11. Intel(R) MPX Overview 2======================== 3 4Intel(R) Memory Protection Extensions (Intel(R) MPX) is a new capability 5introduced into Intel Architecture. Intel MPX provides hardware features 6that can be used in conjunction with compiler changes to check memory 7references, for those references whose compile-time normal intentions are 8usurped at runtime due to buffer overflow or underflow. 9 10You can tell if your CPU supports MPX by looking in /proc/cpuinfo: 11 12 cat /proc/cpuinfo | grep ' mpx ' 13 14For more information, please refer to Intel(R) Architecture Instruction 15Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection 16Extensions. 17 18Note: As of December 2014, no hardware with MPX is available but it is 19possible to use SDE (Intel(R) Software Development Emulator) instead, which 20can be downloaded from 21http://software.intel.com/en-us/articles/intel-software-development-emulator 22 23 242. How to get the advantage of MPX 25================================== 26 27For MPX to work, changes are required in the kernel, binutils and compiler. 28No source changes are required for applications, just a recompile. 29 30There are a lot of moving parts of this to all work right. The following 31is how we expect the compiler, application and kernel to work together. 32 331) Application developer compiles with -fmpx. The compiler will add the 34 instrumentation as well as some setup code called early after the app 35 starts. New instruction prefixes are noops for old CPUs. 362) That setup code allocates (virtual) space for the "bounds directory", 37 points the "bndcfgu" register to the directory (must also set the valid 38 bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) 39 that the app will be using MPX. The app must be careful not to access 40 the bounds tables between the time when it populates "bndcfgu" and 41 when it calls the prctl(). This might be hard to guarantee if the app 42 is compiled with MPX. You can add "__attribute__((bnd_legacy))" to 43 the function to disable MPX instrumentation to help guarantee this. 44 Also be careful not to call out to any other code which might be 45 MPX-instrumented. 463) The kernel detects that the CPU has MPX, allows the new prctl() to 47 succeed, and notes the location of the bounds directory. Userspace is 48 expected to keep the bounds directory at that locationWe note it 49 instead of reading it each time because the 'xsave' operation needed 50 to access the bounds directory register is an expensive operation. 514) If the application needs to spill bounds out of the 4 registers, it 52 issues a bndstx instruction. Since the bounds directory is empty at 53 this point, a bounds fault (#BR) is raised, the kernel allocates a 54 bounds table (in the user address space) and makes the relevant entry 55 in the bounds directory point to the new table. 565) If the application violates the bounds specified in the bounds registers, 57 a separate kind of #BR is raised which will deliver a signal with 58 information about the violation in the 'struct siginfo'. 596) Whenever memory is freed, we know that it can no longer contain valid 60 pointers, and we attempt to free the associated space in the bounds 61 tables. If an entire table becomes unused, we will attempt to free 62 the table and remove the entry in the directory. 63 64To summarize, there are essentially three things interacting here: 65 66GCC with -fmpx: 67 * enables annotation of code with MPX instructions and prefixes 68 * inserts code early in the application to call in to the "gcc runtime" 69GCC MPX Runtime: 70 * Checks for hardware MPX support in cpuid leaf 71 * allocates virtual space for the bounds directory (malloc() essentially) 72 * points the hardware BNDCFGU register at the directory 73 * calls a new prctl(PR_MPX_ENABLE_MANAGEMENT) to notify the kernel to 74 start managing the bounds directories 75Kernel MPX Code: 76 * Checks for hardware MPX support in cpuid leaf 77 * Handles #BR exceptions and sends SIGSEGV to the app when it violates 78 bounds, like during a buffer overflow. 79 * When bounds are spilled in to an unallocated bounds table, the kernel 80 notices in the #BR exception, allocates the virtual space, then 81 updates the bounds directory to point to the new table. It keeps 82 special track of the memory with a VM_MPX flag. 83 * Frees unused bounds tables at the time that the memory they described 84 is unmapped. 85 86 873. How does MPX kernel code work 88================================ 89 90Handling #BR faults caused by MPX 91--------------------------------- 92 93When MPX is enabled, there are 2 new situations that can generate 94#BR faults. 95 * new bounds tables (BT) need to be allocated to save bounds. 96 * bounds violation caused by MPX instructions. 97 98We hook #BR handler to handle these two new situations. 99 100On-demand kernel allocation of bounds tables 101-------------------------------------------- 102 103MPX only has 4 hardware registers for storing bounds information. If 104MPX-enabled code needs more than these 4 registers, it needs to spill 105them somewhere. It has two special instructions for this which allow 106the bounds to be moved between the bounds registers and some new "bounds 107tables". 108 109#BR exceptions are a new class of exceptions just for MPX. They are 110similar conceptually to a page fault and will be raised by the MPX 111hardware during both bounds violations or when the tables are not 112present. The kernel handles those #BR exceptions for not-present tables 113by carving the space out of the normal processes address space and then 114pointing the bounds-directory over to it. 115 116The tables need to be accessed and controlled by userspace because 117the instructions for moving bounds in and out of them are extremely 118frequent. They potentially happen every time a register points to 119memory. Any direct kernel involvement (like a syscall) to access the 120tables would obviously destroy performance. 121 122Why not do this in userspace? MPX does not strictly require anything in 123the kernel. It can theoretically be done completely from userspace. Here 124are a few ways this could be done. We don't think any of them are practical 125in the real-world, but here they are. 126 127Q: Can virtual space simply be reserved for the bounds tables so that we 128 never have to allocate them? 129A: MPX-enabled application will possibly create a lot of bounds tables in 130 process address space to save bounds information. These tables can take 131 up huge swaths of memory (as much as 80% of the memory on the system) 132 even if we clean them up aggressively. In the worst-case scenario, the 133 tables can be 4x the size of the data structure being tracked. IOW, a 134 1-page structure can require 4 bounds-table pages. An X-GB virtual 135 area needs 4*X GB of virtual space, plus 2GB for the bounds directory. 136 If we were to preallocate them for the 128TB of user virtual address 137 space, we would need to reserve 512TB+2GB, which is larger than the 138 entire virtual address space today. This means they can not be reserved 139 ahead of time. Also, a single process's pre-popualated bounds directory 140 consumes 2GB of virtual *AND* physical memory. IOW, it's completely 141 infeasible to prepopulate bounds directories. 142 143Q: Can we preallocate bounds table space at the same time memory is 144 allocated which might contain pointers that might eventually need 145 bounds tables? 146A: This would work if we could hook the site of each and every memory 147 allocation syscall. This can be done for small, constrained applications. 148 But, it isn't practical at a larger scale since a given app has no 149 way of controlling how all the parts of the app might allocate memory 150 (think libraries). The kernel is really the only place to intercept 151 these calls. 152 153Q: Could a bounds fault be handed to userspace and the tables allocated 154 there in a signal handler intead of in the kernel? 155A: mmap() is not on the list of safe async handler functions and even 156 if mmap() would work it still requires locking or nasty tricks to 157 keep track of the allocation state there. 158 159Having ruled out all of the userspace-only approaches for managing 160bounds tables that we could think of, we create them on demand in 161the kernel. 162 163Decoding MPX instructions 164------------------------- 165 166If a #BR is generated due to a bounds violation caused by MPX. 167We need to decode MPX instructions to get violation address and 168set this address into extended struct siginfo. 169 170The _sigfault feild of struct siginfo is extended as follow: 171 17287 /* SIGILL, SIGFPE, SIGSEGV, SIGBUS */ 17388 struct { 17489 void __user *_addr; /* faulting insn/memory ref. */ 17590 #ifdef __ARCH_SI_TRAPNO 17691 int _trapno; /* TRAP # which caused the signal */ 17792 #endif 17893 short _addr_lsb; /* LSB of the reported address */ 17994 struct { 18095 void __user *_lower; 18196 void __user *_upper; 18297 } _addr_bnd; 18398 } _sigfault; 184 185The '_addr' field refers to violation address, and new '_addr_and' 186field refers to the upper/lower bounds when a #BR is caused. 187 188Glibc will be also updated to support this new siginfo. So user 189can get violation address and bounds when bounds violations occur. 190 191Cleanup unused bounds tables 192---------------------------- 193 194When a BNDSTX instruction attempts to save bounds to a bounds directory 195entry marked as invalid, a #BR is generated. This is an indication that 196no bounds table exists for this entry. In this case the fault handler 197will allocate a new bounds table on demand. 198 199Since the kernel allocated those tables on-demand without userspace 200knowledge, it is also responsible for freeing them when the associated 201mappings go away. 202 203Here, the solution for this issue is to hook do_munmap() to check 204whether one process is MPX enabled. If yes, those bounds tables covered 205in the virtual address region which is being unmapped will be freed also. 206 207Adding new prctl commands 208------------------------- 209 210Two new prctl commands are added to enable and disable MPX bounds tables 211management in kernel. 212 213155 #define PR_MPX_ENABLE_MANAGEMENT 43 214156 #define PR_MPX_DISABLE_MANAGEMENT 44 215 216Runtime library in userspace is responsible for allocation of bounds 217directory. So kernel have to use XSAVE instruction to get the base 218of bounds directory from BNDCFG register. 219 220But XSAVE is expected to be very expensive. In order to do performance 221optimization, we have to get the base of bounds directory and save it 222into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT 223command execution. 224 225 2264. Special rules 227================ 228 2291) If userspace is requesting help from the kernel to do the management 230of bounds tables, it may not create or modify entries in the bounds directory. 231 232Certainly users can allocate bounds tables and forcibly point the bounds 233directory at them through XSAVE instruction, and then set valid bit 234of bounds entry to have this entry valid. But, the kernel will decline 235to assist in managing these tables. 236 2372) Userspace may not take multiple bounds directory entries and point 238them at the same bounds table. 239 240This is allowed architecturally. See more information "Intel(R) Architecture 241Instruction Set Extensions Programming Reference" (9.3.4). 242 243However, if users did this, the kernel might be fooled in to unmaping an 244in-use bounds table since it does not recognize sharing. 245