Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-nextgrafted

Pull networking updates from Jakub Kicinski:
 "Core:

   - Add dedicated kmem_cache for typical/small skb->head, avoid having
     to access struct page at kfree time, and improve memory use.

   - Introduce sysctl to set default RPS configuration for new netdevs.

   - Define Netlink protocol specification format which can be used to
     describe messages used by each family and auto-generate parsers.
     Add tools for generating kernel data structures and uAPI headers.

   - Expose all net/core sysctls inside netns.

   - Remove 4s sleep in netpoll if carrier is instantly detected on
     boot.

   - Add configurable limit of MDB entries per port, and port-vlan.

   - Continue populating drop reasons throughout the stack.

   - Retire a handful of legacy Qdiscs and classifiers.

  Protocols:

   - Support IPv4 big TCP (TSO frames larger than 64kB).

   - Add IP_LOCAL_PORT_RANGE socket option, to control local port range
     on socket by socket basis.

   - Track and report in procfs number of MPTCP sockets used.

   - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path
     manager.

   - IPv6: don't check net.ipv6.route.max_size and rely on garbage
     collection to free memory (similarly to IPv4).

   - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986).

   - ICMP: add per-rate limit counters.

   - Add support for user scanning requests in ieee802154.

   - Remove static WEP support.

   - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate
     reporting.

   - WiFi 7 EHT channel puncturing support (client & AP).

  BPF:

   - Add a rbtree data structure following the "next-gen data structure"
     precedent set by recently added linked list, that is, by using
     kfunc + kptr instead of adding a new BPF map type.

   - Expose XDP hints via kfuncs with initial support for RX hash and
     timestamp metadata.

   - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to
     better support decap on GRE tunnel devices not operating in collect
     metadata.

   - Improve x86 JIT's codegen for PROBE_MEM runtime error checks.

   - Remove the need for trace_printk_lock for bpf_trace_printk and
     bpf_trace_vprintk helpers.

   - Extend libbpf's bpf_tracing.h support for tracing arguments of
     kprobes/uprobes and syscall as a special case.

   - Significantly reduce the search time for module symbols by
     livepatch and BPF.

   - Enable cpumasks to be used as kptrs, which is useful for tracing
     programs tracking which tasks end up running on which CPUs in
     different time intervals.

   - Add support for BPF trampoline on s390x and riscv64.

   - Add capability to export the XDP features supported by the NIC.

   - Add __bpf_kfunc tag for marking kernel functions as kfuncs.

   - Add cgroup.memory=nobpf kernel parameter option to disable BPF
     memory accounting for container environments.

  Netfilter:

   - Remove the CLUSTERIP target. It has been marked as obsolete for
     years, and we still have WARN splats wrt races of the out-of-band
     /proc interface installed by this target.

   - Add 'destroy' commands to nf_tables. They are identical to the
     existing 'delete' commands, but do not return an error if the
     referenced object (set, chain, rule...) did not exist.

  Driver API:

   - Improve cpumask_local_spread() locality to help NICs set the right
     IRQ affinity on AMD platforms.

   - Separate C22 and C45 MDIO bus transactions more clearly.

   - Introduce new DCB table to control DSCP rewrite on egress.

   - Support configuration of Physical Layer Collision Avoidance (PLCA)
     Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of
     shared medium Ethernet.

   - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing
     preemption of low priority frames by high priority frames.

   - Add support for controlling MACSec offload using netlink SET.

   - Rework devlink instance refcounts to allow registration and
     de-registration under the instance lock. Split the code into
     multiple files, drop some of the unnecessarily granular locks and
     factor out common parts of netlink operation handling.

   - Add TX frame aggregation parameters (for USB drivers).

   - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning
     messages with notifications for debug.

   - Allow offloading of UDP NEW connections via act_ct.

   - Add support for per action HW stats in TC.

   - Support hardware miss to TC action (continue processing in SW from
     a specific point in the action chain).

   - Warn if old Wireless Extension user space interface is used with
     modern cfg80211/mac80211 drivers. Do not support Wireless
     Extensions for Wi-Fi 7 devices at all. Everyone should switch to
     using nl80211 interface instead.

   - Improve the CAN bit timing configuration. Use extack to return
     error messages directly to user space, update the SJW handling,
     including the definition of a new default value that will benefit
     CAN-FD controllers, by increasing their oscillator tolerance.

  New hardware / drivers:

   - Ethernet:
      - nVidia BlueField-3 support (control traffic driver)
      - Ethernet support for imx93 SoCs
      - Motorcomm yt8531 gigabit Ethernet PHY
      - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA)
      - Microchip LAN8841 PHY (incl. cable diagnostics and PTP)
      - Amlogic gxl MDIO mux

   - WiFi:
      - RealTek RTL8188EU (rtl8xxxu)
      - Qualcomm Wi-Fi 7 devices (ath12k)

   - CAN:
      - Renesas R-Car V4H

  Drivers:

   - Bluetooth:
      - Set Per Platform Antenna Gain (PPAG) for Intel controllers.

   - Ethernet NICs:
      - Intel (1G, igc):
         - support TSN / Qbv / packet scheduling features of i226 model
      - Intel (100G, ice):
         - use GNSS subsystem instead of TTY
         - multi-buffer XDP support
         - extend support for GPIO pins to E823 devices
      - nVidia/Mellanox:
         - update the shared buffer configuration on PFC commands
         - implement PTP adjphase function for HW offset control
         - TC support for Geneve and GRE with VF tunnel offload
         - more efficient crypto key management method
         - multi-port eswitch support
      - Netronome/Corigine:
         - add DCB IEEE support
         - support IPsec offloading for NFP3800
      - Freescale/NXP (enetc):
         - support XDP_REDIRECT for XDP non-linear buffers
         - improve reconfig, avoid link flap and waiting for idle
         - support MAC Merge layer
      - Other NICs:
         - sfc/ef100: add basic devlink support for ef100
         - ionic: rx_push mode operation (writing descriptors via MMIO)
         - bnxt: use the auxiliary bus abstraction for RDMA
         - r8169: disable ASPM and reset bus in case of tx timeout
         - cpsw: support QSGMII mode for J721e CPSW9G
         - cpts: support pulse-per-second output
         - ngbe: add an mdio bus driver
         - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing
         - r8152: handle devices with FW with NCM support
         - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation
         - virtio-net: support multi buffer XDP
         - virtio/vsock: replace virtio_vsock_pkt with sk_buff
         - tsnep: XDP support

   - Ethernet high-speed switches:
      - nVidia/Mellanox (mlxsw):
         - add support for latency TLV (in FW control messages)
      - Microchip (sparx5):
         - separate explicit and implicit traffic forwarding rules, make
           the implicit rules always active
         - add support for egress DSCP rewrite
         - IS0 VCAP support (Ingress Classification)
         - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS
           etc.)
         - ES2 VCAP support (Egress Access Control)
         - support for Per-Stream Filtering and Policing (802.1Q,
           8.6.5.1)

   - Ethernet embedded switches:
      - Marvell (mv88e6xxx):
         - add MAB (port auth) offload support
         - enable PTP receive for mv88e6390
      - NXP (ocelot):
         - support MAC Merge layer
         - support for the the vsc7512 internal copper phys
      - Microchip:
         - lan9303: convert to PHYLINK
         - lan966x: support TC flower filter statistics
         - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x
         - lan937x: support Credit Based Shaper configuration
         - ksz9477: support Energy Efficient Ethernet
      - other:
         - qca8k: convert to regmap read/write API, use bulk operations
         - rswitch: Improve TX timestamp accuracy

   - Intel WiFi (iwlwifi):
      - EHT (Wi-Fi 7) rate reporting
      - STEP equalizer support: transfer some STEP (connection to radio
        on platforms with integrated wifi) related parameters from the
        BIOS to the firmware.

   - Qualcomm 802.11ax WiFi (ath11k):
      - IPQ5018 support
      - Fine Timing Measurement (FTM) responder role support
      - channel 177 support

   - MediaTek WiFi (mt76):
      - per-PHY LED support
      - mt7996: EHT (Wi-Fi 7) support
      - Wireless Ethernet Dispatch (WED) reset support
      - switch to using page pool allocator

   - RealTek WiFi (rtw89):
      - support new version of Bluetooth co-existance

   - Mobile:
      - rmnet: support TX aggregation"

* tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits)
  page_pool: add a comment explaining the fragment counter usage
  net: ethtool: fix __ethtool_dev_mm_supported() implementation
  ethtool: pse-pd: Fix double word in comments
  xsk: add linux/vmalloc.h to xsk.c
  sefltests: netdevsim: wait for devlink instance after netns removal
  selftest: fib_tests: Always cleanup before exit
  net/mlx5e: Align IPsec ASO result memory to be as required by hardware
  net/mlx5e: TC, Set CT miss to the specific ct action instance
  net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG
  net/mlx5: Refactor tc miss handling to a single function
  net/mlx5: Kconfig: Make tc offload depend on tc skb extension
  net/sched: flower: Support hardware miss to tc action
  net/sched: flower: Move filter handle initialization earlier
  net/sched: cls_api: Support hardware miss to tc action
  net/sched: Rename user cookie and act cookie
  sfc: fix builds without CONFIG_RTC_LIB
  sfc: clean up some inconsistent indentings
  net/mlx4_en: Introduce flexible array to silence overflow warning
  net: lan966x: Fix possible deadlock inside PTP
  net/ulp: Remove redundant ->clone() test in inet_clone_ulp().
  ...

1 files changed, 428 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kmsan.rst b/Documentation/dev-tools/kmsan.rst
new file mode 100644
index 000000000..55fa82212
--- /dev/null
+++ b/Documentation/dev-tools/kmsan.rst
@@ -0,0 +1,428 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright (C) 2022, Google LLC.
+
+===================================
+The Kernel Memory Sanitizer (KMSAN)
+===================================
+
+KMSAN is a dynamic error detector aimed at finding uses of uninitialized
+values. It is based on compiler instrumentation, and is quite similar to the
+userspace `MemorySanitizer tool`_.
+
+An important note is that KMSAN is not intended for production use, because it
+drastically increases kernel memory footprint and slows the whole system down.
+
+Usage
+=====
+
+Building the kernel
+-------------------
+
+In order to build a kernel with KMSAN you will need a fresh Clang (14.0.6+).
+Please refer to `LLVM documentation`_ for the instructions on how to build Clang.
+
+Now configure and build the kernel with CONFIG_KMSAN enabled.
+
+Example report
+--------------
+
+Here is an example of a KMSAN report::
+
+  =====================================================
+  BUG: KMSAN: uninit-value in test_uninit_kmsan_check_memory+0x1be/0x380 [kmsan_test]
+   test_uninit_kmsan_check_memory+0x1be/0x380 mm/kmsan/kmsan_test.c:273
+   kunit_run_case_internal lib/kunit/test.c:333
+   kunit_try_run_case+0x206/0x420 lib/kunit/test.c:374
+   kunit_generic_run_threadfn_adapter+0x6d/0xc0 lib/kunit/try-catch.c:28
+   kthread+0x721/0x850 kernel/kthread.c:327
+   ret_from_fork+0x1f/0x30 ??:?
+
+  Uninit was stored to memory at:
+   do_uninit_local_array+0xfa/0x110 mm/kmsan/kmsan_test.c:260
+   test_uninit_kmsan_check_memory+0x1a2/0x380 mm/kmsan/kmsan_test.c:271
+   kunit_run_case_internal lib/kunit/test.c:333
+   kunit_try_run_case+0x206/0x420 lib/kunit/test.c:374
+   kunit_generic_run_threadfn_adapter+0x6d/0xc0 lib/kunit/try-catch.c:28
+   kthread+0x721/0x850 kernel/kthread.c:327
+   ret_from_fork+0x1f/0x30 ??:?
+
+  Local variable uninit created at:
+   do_uninit_local_array+0x4a/0x110 mm/kmsan/kmsan_test.c:256
+   test_uninit_kmsan_check_memory+0x1a2/0x380 mm/kmsan/kmsan_test.c:271
+
+  Bytes 4-7 of 8 are uninitialized
+  Memory access of size 8 starts at ffff888083fe3da0
+
+  CPU: 0 PID: 6731 Comm: kunit_try_catch Tainted: G    B       E     5.16.0-rc3+ #104
+  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
+  =====================================================
+
+The report says that the local variable ``uninit`` was created uninitialized in
+``do_uninit_local_array()``. The third stack trace corresponds to the place
+where this variable was created.
+
+The first stack trace shows where the uninit value was used (in
+``test_uninit_kmsan_check_memory()``). The tool shows the bytes which were left
+uninitialized in the local variable, as well as the stack where the value was
+copied to another memory location before use.
+
+A use of uninitialized value ``v`` is reported by KMSAN in the following cases:
+
+ - in a condition, e.g. ``if (v) { ... }``;
+ - in an indexing or pointer dereferencing, e.g. ``array[v]`` or ``*v``;
+ - when it is copied to userspace or hardware, e.g. ``copy_to_user(..., &v, ...)``;
+ - when it is passed as an argument to a function, and
+   ``CONFIG_KMSAN_CHECK_PARAM_RETVAL`` is enabled (see below).
+
+The mentioned cases (apart from copying data to userspace or hardware, which is
+a security issue) are considered undefined behavior from the C11 Standard point
+of view.
+
+Disabling the instrumentation
+-----------------------------
+
+A function can be marked with ``__no_kmsan_checks``. Doing so makes KMSAN
+ignore uninitialized values in that function and mark its output as initialized.
+As a result, the user will not get KMSAN reports related to that function.
+
+Another function attribute supported by KMSAN is ``__no_sanitize_memory``.
+Applying this attribute to a function will result in KMSAN not instrumenting
+it, which can be helpful if we do not want the compiler to interfere with some
+low-level code (e.g. that marked with ``noinstr`` which implicitly adds
+``__no_sanitize_memory``).
+
+This however comes at a cost: stack allocations from such functions will have
+incorrect shadow/origin values, likely leading to false positives. Functions
+called from non-instrumented code may also receive incorrect metadata for their
+parameters.
+
+As a rule of thumb, avoid using ``__no_sanitize_memory`` explicitly.
+
+It is also possible to disable KMSAN for a single file (e.g. main.o)::
+
+  KMSAN_SANITIZE_main.o := n
+
+or for the whole directory::
+
+  KMSAN_SANITIZE := n
+
+in the Makefile. Think of this as applying ``__no_sanitize_memory`` to every
+function in the file or directory. Most users won't need KMSAN_SANITIZE, unless
+their code gets broken by KMSAN (e.g. runs at early boot time).
+
+Support
+=======
+
+In order for KMSAN to work the kernel must be built with Clang, which so far is
+the only compiler that has KMSAN support. The kernel instrumentation pass is
+based on the userspace `MemorySanitizer tool`_.
+
+The runtime library only supports x86_64 at the moment.
+
+How KMSAN works
+===============
+
+KMSAN shadow memory
+-------------------
+
+KMSAN associates a metadata byte (also called shadow byte) with every byte of
+kernel memory. A bit in the shadow byte is set iff the corresponding bit of the
+kernel memory byte is uninitialized. Marking the memory uninitialized (i.e.
+setting its shadow bytes to ``0xff``) is called poisoning, marking it
+initialized (setting the shadow bytes to ``0x00``) is called unpoisoning.
+
+When a new variable is allocated on the stack, it is poisoned by default by
+instrumentation code inserted by the compiler (unless it is a stack variable
+that is immediately initialized). Any new heap allocation done without
+``__GFP_ZERO`` is also poisoned.
+
+Compiler instrumentation also tracks the shadow values as they are used along
+the code. When needed, instrumentation code invokes the runtime library in
+``mm/kmsan/`` to persist shadow values.
+
+The shadow value of a basic or compound type is an array of bytes of the same
+length. When a constant value is written into memory, that memory is unpoisoned.
+When a value is read from memory, its shadow memory is also obtained and
+propagated into all the operations which use that value. For every instruction
+that takes one or more values the compiler generates code that calculates the
+shadow of the result depending on those values and their shadows.
+
+Example::
+
+  int a = 0xff;  // i.e. 0x000000ff
+  int b;
+  int c = a | b;
+
+In this case the shadow of ``a`` is ``0``, shadow of ``b`` is ``0xffffffff``,
+shadow of ``c`` is ``0xffffff00``. This means that the upper three bytes of
+``c`` are uninitialized, while the lower byte is initialized.
+
+Origin tracking
+---------------
+
+Every four bytes of kernel memory also have a so-called origin mapped to them.
+This origin describes the point in program execution at which the uninitialized
+value was created. Every origin is associated with either the full allocation
+stack (for heap-allocated memory), or the function containing the uninitialized
+variable (for locals).
+
+When an uninitialized variable is allocated on stack or heap, a new origin
+value is created, and that variable's origin is filled with that value. When a
+value is read from memory, its origin is also read and kept together with the
+shadow. For every instruction that takes one or more values, the origin of the
+result is one of the origins corresponding to any of the uninitialized inputs.
+If a poisoned value is written into memory, its origin is written to the
+corresponding storage as well.
+
+Example 1::
+
+  int a = 42;
+  int b;
+  int c = a + b;
+
+In this case the origin of ``b`` is generated upon function entry, and is
+stored to the origin of ``c`` right before the addition result is written into
+memory.
+
+Several variables may share the same origin address, if they are stored in the
+same four-byte chunk. In this case every write to either variable updates the
+origin for all of them. We have to sacrifice precision in this case, because
+storing origins for individual bits (and even bytes) would be too costly.
+
+Example 2::
+
+  int combine(short a, short b) {
+    union ret_t {
+      int i;
+      short s[2];
+    } ret;
+    ret.s[0] = a;
+    ret.s[1] = b;
+    return ret.i;
+  }
+
+If ``a`` is initialized and ``b`` is not, the shadow of the result would be
+0xffff0000, and the origin of the result would be the origin of ``b``.
+``ret.s[0]`` would have the same origin, but it will never be used, because
+that variable is initialized.
+
+If both function arguments are uninitialized, only the origin of the second
+argument is preserved.
+
+Origin chaining
+~~~~~~~~~~~~~~~
+
+To ease debugging, KMSAN creates a new origin for every store of an
+uninitialized value to memory. The new origin references both its creation stack
+and the previous origin the value had. This may cause increased memory
+consumption, so we limit the length of origin chains in the runtime.
+
+Clang instrumentation API
+-------------------------
+
+Clang instrumentation pass inserts calls to functions defined in
+``mm/kmsan/nstrumentation.c`` into the kernel code.
+
+Shadow manipulation
+~~~~~~~~~~~~~~~~~~~
+
+For every memory access the compiler emits a call to a function that returns a
+pair of pointers to the shadow and origin addresses of the given memory::
+
+  typedef struct {
+    void *shadow, *origin;
+  } shadow_origin_ptr_t
+
+  shadow_origin_ptr_t __msan_metadata_ptr_for_load_{1,2,4,8}(void *addr)
+  shadow_origin_ptr_t __msan_metadata_ptr_for_store_{1,2,4,8}(void *addr)
+  shadow_origin_ptr_t __msan_metadata_ptr_for_load_n(void *addr, uintptr_t size)
+  shadow_origin_ptr_t __msan_metadata_ptr_for_store_n(void *addr, uintptr_t size)
+
+The function name depends on the memory access size.
+
+The compiler makes sure that for every loaded value its shadow and origin
+values are read from memory. When a value is stored to memory, its shadow and
+origin are also stored using the metadata pointers.
+
+Handling locals
+~~~~~~~~~~~~~~~
+
+A special function is used to create a new origin value for a local variable and
+set the origin of that variable to that value::
+
+  void __msan_poison_alloca(void *addr, uintptr_t size, char *descr)
+
+Access to per-task data
+~~~~~~~~~~~~~~~~~~~~~~~
+
+At the beginning of every instrumented function KMSAN inserts a call to
+``__msan_get_context_state()``::
+
+  kmsan_context_state *__msan_get_context_state(void)
+
+``kmsan_context_state`` is declared in ``include/linux/kmsan.h``::
+
+  struct kmsan_context_state {
+    char param_tls[KMSAN_PARAM_SIZE];
+    char retval_tls[KMSAN_RETVAL_SIZE];
+    char va_arg_tls[KMSAN_PARAM_SIZE];
+    char va_arg_origin_tls[KMSAN_PARAM_SIZE];
+    u64 va_arg_overflow_size_tls;
+    char param_origin_tls[KMSAN_PARAM_SIZE];
+    depot_stack_handle_t retval_origin_tls;
+  };
+
+This structure is used by KMSAN to pass parameter shadows and origins between
+instrumented functions (unless the parameters are checked immediately by
+``CONFIG_KMSAN_CHECK_PARAM_RETVAL``).
+
+Passing uninitialized values to functions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Clang's MemorySanitizer instrumentation has an option,
+``-fsanitize-memory-param-retval``, which makes the compiler check function
+parameters passed by value, as well as function return values.
+
+The option is controlled by ``CONFIG_KMSAN_CHECK_PARAM_RETVAL``, which is
+enabled by default to let KMSAN report uninitialized values earlier.
+Please refer to the `LKML discussion`_ for more details.
+
+Because of the way the checks are implemented in LLVM (they are only applied to
+parameters marked as ``noundef``), not all parameters are guaranteed to be
+checked, so we cannot give up the metadata storage in ``kmsan_context_state``.
+
+String functions
+~~~~~~~~~~~~~~~~
+
+The compiler replaces calls to ``memcpy()``/``memmove()``/``memset()`` with the
+following functions. These functions are also called when data structures are
+initialized or copied, making sure shadow and origin values are copied alongside
+with the data::
+
+  void *__msan_memcpy(void *dst, void *src, uintptr_t n)
+  void *__msan_memmove(void *dst, void *src, uintptr_t n)
+  void *__msan_memset(void *dst, int c, uintptr_t n)
+
+Error reporting
+~~~~~~~~~~~~~~~
+
+For each use of a value the compiler emits a shadow check that calls
+``__msan_warning()`` in the case that value is poisoned::
+
+  void __msan_warning(u32 origin)
+
+``__msan_warning()`` causes KMSAN runtime to print an error report.
+
+Inline assembly instrumentation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+KMSAN instruments every inline assembly output with a call to::
+
+  void __msan_instrument_asm_store(void *addr, uintptr_t size)
+
+, which unpoisons the memory region.
+
+This approach may mask certain errors, but it also helps to avoid a lot of
+false positives in bitwise operations, atomics etc.
+
+Sometimes the pointers passed into inline assembly do not point to valid memory.
+In such cases they are ignored at runtime.
+
+
+Runtime library
+---------------
+
+The code is located in ``mm/kmsan/``.
+
+Per-task KMSAN state
+~~~~~~~~~~~~~~~~~~~~
+
+Every task_struct has an associated KMSAN task state that holds the KMSAN
+context (see above) and a per-task flag disallowing KMSAN reports::
+
+  struct kmsan_context {
+    ...
+    bool allow_reporting;
+    struct kmsan_context_state cstate;
+    ...
+  }
+
+  struct task_struct {
+    ...
+    struct kmsan_context kmsan;
+    ...
+  }
+
+KMSAN contexts
+~~~~~~~~~~~~~~
+
+When running in a kernel task context, KMSAN uses ``current->kmsan.cstate`` to
+hold the metadata for function parameters and return values.
+
+But in the case the kernel is running in the interrupt, softirq or NMI context,
+where ``current`` is unavailable, KMSAN switches to per-cpu interrupt state::
+
+  DEFINE_PER_CPU(struct kmsan_ctx, kmsan_percpu_ctx);
+
+Metadata allocation
+~~~~~~~~~~~~~~~~~~~
+
+There are several places in the kernel for which the metadata is stored.
+
+1. Each ``struct page`` instance contains two pointers to its shadow and
+origin pages::
+
+  struct page {
+    ...
+    struct page *shadow, *origin;
+    ...
+  };
+
+At boot-time, the kernel allocates shadow and origin pages for every available
+kernel page. This is done quite late, when the kernel address space is already
+fragmented, so normal data pages may arbitrarily interleave with the metadata
+pages.
+
+This means that in general for two contiguous memory pages their shadow/origin
+pages may not be contiguous. Consequently, if a memory access crosses the
+boundary of a memory block, accesses to shadow/origin memory may potentially
+corrupt other pages or read incorrect values from them.
+
+In practice, contiguous memory pages returned by the same ``alloc_pages()``
+call will have contiguous metadata, whereas if these pages belong to two
+different allocations their metadata pages can be fragmented.
+
+For the kernel data (``.data``, ``.bss`` etc.) and percpu memory regions
+there also are no guarantees on metadata contiguity.
+
+In the case ``__msan_metadata_ptr_for_XXX_YYY()`` hits the border between two
+pages with non-contiguous metadata, it returns pointers to fake shadow/origin regions::
+
+  char dummy_load_page[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
+  char dummy_store_page[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
+
+``dummy_load_page`` is zero-initialized, so reads from it always yield zeroes.
+All stores to ``dummy_store_page`` are ignored.
+
+2. For vmalloc memory and modules, there is a direct mapping between the memory
+range, its shadow and origin. KMSAN reduces the vmalloc area by 3/4, making only
+the first quarter available to ``vmalloc()``. The second quarter of the vmalloc
+area contains shadow memory for the first quarter, the third one holds the
+origins. A small part of the fourth quarter contains shadow and origins for the
+kernel modules. Please refer to ``arch/x86/include/asm/pgtable_64_types.h`` for
+more details.
+
+When an array of pages is mapped into a contiguous virtual memory space, their
+shadow and origin pages are similarly mapped into contiguous regions.
+
+References
+==========
+
+E. Stepanov, K. Serebryany. `MemorySanitizer: fast detector of uninitialized
+memory use in C++
+<https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43308.pdf>`_.
+In Proceedings of CGO 2015.
+
+.. _MemorySanitizer tool: https://clang.llvm.org/docs/MemorySanitizer.html
+.. _LLVM documentation: https://llvm.org/docs/GettingStarted.html
+.. _LKML discussion: https://lore.kernel.org/all/20220614144853.3693273-1-glider@google.com/