[PATCH v6 1/2] arm64: Define Documentation/arm64/tagged-address-abi.rst

[Vincenzo Frascino]

On arm64 the TCR_EL1.TBI0 bit has been always enabled hence
the userspace (EL0) is allowed to set a non-zero value in the
top byte but the resulting pointers are not allowed at the
user-kernel syscall ABI boundary.

With the relaxed ABI proposed through this document, it is now possible
to pass tagged pointers to the syscalls, when these pointers are in
memory ranges obtained by an anonymous (MAP_ANONYMOUS) mmap().

This change in the ABI requires a mechanism to requires the userspace
to opt-in to such an option.

Specify and document the way in which sysctl and prctl() can be used
in combination to allow the userspace to opt-in this feature.

Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
CC: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Acked-by: Szabolcs Nagy <szabolcs.nagy@xxxxxxx>
---
 Documentation/arm64/tagged-address-abi.rst | 148 +++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 Documentation/arm64/tagged-address-abi.rst

diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
new file mode 100644
index 000000000000..a8ecb991de82
--- /dev/null
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@@ -0,0 +1,148 @@
+========================
+ARM64 TAGGED ADDRESS ABI
+========================
+
+Author: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
+
+Date: 25 July 2019
+
+This document describes the usage and semantics of the Tagged Address
+ABI on arm64.
+
+1. Introduction
+---------------
+
+On arm64 the TCR_EL1.TBI0 bit has always been enabled on the kernel, hence
+the userspace (EL0) is entitled to perform a user memory access through a
+64-bit pointer with a non-zero top byte but the resulting pointers are not
+allowed at the user-kernel syscall ABI boundary.
+
+This document describes a relaxation of the ABI that makes it possible to
+to pass tagged pointers to the syscalls, when these pointers are in memory
+ranges obtained as described in section 2.
+
+Since it is not desirable to relax the ABI to allow tagged user addresses
+into the kernel indiscriminately, arm64 provides a new sysctl interface
+(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
+enabling the relaxed ABI and a new prctl() interface that can be used to
+enable or disable the relaxed ABI.
+A detailed description of the newly introduced mechanisms will be provided
+in section 2.
+
+2. ARM64 Tagged Address ABI
+---------------------------
+
+From the kernel syscall interface perspective, we define, for the purposes
+of this document, a "valid tagged pointer" as a pointer that either has a
+zero value set in the top byte or has a non-zero value, is in memory ranges
+privately owned by a userspace process and is obtained in one of the
+following ways:
+- mmap() done by the process itself, where either:
+
+  - flags have **MAP_PRIVATE** and **MAP_ANONYMOUS**
+  - flags have **MAP_PRIVATE** and the file descriptor refers to a regular
+    file or **/dev/zero**
+
+- brk() system call done by the process itself (i.e. the heap area between
+  the initial location of the program break at process creation and its
+  current location).
+- any memory mapped by the kernel in the process's address space during
+  creation and with the same restrictions as for mmap() (e.g. data, bss,
+  stack).
+
+The ARM64 Tagged Address ABI is an opt-in feature, and an application can
+control it using the following:
+
+- **/proc/sys/abi/tagged_addr**: a new sysctl interface that can be used to
+  prevent the applications from enabling the access to the relaxed ABI.
+  The sysctl supports the following configuration options:
+
+  - **0**: Disable the access to the ARM64 Tagged Address ABI for all
+    the applications.
+  - **1** (Default): Enable the access to the ARM64 Tagged Address ABI for
+    all the applications.
+
+   If the access to the ARM64 Tagged Address ABI is disabled at a certain
+   point in time, all the applications that were using tagging before this
+   event occurs, will continue to use tagging.
+- **prctl()s**:
+
+  - **PR_SET_TAGGED_ADDR_CTRL**: Invoked by a process, can be used to enable or
+    disable its access to the ARM64 Tagged Address ABI.
+
+    The (unsigned int) arg2 argument is a bit mask describing the control mode
+    used:
+
+    - **PR_TAGGED_ADDR_ENABLE**: Enable ARM64 Tagged Address ABI.
+
+    The prctl(PR_SET_TAGGED_ADDR_CTRL, ...) will return -EINVAL if the ARM64
+    Tagged Address ABI is not available.
+
+    The arguments arg3, arg4, and arg5 are ignored.
+  - **PR_GET_TAGGED_ADDR_CTRL**: can be used to check the status of the Tagged
+    Address ABI.
+
+    The arguments arg2, arg3, arg4, and arg5 are ignored.
+
+The ABI properties set by the mechanisms described above are inherited by threads
+of the same application and fork()'ed children but cleared by execve().
+
+When a process has successfully opted into the new ABI by invoking
+PR_SET_TAGGED_ADDR_CTRL prctl(), this guarantees the following behaviours:
+
+ - Every currently available syscall, except the cases mentioned in section 3, can
+   accept any valid tagged pointer. The same rule is applicable to any syscall
+   introduced in the future.
+ - If a non valid tagged pointer is passed to a syscall then the behaviour
+   is undefined.
+ - Every valid tagged pointer is expected to work as an untagged one.
+ - The kernel preserves any valid tagged pointer and returns it to the
+   userspace unchanged (i.e. on syscall return) in all the cases except the
+   ones documented in the "Preserving tags" section of tagged-pointers.txt.
+
+A definition of the meaning of tagged pointers on arm64 can be found in:
+Documentation/arm64/tagged-pointers.txt.
+
+3. ARM64 Tagged Address ABI Exceptions
+--------------------------------------
+
+The behaviours described in section 2, with particular reference to the
+acceptance by the syscalls of any valid tagged pointer are not applicable
+to the following cases:
+
+ - mmap() addr parameter.
+ - mremap() new_address parameter.
+ - prctl(PR_SET_MM, PR_SET_MM_MAP, ...) struct prctl_mm_map fields.
+ - prctl(PR_SET_MM, PR_SET_MM_MAP_SIZE, ...) struct prctl_mm_map fields.
+
+Any attempt to use non-zero tagged pointers will lead to undefined behaviour.
+
+4. Example of correct usage
+---------------------------
+.. code-block:: c
+
+   void main(void)
+   {
+           static int tbi_enabled = 0;
+           unsigned long tag = 0;
+
+           char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
+                            MAP_ANONYMOUS, -1, 0);
+
+           if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE,
+                     0, 0, 0) == 0)
+                   tbi_enabled = 1;
+
+           if (ptr == (void *)-1) /* MAP_FAILED */
+                   return -1;
+
+           if (tbi_enabled)
+                   tag = rand() & 0xff;
+
+           ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
+
+           *ptr = 'a';
+
+           ...
+   }
+
-- 
2.22.0