12. Intel(R) Memory Protection Extensions (MPX)

12.1. Intel(R) MPX Overview

Intel(R) Memory Protection Extensions (Intel(R) MPX) is a new capability introduced into Intel Architecture. Intel MPX provides hardware features that can be used in conjunction with compiler changes to check memory references, for those references whose compile-time normal intentions are usurped at runtime due to buffer overflow or underflow.

You can tell if your CPU supports MPX by looking in /proc/cpuinfo:

cat /proc/cpuinfo  | grep ' mpx '

For more information, please refer to Intel(R) Architecture Instruction Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection Extensions.

Note: As of December 2014, no hardware with MPX is available but it is possible to use SDE (Intel(R) Software Development Emulator) instead, which can be downloaded from http://software.intel.com/en-us/articles/intel-software-development-emulator

12.2. How to get the advantage of MPX

For MPX to work, changes are required in the kernel, binutils and compiler. No source changes are required for applications, just a recompile.

There are a lot of moving parts of this to all work right. The following is how we expect the compiler, application and kernel to work together.

  1. Application developer compiles with -fmpx. The compiler will add the instrumentation as well as some setup code called early after the app starts. New instruction prefixes are noops for old CPUs.

  2. That setup code allocates (virtual) space for the "bounds directory", points the "bndcfgu" register to the directory (must also set the valid bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) that the app will be using MPX. The app must be careful not to access the bounds tables between the time when it populates "bndcfgu" and when it calls the prctl(). This might be hard to guarantee if the app is compiled with MPX. You can add "__attribute__((bnd_legacy))" to the function to disable MPX instrumentation to help guarantee this. Also be careful not to call out to any other code which might be MPX-instrumented.

  3. The kernel detects that the CPU has MPX, allows the new prctl() to succeed, and notes the location of the bounds directory. Userspace is expected to keep the bounds directory at that location. We note it instead of reading it each time because the 'xsave' operation needed to access the bounds directory register is an expensive operation.

  4. If the application needs to spill bounds out of the 4 registers, it issues a bndstx instruction. Since the bounds directory is empty at this point, a bounds fault (#BR) is raised, the kernel allocates a bounds table (in the user address space) and makes the relevant entry in the bounds directory point to the new table.

  5. If the application violates the bounds specified in the bounds registers, a separate kind of #BR is raised which will deliver a signal with information about the violation in the 'struct siginfo'.

  6. Whenever memory is freed, we know that it can no longer contain valid pointers, and we attempt to free the associated space in the bounds tables. If an entire table becomes unused, we will attempt to free the table and remove the entry in the directory.

To summarize, there are essentially three things interacting here:

GCC with -fmpx:

  • enables annotation of code with MPX instructions and prefixes

  • inserts code early in the application to call in to the "gcc runtime"

GCC MPX Runtime:

  • Checks for hardware MPX support in cpuid leaf

  • allocates virtual space for the bounds directory (malloc() essentially)

  • points the hardware BNDCFGU register at the directory

  • calls a new prctl(PR_MPX_ENABLE_MANAGEMENT) to notify the kernel to start managing the bounds directories

Kernel MPX Code:

  • Checks for hardware MPX support in cpuid leaf

  • Handles #BR exceptions and sends SIGSEGV to the app when it violates bounds, like during a buffer overflow.

  • When bounds are spilled in to an unallocated bounds table, the kernel notices in the #BR exception, allocates the virtual space, then updates the bounds directory to point to the new table. It keeps special track of the memory with a VM_MPX flag.

  • Frees unused bounds tables at the time that the memory they described is unmapped.

12.3. How does MPX kernel code work

12.3.1. Handling #BR faults caused by MPX

When MPX is enabled, there are 2 new situations that can generate #BR faults.

  • new bounds tables (BT) need to be allocated to save bounds.

  • bounds violation caused by MPX instructions.

We hook #BR handler to handle these two new situations.

12.3.2. On-demand kernel allocation of bounds tables

MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables".

#BR exceptions are a new class of exceptions just for MPX. They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. The kernel handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space and then pointing the bounds-directory over to it.

The tables need to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register points to memory. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance.

Why not do this in userspace? MPX does not strictly require anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this could be done. We don't think any of them are practical in the real-world, but here they are.

Q

Can virtual space simply be reserved for the bounds tables so that we never have to allocate them?

A

MPX-enabled application will possibly create a lot of bounds tables in process address space to save bounds information. These tables can take up huge swaths of memory (as much as 80% of the memory on the system) even if we clean them up aggressively. In the worst-case scenario, the tables can be 4x the size of the data structure being tracked. IOW, a 1-page structure can require 4 bounds-table pages. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-populated bounds directory consumes 2GB of virtual AND physical memory. IOW, it's completely infeasible to prepopulate bounds directories.

Q

Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables?

A

This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls.

Q

Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel?

A

mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there.

Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel.

12.3.3. Decoding MPX instructions

If a #BR is generated due to a bounds violation caused by MPX. We need to decode MPX instructions to get violation address and set this address into extended struct siginfo.

The _sigfault field of struct siginfo is extended as follow:

87            /* SIGILL, SIGFPE, SIGSEGV, SIGBUS */
88            struct {
89                    void __user *_addr; /* faulting insn/memory ref. */
90 #ifdef __ARCH_SI_TRAPNO
91                    int _trapno;    /* TRAP # which caused the signal */
92 #endif
93                    short _addr_lsb; /* LSB of the reported address */
94                    struct {
95                            void __user *_lower;
96                            void __user *_upper;
97                    } _addr_bnd;
98            } _sigfault;

The '_addr' field refers to violation address, and new '_addr_and' field refers to the upper/lower bounds when a #BR is caused.

Glibc will be also updated to support this new siginfo. So user can get violation address and bounds when bounds violations occur.

12.3.4. Cleanup unused bounds tables

When a BNDSTX instruction attempts to save bounds to a bounds directory entry marked as invalid, a #BR is generated. This is an indication that no bounds table exists for this entry. In this case the fault handler will allocate a new bounds table on demand.

Since the kernel allocated those tables on-demand without userspace knowledge, it is also responsible for freeing them when the associated mappings go away.

Here, the solution for this issue is to hook do_munmap() to check whether one process is MPX enabled. If yes, those bounds tables covered in the virtual address region which is being unmapped will be freed also.

12.3.5. Adding new prctl commands

Two new prctl commands are added to enable and disable MPX bounds tables management in kernel.

155   #define PR_MPX_ENABLE_MANAGEMENT        43
156   #define PR_MPX_DISABLE_MANAGEMENT       44

Runtime library in userspace is responsible for allocation of bounds directory. So kernel have to use XSAVE instruction to get the base of bounds directory from BNDCFG register.

But XSAVE is expected to be very expensive. In order to do performance optimization, we have to get the base of bounds directory and save it into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT command execution.

12.4. Special rules

1) If userspace is requesting help from the kernel to do the management of bounds tables, it may not create or modify entries in the bounds directory.

Certainly users can allocate bounds tables and forcibly point the bounds directory at them through XSAVE instruction, and then set valid bit of bounds entry to have this entry valid. But, the kernel will decline to assist in managing these tables.

2) Userspace may not take multiple bounds directory entries and point them at the same bounds table.

This is allowed architecturally. See more information "Intel(R) Architecture Instruction Set Extensions Programming Reference" (9.3.4).

However, if users did this, the kernel might be fooled in to unmapping an in-use bounds table since it does not recognize sharing.