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diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt new file mode 100644 index 000000000..85d0549ad --- /dev/null +++ b/Documentation/x86/intel_mpx.txt @@ -0,0 +1,244 @@ +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 + + +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. + + +3. How does MPX kernel code work +================================ + +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. + +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. + +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. + +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. + +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. + + +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. |