clang/docs/HardwareAssistedAddressSanitizerDesign.rst - llvm-project - Git at Google

 =======================================================
 Hardware-assisted AddressSanitizer Design Documentation
 =======================================================

 This page is a design document for
 **hardware-assisted AddressSanitizer** (or **HWASAN**)
 a tool similar to :doc:`AddressSanitizer`,
 but based on partial hardware assistance.


 Introduction
 ============

 :doc:`AddressSanitizer`
 tags every 8 bytes of the application memory with a 1 byte tag (using *shadow memory*),
 uses *redzones* to find buffer-overflows and
 *quarantine* to find use-after-free.
 The redzones, the quarantine, and, to a less extent, the shadow, are the
 sources of AddressSanitizer's memory overhead.
 See the `AddressSanitizer paper`_ for details.

 AArch64 has `Address Tagging`_ (or top-byte-ignore, TBI), a hardware feature that allows
 software to use the 8 most significant bits of a 64-bit pointer as
 a tag. HWASAN uses `Address Tagging`_
 to implement a memory safety tool, similar to :doc:`AddressSanitizer`,
 but with smaller memory overhead and slightly different (mostly better)
 accuracy guarantees.

 Intel's `Linear Address Masking`_ (LAM) also provides address tagging for
 x86_64, though it is not widely available in hardware yet.  For x86_64, HWASAN
 has a limited implementation using page aliasing instead.

 Algorithm
 =========
 * Every heap/stack/global memory object is forcibly aligned by `TG` bytes
   (`TG` is e.g. 16 or 64). We call `TG` the **tagging granularity**.
 * For every such object a random `TS`-bit tag `T` is chosen (`TS`, or tag size, is e.g. 4 or 8)
 * The pointer to the object is tagged with `T`.
 * The memory for the object is also tagged with `T` (using a `TG=>1` shadow memory)
 * Every load and store is instrumented to read the memory tag and compare it
   with the pointer tag, exception is raised on tag mismatch.

 For a more detailed discussion of this approach see https://arxiv.org/pdf/1802.09517.pdf

 Short granules
 --------------

 A short granule is a granule of size between 1 and `TG-1` bytes. The size
 of a short granule is stored at the location in shadow memory where the
 granule's tag is normally stored, while the granule's actual tag is stored
 in the last byte of the granule. This means that in order to verify that a
 pointer tag matches a memory tag, HWASAN must check for two possibilities:

 * the pointer tag is equal to the memory tag in shadow memory, or
 * the shadow memory tag is actually a short granule size, the value being loaded
   is in bounds of the granule and the pointer tag is equal to the last byte of
   the granule.

 Pointer tags between 1 to `TG-1` are possible and are as likely as any other
 tag. This means that these tags in memory have two interpretations: the full
 tag interpretation (where the pointer tag is between 1 and `TG-1` and the
 last byte of the granule is ordinary data) and the short tag interpretation
 (where the pointer tag is stored in the granule).

 When HWASAN detects an error near a memory tag between 1 and `TG-1`, it
 will show both the memory tag and the last byte of the granule. Currently,
 it is up to the user to disambiguate the two possibilities.

 Instrumentation
 ===============

 Memory Accesses
 ---------------
 In the majority of cases, memory accesses are prefixed with a call to
 an outlined instruction sequence that verifies the tags. The code size
 and performance overhead of the call is reduced by using a custom calling
 convention that

 * preserves most registers, and
 * is specialized to the register containing the address, and the type and
   size of the memory access.

 Currently, the following sequence is used:

 .. code-block:: none

   // int foo(int *a) { return *a; }
   // clang -O2 --target=aarch64-linux-android30 -fsanitize=hwaddress -S -o - load.c
   [...]
   foo:
         stp     x30, x20, [sp, #-16]!
         adrp    x20, :got:__hwasan_shadow               // load shadow address from GOT into x20
         ldr     x20, [x20, :got_lo12:__hwasan_shadow]
         bl      __hwasan_check_x0_2_short_v2            // call outlined tag check
                                                         // (arguments: x0 = address, x20 = shadow base;
                                                         // "2" encodes the access type and size)
         ldr     w0, [x0]                                // inline load
         ldp     x30, x20, [sp], #16
         ret

   [...]
   __hwasan_check_x0_2_short_v2:
         sbfx    x16, x0, #4, #52                        // shadow offset
         ldrb    w16, [x20, x16]                         // load shadow tag
         cmp     x16, x0, lsr #56                        // extract address tag, compare with shadow tag
         b.ne    .Ltmp0                                  // jump to short tag handler on mismatch
   .Ltmp1:
         ret
   .Ltmp0:
         cmp     w16, #15                                // is this a short tag?
         b.hi    .Ltmp2                                  // if not, error
         and     x17, x0, #0xf                           // find the address's position in the short granule
         add     x17, x17, #3                            // adjust to the position of the last byte loaded
         cmp     w16, w17                                // check that position is in bounds
         b.ls    .Ltmp2                                  // if not, error
         orr     x16, x0, #0xf                           // compute address of last byte of granule
         ldrb    w16, [x16]                              // load tag from it
         cmp     x16, x0, lsr #56                        // compare with pointer tag
         b.eq    .Ltmp1                                  // if matches, continue
   .Ltmp2:
         stp     x0, x1, [sp, #-256]!                    // save original x0, x1 on stack (they will be overwritten)
         stp     x29, x30, [sp, #232]                    // create frame record
         mov     x1, #2                                  // set x1 to a constant indicating the type of failure
         adrp    x16, :got:__hwasan_tag_mismatch_v2      // call runtime function to save remaining registers and report error
         ldr     x16, [x16, :got_lo12:__hwasan_tag_mismatch_v2] // (load address from GOT to avoid potential register clobbers in delay load handler)
         br      x16

 Heap
 ----

 Tagging the heap memory/pointers is done by `malloc`.
 This can be based on any malloc that forces all objects to be TG-aligned.
 `free` tags the memory with a different tag.

 Stack
 -----

 Stack frames are instrumented by aligning all non-promotable allocas
 by `TG` and tagging stack memory in function prologue and epilogue.

 Tags for different allocas in one function are **not** generated
 independently; doing that in a function with `M` allocas would require
 maintaining `M` live stack pointers, significantly increasing register
 pressure. Instead we generate a single base tag value in the prologue,
 and build the tag for alloca number `M` as `ReTag(BaseTag, M)`, where
 ReTag can be as simple as exclusive-or with constant `M`.

 Stack instrumentation is expected to be a major source of overhead,
 but could be optional.

 Globals
 -------

 Most globals in HWASAN instrumented code are tagged. This is accomplished
 using the following mechanisms:

   * The address of each global has a static tag associated with it. The first
     defined global in a translation unit has a pseudorandom tag associated
     with it, based on the hash of the file path. Subsequent global tags are
     incremental from the previously-assigned tag.

   * The global's tag is added to its symbol address in the object file's symbol
     table. This causes the global's address to be tagged when its address is
     taken.

   * When the address of a global is taken directly (i.e. not via the GOT), a special
     instruction sequence needs to be used to add the tag to the address,
     because the tag would otherwise take the address outside of the small code
     model (4GB on AArch64). No changes are required when the address is taken
     via the GOT because the address stored in the GOT will contain the tag.

   * An associated ``hwasan_globals`` section is emitted for each tagged global,
     which indicates the address of the global, its size and its tag.  These
     sections are concatenated by the linker into a single ``hwasan_globals``
     section that is enumerated by the runtime (via an ELF note) when a binary
     is loaded and the memory is tagged accordingly.

 A complete example is given below:

 .. code-block:: none

   // int x = 1; int *f() { return &x; }
   // clang -O2 --target=aarch64-linux-android30 -fsanitize=hwaddress -S -o - global.c

   [...]
   f:
         adrp    x0, :pg_hi21_nc:x            // set bits 12-63 to upper bits of untagged address
         movk    x0, #:prel_g3:x+0x100000000  // set bits 48-63 to tag
         add     x0, x0, :lo12:x              // set bits 0-11 to lower bits of address
         ret

   [...]
         .data
   .Lx.hwasan:
         .word   1

         .globl  x
         .set x, .Lx.hwasan+0x2d00000000000000

   [...]
         .section        .note.hwasan.globals,"aG",@note,hwasan.module_ctor,comdat
   .Lhwasan.note:
         .word   8                            // namesz
         .word   8                            // descsz
         .word   3                            // NT_LLVM_HWASAN_GLOBALS
         .asciz  "LLVM\000\000\000"
         .word   __start_hwasan_globals-.Lhwasan.note
         .word   __stop_hwasan_globals-.Lhwasan.note

   [...]
         .section        hwasan_globals,"ao",@progbits,.Lx.hwasan,unique,2
   .Lx.hwasan.descriptor:
         .word   .Lx.hwasan-.Lx.hwasan.descriptor
         .word   0x2d000004                   // tag = 0x2d, size = 4

 Error reporting
 ---------------

 Errors are generated by the `HLT` instruction and are handled by a signal handler.

 Attribute
 ---------

 HWASAN uses its own LLVM IR Attribute `sanitize_hwaddress` and a matching
 C function attribute. An alternative would be to re-use ASAN's attribute
 `sanitize_address`. The reasons to use a separate attribute are:

   * Users may need to disable ASAN but not HWASAN, or vise versa,
     because the tools have different trade-offs and compatibility issues.
   * LLVM (ideally) does not use flags to decide which pass is being used,
     ASAN or HWASAN are being applied, based on the function attributes.

 This does mean that users of HWASAN may need to add the new attribute
 to the code that already uses the old attribute.


 Comparison with AddressSanitizer
 ================================

 HWASAN:
   * Is less portable than :doc:`AddressSanitizer`
     as it relies on hardware `Address Tagging`_ (AArch64).
     Address Tagging can be emulated with compiler instrumentation,
     but it will require the instrumentation to remove the tags before
     any load or store, which is infeasible in any realistic environment
     that contains non-instrumented code.
   * May have compatibility problems if the target code uses higher
     pointer bits for other purposes.
   * May require changes in the OS kernels (e.g. Linux seems to dislike
     tagged pointers passed from address space:
     https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt).
   * **Does not require redzones to detect buffer overflows**,
     but the buffer overflow detection is probabilistic, with roughly
     `1/(2**TS)` chance of missing a bug (6.25% or 0.39% with 4 and 8-bit TS
     respectively).
   * **Does not require quarantine to detect heap-use-after-free,
     or stack-use-after-return**.
     The detection is similarly probabilistic.

 The memory overhead of HWASAN is expected to be much smaller
 than that of AddressSanitizer:
 `1/TG` extra memory for the shadow
 and some overhead due to `TG`-aligning all objects.

 Security Considerations
 =======================

 HWASAN is a bug detection tool and its runtime is not meant to be
 linked against production executables. While it may be useful for testing,
 HWASAN's runtime was not developed with security-sensitive
 constraints in mind and may compromise the security of the resulting executable.

 Supported architectures
 =======================
 HWASAN relies on `Address Tagging`_ which is only available on AArch64.
 For other 64-bit architectures it is possible to remove the address tags
 before every load and store by compiler instrumentation, but this variant
 will have limited deployability since not all of the code is
 typically instrumented.

 On x86_64, HWASAN utilizes page aliasing to place tags in userspace address
 bits.  Currently only heap tagging is supported.  The page aliases rely on
 shared memory, which will cause heap memory to be shared between processes if
 the application calls ``fork()``.  Therefore x86_64 is really only safe for
 applications that do not fork.

 HWASAN does not currently support 32-bit architectures since they do not
 support `Address Tagging`_ and the address space is too constrained to easily
 implement page aliasing.


 Related Work
 ============
 * `SPARC ADI`_ implements a similar tool mostly in hardware.
 * `Effective and Efficient Memory Protection Using Dynamic Tainting`_ discusses
   similar approaches ("lock & key").
 * `Watchdog`_ discussed a heavier, but still somewhat similar
   "lock & key" approach.
 * *TODO: add more "related work" links. Suggestions are welcome.*


 .. _Watchdog: https://www.cis.upenn.edu/acg/papers/isca12_watchdog.pdf
 .. _Effective and Efficient Memory Protection Using Dynamic Tainting: https://www.cc.gatech.edu/~orso/papers/clause.doudalis.orso.prvulovic.pdf
 .. _SPARC ADI: https://lazytyped.blogspot.com/2017/09/getting-started-with-adi.html
 .. _AddressSanitizer paper: https://www.usenix.org/system/files/conference/atc12/atc12-final39.pdf
 .. _Address Tagging: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0024a/ch12s05s01.html
 .. _Linear Address Masking: https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html
	=======================================================
	Hardware-assisted AddressSanitizer Design Documentation
	=======================================================

	This page is a design document for
	hardware-assisted AddressSanitizer (or HWASAN)
	a tool similar to :doc:`AddressSanitizer`,
	but based on partial hardware assistance.


	Introduction
	============

	:doc:`AddressSanitizer`
	tags every 8 bytes of the application memory with a 1 byte tag (using shadow memory),
	uses redzones to find buffer-overflows and
	quarantine to find use-after-free.
	The redzones, the quarantine, and, to a less extent, the shadow, are the
	sources of AddressSanitizer's memory overhead.
	See the `AddressSanitizer paper`_ for details.

	AArch64 has `Address Tagging`_ (or top-byte-ignore, TBI), a hardware feature that allows
	software to use the 8 most significant bits of a 64-bit pointer as
	a tag. HWASAN uses `Address Tagging`_
	to implement a memory safety tool, similar to :doc:`AddressSanitizer`,
	but with smaller memory overhead and slightly different (mostly better)
	accuracy guarantees.

	Intel's `Linear Address Masking`_ (LAM) also provides address tagging for
	x86_64, though it is not widely available in hardware yet. For x86_64, HWASAN
	has a limited implementation using page aliasing instead.

	Algorithm
	=========
	* Every heap/stack/global memory object is forcibly aligned by `TG` bytes
	(`TG` is e.g. 16 or 64). We call `TG` the tagging granularity.
	* For every such object a random `TS`-bit tag `T` is chosen (`TS`, or tag size, is e.g. 4 or 8)
	* The pointer to the object is tagged with `T`.
	* The memory for the object is also tagged with `T` (using a `TG=>1` shadow memory)
	* Every load and store is instrumented to read the memory tag and compare it
	with the pointer tag, exception is raised on tag mismatch.

	For a more detailed discussion of this approach see https://arxiv.org/pdf/1802.09517.pdf

	Short granules
	--------------

	A short granule is a granule of size between 1 and `TG-1` bytes. The size
	of a short granule is stored at the location in shadow memory where the
	granule's tag is normally stored, while the granule's actual tag is stored
	in the last byte of the granule. This means that in order to verify that a
	pointer tag matches a memory tag, HWASAN must check for two possibilities:

	* the pointer tag is equal to the memory tag in shadow memory, or
	* the shadow memory tag is actually a short granule size, the value being loaded
	is in bounds of the granule and the pointer tag is equal to the last byte of
	the granule.

	Pointer tags between 1 to `TG-1` are possible and are as likely as any other
	tag. This means that these tags in memory have two interpretations: the full
	tag interpretation (where the pointer tag is between 1 and `TG-1` and the
	last byte of the granule is ordinary data) and the short tag interpretation
	(where the pointer tag is stored in the granule).

	When HWASAN detects an error near a memory tag between 1 and `TG-1`, it
	will show both the memory tag and the last byte of the granule. Currently,
	it is up to the user to disambiguate the two possibilities.

	Instrumentation
	===============

	Memory Accesses
	---------------
	In the majority of cases, memory accesses are prefixed with a call to
	an outlined instruction sequence that verifies the tags. The code size
	and performance overhead of the call is reduced by using a custom calling
	convention that

	* preserves most registers, and
	* is specialized to the register containing the address, and the type and
	size of the memory access.

	Currently, the following sequence is used:

	.. code-block:: none

	// int foo(int a) { return a; }
	// clang -O2 --target=aarch64-linux-android30 -fsanitize=hwaddress -S -o - load.c
	[...]
	foo:
	stp x30, x20, [sp, #-16]!
	adrp x20, :got:__hwasan_shadow // load shadow address from GOT into x20
	ldr x20, [x20, :got_lo12:__hwasan_shadow]
	bl __hwasan_check_x0_2_short_v2 // call outlined tag check
	// (arguments: x0 = address, x20 = shadow base;
	// "2" encodes the access type and size)
	ldr w0, [x0] // inline load
	ldp x30, x20, [sp], #16
	ret

	[...]
	__hwasan_check_x0_2_short_v2:
	sbfx x16, x0, #4, #52 // shadow offset
	ldrb w16, [x20, x16] // load shadow tag
	cmp x16, x0, lsr #56 // extract address tag, compare with shadow tag
	b.ne .Ltmp0 // jump to short tag handler on mismatch
	.Ltmp1:
	ret
	.Ltmp0:
	cmp w16, #15 // is this a short tag?
	b.hi .Ltmp2 // if not, error
	and x17, x0, #0xf // find the address's position in the short granule
	add x17, x17, #3 // adjust to the position of the last byte loaded
	cmp w16, w17 // check that position is in bounds
	b.ls .Ltmp2 // if not, error
	orr x16, x0, #0xf // compute address of last byte of granule
	ldrb w16, [x16] // load tag from it
	cmp x16, x0, lsr #56 // compare with pointer tag
	b.eq .Ltmp1 // if matches, continue
	.Ltmp2:
	stp x0, x1, [sp, #-256]! // save original x0, x1 on stack (they will be overwritten)
	stp x29, x30, [sp, #232] // create frame record
	mov x1, #2 // set x1 to a constant indicating the type of failure
	adrp x16, :got:__hwasan_tag_mismatch_v2 // call runtime function to save remaining registers and report error
	ldr x16, [x16, :got_lo12:__hwasan_tag_mismatch_v2] // (load address from GOT to avoid potential register clobbers in delay load handler)
	br x16

	Heap
	----

	Tagging the heap memory/pointers is done by `malloc`.
	This can be based on any malloc that forces all objects to be TG-aligned.
	`free` tags the memory with a different tag.

	Stack
	-----

	Stack frames are instrumented by aligning all non-promotable allocas
	by `TG` and tagging stack memory in function prologue and epilogue.

	Tags for different allocas in one function are not generated
	independently; doing that in a function with `M` allocas would require
	maintaining `M` live stack pointers, significantly increasing register
	pressure. Instead we generate a single base tag value in the prologue,
	and build the tag for alloca number `M` as `ReTag(BaseTag, M)`, where
	ReTag can be as simple as exclusive-or with constant `M`.

	Stack instrumentation is expected to be a major source of overhead,
	but could be optional.

	Globals
	-------

	Most globals in HWASAN instrumented code are tagged. This is accomplished
	using the following mechanisms:

	* The address of each global has a static tag associated with it. The first
	defined global in a translation unit has a pseudorandom tag associated
	with it, based on the hash of the file path. Subsequent global tags are
	incremental from the previously-assigned tag.

	* The global's tag is added to its symbol address in the object file's symbol
	table. This causes the global's address to be tagged when its address is
	taken.

	* When the address of a global is taken directly (i.e. not via the GOT), a special
	instruction sequence needs to be used to add the tag to the address,
	because the tag would otherwise take the address outside of the small code
	model (4GB on AArch64). No changes are required when the address is taken
	via the GOT because the address stored in the GOT will contain the tag.

	* An associated ``hwasan_globals`` section is emitted for each tagged global,
	which indicates the address of the global, its size and its tag. These
	sections are concatenated by the linker into a single ``hwasan_globals``
	section that is enumerated by the runtime (via an ELF note) when a binary
	is loaded and the memory is tagged accordingly.

	A complete example is given below:

	.. code-block:: none

	// int x = 1; int *f() { return &x; }
	// clang -O2 --target=aarch64-linux-android30 -fsanitize=hwaddress -S -o - global.c

	[...]
	f:
	adrp x0, :pg_hi21_nc:x // set bits 12-63 to upper bits of untagged address
	movk x0, #:prel_g3:x+0x100000000 // set bits 48-63 to tag
	add x0, x0, :lo12:x // set bits 0-11 to lower bits of address
	ret

	[...]
	.data
	.Lx.hwasan:
	.word 1

	.globl x
	.set x, .Lx.hwasan+0x2d00000000000000

	[...]
	.section .note.hwasan.globals,"aG",@note,hwasan.module_ctor,comdat
	.Lhwasan.note:
	.word 8 // namesz
	.word 8 // descsz
	.word 3 // NT_LLVM_HWASAN_GLOBALS
	.asciz "LLVM\000\000\000"
	.word __start_hwasan_globals-.Lhwasan.note
	.word __stop_hwasan_globals-.Lhwasan.note

	[...]
	.section hwasan_globals,"ao",@progbits,.Lx.hwasan,unique,2
	.Lx.hwasan.descriptor:
	.word .Lx.hwasan-.Lx.hwasan.descriptor
	.word 0x2d000004 // tag = 0x2d, size = 4

	Error reporting
	---------------

	Errors are generated by the `HLT` instruction and are handled by a signal handler.

	Attribute
	---------

	HWASAN uses its own LLVM IR Attribute `sanitize_hwaddress` and a matching
	C function attribute. An alternative would be to re-use ASAN's attribute
	`sanitize_address`. The reasons to use a separate attribute are:

	* Users may need to disable ASAN but not HWASAN, or vise versa,
	because the tools have different trade-offs and compatibility issues.
	* LLVM (ideally) does not use flags to decide which pass is being used,
	ASAN or HWASAN are being applied, based on the function attributes.

	This does mean that users of HWASAN may need to add the new attribute
	to the code that already uses the old attribute.


	Comparison with AddressSanitizer
	================================

	HWASAN:
	* Is less portable than :doc:`AddressSanitizer`
	as it relies on hardware `Address Tagging`_ (AArch64).
	Address Tagging can be emulated with compiler instrumentation,
	but it will require the instrumentation to remove the tags before
	any load or store, which is infeasible in any realistic environment
	that contains non-instrumented code.
	* May have compatibility problems if the target code uses higher
	pointer bits for other purposes.
	* May require changes in the OS kernels (e.g. Linux seems to dislike
	tagged pointers passed from address space:
	https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt).
	* Does not require redzones to detect buffer overflows,
	but the buffer overflow detection is probabilistic, with roughly
	`1/(2**TS)` chance of missing a bug (6.25% or 0.39% with 4 and 8-bit TS
	respectively).
	* **Does not require quarantine to detect heap-use-after-free,
	or stack-use-after-return**.
	The detection is similarly probabilistic.

	The memory overhead of HWASAN is expected to be much smaller
	than that of AddressSanitizer:
	`1/TG` extra memory for the shadow
	and some overhead due to `TG`-aligning all objects.

	Security Considerations
	=======================

	HWASAN is a bug detection tool and its runtime is not meant to be
	linked against production executables. While it may be useful for testing,
	HWASAN's runtime was not developed with security-sensitive
	constraints in mind and may compromise the security of the resulting executable.

	Supported architectures
	=======================
	HWASAN relies on `Address Tagging`_ which is only available on AArch64.
	For other 64-bit architectures it is possible to remove the address tags
	before every load and store by compiler instrumentation, but this variant
	will have limited deployability since not all of the code is
	typically instrumented.

	On x86_64, HWASAN utilizes page aliasing to place tags in userspace address
	bits. Currently only heap tagging is supported. The page aliases rely on
	shared memory, which will cause heap memory to be shared between processes if
	the application calls ``fork()``. Therefore x86_64 is really only safe for
	applications that do not fork.

	HWASAN does not currently support 32-bit architectures since they do not
	support `Address Tagging`_ and the address space is too constrained to easily
	implement page aliasing.


	Related Work
	============
	* `SPARC ADI`_ implements a similar tool mostly in hardware.
	* `Effective and Efficient Memory Protection Using Dynamic Tainting`_ discusses
	similar approaches ("lock & key").
	* `Watchdog`_ discussed a heavier, but still somewhat similar
	"lock & key" approach.
	* TODO: add more "related work" links. Suggestions are welcome.


	.. _Watchdog: https://www.cis.upenn.edu/acg/papers/isca12_watchdog.pdf
	.. _Effective and Efficient Memory Protection Using Dynamic Tainting: https://www.cc.gatech.edu/~orso/papers/clause.doudalis.orso.prvulovic.pdf
	.. _SPARC ADI: https://lazytyped.blogspot.com/2017/09/getting-started-with-adi.html
	.. _AddressSanitizer paper: https://www.usenix.org/system/files/conference/atc12/atc12-final39.pdf
	.. _Address Tagging: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0024a/ch12s05s01.html
	.. _Linear Address Masking: https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html