From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Memory Tagging Extension (part of the ARMv8.5 Extensions) provides
a mechanism to detect the sources of memory related errors which
may be vulnerable to exploitation, including bounds violations,
use-after-free, use-after-return, use-out-of-scope and use before
initialization errors.
Add Memory Tagging Extension documentation for the arm64 linux
kernel support.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
---
Notes:
v2:
- Documented the uaccess kernel tag checking mode.
- Removed the BTI definitions from cpu-feature-registers.rst.
- Removed the paragraph stating that MTE depends on the tagged address
ABI (while the Kconfig entry does, there is no requirement for the
user to enable both).
- Changed the GCR_EL1.Exclude handling description following the change
in the prctl() interface (include vs exclude mask).
- Updated the example code.
Documentation/arm64/cpu-feature-registers.rst | 2 +
Documentation/arm64/elf_hwcaps.rst | 5 +
Documentation/arm64/index.rst | 1 +
.../arm64/memory-tagging-extension.rst | 228 ++++++++++++++++++
4 files changed, 236 insertions(+)
create mode 100644 Documentation/arm64/memory-tagging-extension.rst
diff --git a/Documentation/arm64/cpu-feature-registers.rst b/Documentation/arm64/cpu-feature-registers.rst
index 41937a8091aa..b5679fa85ad9 100644
--- a/Documentation/arm64/cpu-feature-registers.rst
+++ b/Documentation/arm64/cpu-feature-registers.rst
@@ -174,6 +174,8 @@ infrastructure:
+------------------------------+---------+---------+
| Name | bits | visible |
+------------------------------+---------+---------+
+ | MTE | [11-8] | y |
+ +------------------------------+---------+---------+
| SSBS | [7-4] | y |
+------------------------------+---------+---------+
diff --git a/Documentation/arm64/elf_hwcaps.rst b/Documentation/arm64/elf_hwcaps.rst
index 7dfb97dfe416..ca7f90e99e3a 100644
--- a/Documentation/arm64/elf_hwcaps.rst
+++ b/Documentation/arm64/elf_hwcaps.rst
@@ -236,6 +236,11 @@ HWCAP2_RNG
Functionality implied by ID_AA64ISAR0_EL1.RNDR == 0b0001.
+HWCAP2_MTE
+
+ Functionality implied by ID_AA64PFR1_EL1.MTE == 0b0010, as described
+ by Documentation/arm64/memory-tagging-extension.rst.
+
4. Unused AT_HWCAP bits
-----------------------
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index 5c0c69dc58aa..82970c6d384f 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -13,6 +13,7 @@ ARM64 Architecture
hugetlbpage
legacy_instructions
memory
+ memory-tagging-extension
pointer-authentication
silicon-errata
sve
diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
new file mode 100644
index 000000000000..00ac0e22d5e9
--- /dev/null
+++ b/Documentation/arm64/memory-tagging-extension.rst
@@ -0,0 +1,228 @@
+===============================================
+Memory Tagging Extension (MTE) in AArch64 Linux
+===============================================
+
+Authors: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
+ Catalin Marinas <catalin.marinas@xxxxxxx>
+
+Date: 2020-02-25
+
+This document describes the provision of the Memory Tagging Extension
+functionality in AArch64 Linux.
+
+Introduction
+============
+
+ARMv8.5 based processors introduce the Memory Tagging Extension (MTE)
+feature. MTE is built on top of the ARMv8.0 virtual address tagging TBI
+(Top Byte Ignore) feature and allows software to access a 4-bit
+allocation tag for each 16-byte granule in the physical address space.
+Such memory range must be mapped with the Normal-Tagged memory
+attribute. A logical tag is derived from bits 59-56 of the virtual
+address used for the memory access. A CPU with MTE enabled will compare
+the logical tag against the allocation tag and potentially raise an
+exception on mismatch, subject to system registers configuration.
+
+Userspace Support
+=================
+
+When ``CONFIG_ARM64_MTE`` is selected and Memory Tagging Extension is
+supported by the hardware, the kernel advertises the feature to
+userspace via ``HWCAP2_MTE``.
+
+PROT_MTE
+--------
+
+To access the allocation tags, a user process must enable the Tagged
+memory attribute on an address range using a new ``prot`` flag for
+``mmap()`` and ``mprotect()``:
+
+``PROT_MTE`` - Pages allow access to the MTE allocation tags.
+
+The allocation tag is set to 0 when such pages are first mapped in the
+user address space and preserved on copy-on-write. ``MAP_SHARED`` is
+supported and the allocation tags can be shared between processes.
+
+**Note**: ``PROT_MTE`` is only supported on ``MAP_ANONYMOUS`` and
+RAM-based file mappings (``tmpfs``, ``memfd``). Passing it to other
+types of mapping will result in ``-EINVAL`` returned by these system
+calls.
+
+**Note**: The ``PROT_MTE`` flag (and corresponding memory type) cannot
+be cleared by ``mprotect()``.
+
+Tag Check Faults
+----------------
+
+When ``PROT_MTE`` is enabled on an address range and a mismatch between
+the logical and allocation tags occurs on access, there are three
+configurable behaviours:
+
+- *Ignore* - This is the default mode. The CPU (and kernel) ignores the
+ tag check fault.
+
+- *Synchronous* - The kernel raises a ``SIGSEGV`` synchronously, with
+ ``.si_code = SEGV_MTESERR`` and ``.si_addr = <fault-address>``. The
+ memory access is not performed.
+
+- *Asynchronous* - The kernel raises a ``SIGSEGV``, in the current
+ thread, asynchronously following one or multiple tag check faults,
+ with ``.si_code = SEGV_MTEAERR`` and ``.si_addr = 0``.
+
+**Note**: There are no *match-all* logical tags available for user
+applications.
+
+The user can select the above modes, per thread, using the
+``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where
+``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK``
+bit-field:
+
+- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults
+- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode
+- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
+
+Tag checking can also be disabled for a user thread by setting the
+``PSTATE.TCO`` bit with ``MSR TCO, #1``.
+
+**Note**: Signal handlers are always invoked with ``PSTATE.TCO = 0``,
+irrespective of the interrupted context.
+
+**Note**: Kernel accesses to user memory (e.g. ``read()`` system call)
+follow the same tag checking mode as set by the current thread.
+
+Excluding Tags in the ``IRG``, ``ADDG`` and ``SUBG`` instructions
+-----------------------------------------------------------------
+
+The architecture allows excluding certain tags to be randomly generated
+via the ``GCR_EL1.Exclude`` register bit-field. By default, Linux
+excludes all tags other than 0. A user thread can enable specific tags
+in the randomly generated set using the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
+flags, 0, 0, 0)`` system call where ``flags`` contains the tags bitmap
+in the ``PR_MTE_TAG_MASK`` bit-field.
+
+**Note**: The hardware uses an exclude mask but the ``prctl()``
+interface provides an include mask.
+
+Example of correct usage
+========================
+
+*MTE Example code*
+
+.. code-block:: c
+
+ /*
+ * To be compiled with -march=armv8.5-a+memtag
+ */
+ #include <errno.h>
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <unistd.h>
+ #include <sys/auxv.h>
+ #include <sys/mman.h>
+ #include <sys/prctl.h>
+
+ /*
+ * From arch/arm64/include/uapi/asm/hwcap.h
+ */
+ #define HWCAP2_MTE (1 << 18)
+
+ /*
+ * From arch/arm64/include/uapi/asm/mman.h
+ */
+ #define PROT_MTE 0x20
+
+ /*
+ * From include/uapi/linux/prctl.h
+ */
+ #define PR_SET_TAGGED_ADDR_CTRL 55
+ #define PR_GET_TAGGED_ADDR_CTRL 56
+ # define PR_TAGGED_ADDR_ENABLE (1UL << 0)
+ # define PR_MTE_TCF_SHIFT 1
+ # define PR_MTE_TCF_NONE (0UL << PR_MTE_TCF_SHIFT)
+ # define PR_MTE_TCF_SYNC (1UL << PR_MTE_TCF_SHIFT)
+ # define PR_MTE_TCF_ASYNC (2UL << PR_MTE_TCF_SHIFT)
+ # define PR_MTE_TCF_MASK (3UL << PR_MTE_TCF_SHIFT)
+ # define PR_MTE_TAG_SHIFT 3
+ # define PR_MTE_TAG_MASK (0xffffUL << PR_MTE_TAG_SHIFT)
+
+ /*
+ * Insert a random logical tag into the given pointer.
+ */
+ #define insert_random_tag(ptr) ({ \
+ __u64 __val; \
+ asm("irg %0, %1" : "=r" (__val) : "r" (ptr)); \
+ __val; \
+ })
+
+ /*
+ * Set the allocation tag on the destination address.
+ */
+ #define set_tag(tagged_addr) do { \
+ asm volatile("stg %0, [%0]" : : "r" (tagged_addr) : "memory"); \
+ } while (0)
+
+ int main()
+ {
+ unsigned long *a;
+ unsigned long page_sz = getpagesize();
+ unsigned long hwcap2 = getauxval(AT_HWCAP2);
+
+ /* check if MTE is present */
+ if (!(hwcap2 & HWCAP2_MTE))
+ return -1;
+
+ /*
+ * Enable the tagged address ABI, synchronous MTE tag check faults and
+ * allow all non-zero tags in the randomly generated set.
+ */
+ if (prctl(PR_SET_TAGGED_ADDR_CTRL,
+ PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | (0xfffe << PR_MTE_TAG_SHIFT),
+ 0, 0, 0)) {
+ perror("prctl() failed");
+ return -1;
+ }
+
+ a = mmap(0, page_sz, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ if (a == MAP_FAILED) {
+ perror("mmap() failed");
+ return -1;
+ }
+
+ /*
+ * Enable MTE on the above anonymous mmap. The flag could be passed
+ * directly to mmap() and skip this step.
+ */
+ if (mprotect(a, page_sz, PROT_READ | PROT_WRITE | PROT_MTE)) {
+ perror("mprotect() failed");
+ return -1;
+ }
+
+ /* access with the default tag (0) */
+ a[0] = 1;
+ a[1] = 2;
+
+ printf("a[0] = %lu a[1] = %lu\n", a[0], a[1]);
+
+ /* set the logical and allocation tags */
+ a = (unsigned long *)insert_random_tag(a);
+ set_tag(a);
+
+ printf("%p\n", a);
+
+ /* non-zero tag access */
+ a[0] = 3;
+ printf("a[0] = %lu a[1] = %lu\n", a[0], a[1]);
+
+ /*
+ * If MTE is enabled correctly the next instruction will generate an
+ * exception.
+ */
+ printf("Expecting SIGSEGV...\n");
+ a[2] = 0xdead;
+
+ /* this should not be printed in the PR_MTE_TCF_SYNC mode */
+ printf("...done\n");
+
+ return 0;
+ }
