[HWASan] Mention x86_64 aliasing mode in design doc.

Reviewed By: eugenis Differential Revision: https://reviews.llvm.org/D98892

Author: morehouse

clang/docs/HardwareAssistedAddressSanitizerDesign.rst

@@ -19,13 +19,17 @@ 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 the `Address Tagging`_ (or top-byte-ignore, TBI), a hardware feature that allows
-software to use 8 most significant bits of a 64-bit pointer as
+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
@@ -266,7 +270,15 @@ before every load and store by compiler instrumentation, but this variant
 will have limited deployability since not all of the code is
 typically instrumented.
 
-The HWASAN's approach is not applicable to 32-bit architectures.
+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
@@ -284,4 +296,4 @@ Related Work
 .. _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