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1 1. Intel(R) MPX Overview
2 ========================
4 Intel(R) Memory Protection Extensions (Intel(R) MPX) is a new capability
5 introduced into Intel Architecture. Intel MPX provides hardware features
6 that can be used in conjunction with compiler changes to check memory
7 references, for those references whose compile-time normal intentions are
8 usurped at runtime due to buffer overflow or underflow.
10 You can tell if your CPU supports MPX by looking in /proc/cpuinfo:
12 cat /proc/cpuinfo | grep ' mpx '
14 For more information, please refer to Intel(R) Architecture Instruction
15 Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection
16 Extensions.
18 Note: As of December 2014, no hardware with MPX is available but it is
19 possible to use SDE (Intel(R) Software Development Emulator) instead, which
20 can be downloaded from
21 http://software.intel.com/en-us/articles/intel-software-development-emulator
24 2. How to get the advantage of MPX
25 ==================================
27 For MPX to work, changes are required in the kernel, binutils and compiler.
28 No source changes are required for applications, just a recompile.
30 There are a lot of moving parts of this to all work right. The following
31 is how we expect the compiler, application and kernel to work together.
33 1) 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.
36 2) 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.
46 3) 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.
51 4) 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.
56 5) 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'.
59 6) 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.
64 To summarize, there are essentially three things interacting here:
66 GCC 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"
69 GCC 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
75 Kernel 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.
87 3. How does MPX kernel code work
88 ================================
90 Handling #BR faults caused by MPX
91 ---------------------------------
93 When 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.
98 We hook #BR handler to handle these two new situations.
100 On-demand kernel allocation of bounds tables
101 --------------------------------------------
103 MPX only has 4 hardware registers for storing bounds information. If
104 MPX-enabled code needs more than these 4 registers, it needs to spill
105 them somewhere. It has two special instructions for this which allow
106 the bounds to be moved between the bounds registers and some new "bounds
107 tables".
109 #BR exceptions are a new class of exceptions just for MPX. They are
110 similar conceptually to a page fault and will be raised by the MPX
111 hardware during both bounds violations or when the tables are not
112 present. The kernel handles those #BR exceptions for not-present tables
113 by carving the space out of the normal processes address space and then
114 pointing the bounds-directory over to it.
116 The tables need to be accessed and controlled by userspace because
117 the instructions for moving bounds in and out of them are extremely
118 frequent. They potentially happen every time a register points to
119 memory. Any direct kernel involvement (like a syscall) to access the
120 tables would obviously destroy performance.
122 Why not do this in userspace? MPX does not strictly require anything in
123 the kernel. It can theoretically be done completely from userspace. Here
124 are a few ways this could be done. We don't think any of them are practical
125 in the real-world, but here they are.
127 Q: Can virtual space simply be reserved for the bounds tables so that we
128 never have to allocate them?
129 A: 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-populated bounds directory
140 consumes 2GB of virtual *AND* physical memory. IOW, it's completely
141 infeasible to prepopulate bounds directories.
143 Q: 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?
146 A: 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.
153 Q: Could a bounds fault be handed to userspace and the tables allocated
154 there in a signal handler instead of in the kernel?
155 A: 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.
159 Having ruled out all of the userspace-only approaches for managing
160 bounds tables that we could think of, we create them on demand in
161 the kernel.
163 Decoding MPX instructions
164 -------------------------
166 If a #BR is generated due to a bounds violation caused by MPX.
167 We need to decode MPX instructions to get violation address and
168 set this address into extended struct siginfo.
170 The _sigfault feild of struct siginfo is extended as follow:
172 87 /* SIGILL, SIGFPE, SIGSEGV, SIGBUS */
173 88 struct {
174 89 void __user *_addr; /* faulting insn/memory ref. */
175 90 #ifdef __ARCH_SI_TRAPNO
176 91 int _trapno; /* TRAP # which caused the signal */
177 92 #endif
178 93 short _addr_lsb; /* LSB of the reported address */
179 94 struct {
180 95 void __user *_lower;
181 96 void __user *_upper;
182 97 } _addr_bnd;
183 98 } _sigfault;
185 The '_addr' field refers to violation address, and new '_addr_and'
186 field refers to the upper/lower bounds when a #BR is caused.
188 Glibc will be also updated to support this new siginfo. So user
189 can get violation address and bounds when bounds violations occur.
191 Cleanup unused bounds tables
192 ----------------------------
194 When a BNDSTX instruction attempts to save bounds to a bounds directory
195 entry marked as invalid, a #BR is generated. This is an indication that
196 no bounds table exists for this entry. In this case the fault handler
197 will allocate a new bounds table on demand.
199 Since the kernel allocated those tables on-demand without userspace
200 knowledge, it is also responsible for freeing them when the associated
201 mappings go away.
203 Here, the solution for this issue is to hook do_munmap() to check
204 whether one process is MPX enabled. If yes, those bounds tables covered
205 in the virtual address region which is being unmapped will be freed also.
207 Adding new prctl commands
208 -------------------------
210 Two new prctl commands are added to enable and disable MPX bounds tables
211 management in kernel.
213 155 #define PR_MPX_ENABLE_MANAGEMENT 43
214 156 #define PR_MPX_DISABLE_MANAGEMENT 44
216 Runtime library in userspace is responsible for allocation of bounds
217 directory. So kernel have to use XSAVE instruction to get the base
218 of bounds directory from BNDCFG register.
220 But XSAVE is expected to be very expensive. In order to do performance
221 optimization, we have to get the base of bounds directory and save it
222 into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT
223 command execution.
226 4. Special rules
227 ================
229 1) If userspace is requesting help from the kernel to do the management
230 of bounds tables, it may not create or modify entries in the bounds directory.
232 Certainly users can allocate bounds tables and forcibly point the bounds
233 directory at them through XSAVE instruction, and then set valid bit
234 of bounds entry to have this entry valid. But, the kernel will decline
235 to assist in managing these tables.
237 2) Userspace may not take multiple bounds directory entries and point
238 them at the same bounds table.
240 This is allowed architecturally. See more information "Intel(R) Architecture
241 Instruction Set Extensions Programming Reference" (9.3.4).
243 However, if users did this, the kernel might be fooled in to unmaping an
244 in-use bounds table since it does not recognize sharing.