loading
> -the module with insmod or modprobe. The module is called ``test_kasan``.
> +   With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
> +   module and run by loading the `test_kasan.ko`` with ``insmod`` or

s/`test_kasan.ko``/``test_kasan.ko``/
(Missing `)

Also, "the" before test_kasan.ko is incorrect if nothing follows ("the
test_kasan.ko module" on the other hand would be fine).

> +   ``modprobe``.

Re: [PATCH 09/11] kasan: docs: update shadow memory section

[Marco Elver]

On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
[...]
> -The kernel maps memory in a number of different parts of the address
> -space. This poses something of a problem for KASAN, which requires
> -that all addresses accessed by instrumented code have a valid shadow
> -region.
> -
> -The range of kernel virtual addresses is large: there is not enough
> -real memory to support a real shadow region for every address that
> -could be accessed by the kernel.
> +The kernel maps memory in several different parts of the address space.
> +The range of kernel virtual addresses is large: there is not enough real
> +memory to support a real shadow region for every address that could be
> +accessed by the kernel. Therefore, KASAN only maps real shadow for certain
> +parts of the address space.
>  
>  By default
>  ~~~~~~~~~~

While we're here, can we change this "By default" heading which seems
wrong -- the paragraph starts with "By default, ..." as well.

Perhaps "Default Behaviour"?

Re: [PATCH 10/11] kasan: docs: update ignoring accesses section

[Marco Elver]

On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
[...]  
> +Other parts of the kernel might access metadata for allocated objects. Normally,
> +KASAN detects and reports such accesses, but in certain cases (e.g., in memory
> +allocators) these accesses are valid. Disabling instrumentation for memory
> +allocators files helps with accesses that happen directly in that code for
> +software KASAN modes. But it does not help when the accesses happen indirectly
> +(through generic function calls) or with the hardware tag-based mode that does
> +not use compiler instrumentation.
> +
> +To disable KASAN reports in a certain part of the kernel code:
> +
> +- For software modes, add a
> +  ``kasan_disable_current()``/``kasan_enable_current()`` critical section.

Should we mention function attribute __no_sanitize_address (and noinstr,
which just applies to any kind of instrumentation) here? Perhaps with
the note that called functions may still be instrumented, and in such
cases would require combining with kasan_{disable,enable}_current().

> +- For tag-based modes, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``.

Re: [PATCH 02/11] kasan: docs: update overview section

[Andrey Konovalov]

On Fri, Mar 12, 2021 at 11:18 AM Marco Elver <elver@google.com> wrote:
>
> > -Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
> > +The hardware KASAN mode (#3) relies on hardware to perform the checks but
> > +still requires a compiler version that supports memory tagging instructions.
> > +This mode is supported in Clang 11+.
>
> Doesn't HW_TAGS mode work with GCC as well? While the sentence doesn't
> say "exclusively", the mention of Clang 11+ makes me think it's only
> Clang.

I never tried it with GCC until just now. But looks like it works.
Will add GCC here, thanks!

Re: [PATCH 09/11] kasan: docs: update shadow memory section

[Andrey Konovalov]

On Fri, Mar 12, 2021 at 11:52 AM Marco Elver <elver@google.com> wrote:
>
> On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
> [...]
> > -The kernel maps memory in a number of different parts of the address
> > -space. This poses something of a problem for KASAN, which requires
> > -that all addresses accessed by instrumented code have a valid shadow
> > -region.
> > -
> > -The range of kernel virtual addresses is large: there is not enough
> > -real memory to support a real shadow region for every address that
> > -could be accessed by the kernel.
> > +The kernel maps memory in several different parts of the address space.
> > +The range of kernel virtual addresses is large: there is not enough real
> > +memory to support a real shadow region for every address that could be
> > +accessed by the kernel. Therefore, KASAN only maps real shadow for certain
> > +parts of the address space.
> >
> >  By default
> >  ~~~~~~~~~~
>
> While we're here, can we change this "By default" heading which seems
> wrong -- the paragraph starts with "By default, ..." as well.
>
> Perhaps "Default Behaviour"?

Sounds good, thanks!

Re: [PATCH 11/11] kasan: docs: update tests section

[Andrey Konovalov]

On Fri, Mar 12, 2021 at 11:47 AM Marco Elver <elver@google.com> wrote:
>
> On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
> [...]
> > -With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
> > -a loadable module and run on any architecture that supports KASAN by loading
> > -the module with insmod or modprobe. The module is called ``test_kasan``.
> > +   With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
> > +   module and run by loading the `test_kasan.ko`` with ``insmod`` or
>
> s/`test_kasan.ko``/``test_kasan.ko``/
> (Missing `)
>
> Also, "the" before test_kasan.ko is incorrect if nothing follows ("the
> test_kasan.ko module" on the other hand would be fine).

Will fix both, thanks!

Re: [PATCH 10/11] kasan: docs: update ignoring accesses section

[Andrey Konovalov]

On Fri, Mar 12, 2021 at 12:02 PM Marco Elver <elver@google.com> wrote:
>
> On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
> [...]
> > +Other parts of the kernel might access metadata for allocated objects. Normally,
> > +KASAN detects and reports such accesses, but in certain cases (e.g., in memory
> > +allocators) these accesses are valid. Disabling instrumentation for memory
> > +allocators files helps with accesses that happen directly in that code for
> > +software KASAN modes. But it does not help when the accesses happen indirectly
> > +(through generic function calls) or with the hardware tag-based mode that does
> > +not use compiler instrumentation.
> > +
> > +To disable KASAN reports in a certain part of the kernel code:
> > +
> > +- For software modes, add a
> > +  ``kasan_disable_current()``/``kasan_enable_current()`` critical section.
>
> Should we mention function attribute __no_sanitize_address (and noinstr,
> which just applies to any kind of instrumentation) here? Perhaps with
> the note that called functions may still be instrumented, and in such
> cases would require combining with kasan_{disable,enable}_current().

Indeed, forgot about the attributes.. Will add, thank you!