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| -rw-r--r-- | Documentation/x86/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/x86/intel_mpx.rst (renamed from Documentation/x86/intel_mpx.txt) | 120 |
2 files changed, 65 insertions, 56 deletions
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index e2c0db9fcd4e..b5cdc0d889b3 100644 --- a/Documentation/x86/index.rst +++ b/Documentation/x86/index.rst @@ -19,3 +19,4 @@ x86-specific Documentation mtrr pat protection-keys + intel_mpx diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.rst index 85d0549ad846..387a640941a6 100644 --- a/Documentation/x86/intel_mpx.txt +++ b/Documentation/x86/intel_mpx.rst @@ -1,5 +1,11 @@ -1. Intel(R) MPX Overview -======================== +.. SPDX-License-Identifier: GPL-2.0 + +=========================================== +Intel(R) Memory Protection Extensions (MPX) +=========================================== + +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 @@ -7,7 +13,7 @@ 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: +You can tell if your CPU supports MPX by looking in /proc/cpuinfo:: cat /proc/cpuinfo | grep ' mpx ' @@ -21,8 +27,8 @@ can be downloaded from http://software.intel.com/en-us/articles/intel-software-development-emulator -2. How to get the advantage of MPX -================================== +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. @@ -84,14 +90,15 @@ Kernel MPX Code: is unmapped. -3. How does MPX kernel code work -================================ +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. @@ -124,37 +131,37 @@ 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. +: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 @@ -167,20 +174,20 @@ 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 _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. @@ -209,9 +216,10 @@ 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 + 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 @@ -223,8 +231,8 @@ into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT command execution. -4. Special rules -================ +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. |