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From 26057cba30205ed659094a2816557b439c651286 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Nikola=20Forr=C3=B3?= <nforro@redhat.com>
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Date: Tue, 28 Jun 2016 13:28:29 +0200
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Subject: [PATCH 11/17] prctl.2: add description of Intel MPX calls
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---
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man-pages/man2/prctl.2 | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++
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1 file changed, 81 insertions(+)
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diff --git a/man-pages/man2/prctl.2 b/man-pages/man2/prctl.2
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index 24e56d2..92eecf9 100644
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--- a/man-pages/man2/prctl.2
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+++ b/man-pages/man2/prctl.2
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@@ -47,6 +47,7 @@
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.\" PR_GET_TIMERSLACK
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.\" 2013-01-10 Kees Cook, document PR_SET_PTRACER
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.\" 2012-02-04 Michael kerrisk, document PR_{SET,GET}_CHILD_SUBREAPER
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+.\" 2014-11-10 Dave Hansen, document PR_MPX_{EN,DIS}ABLE_MANAGEMENT
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.\"
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.\"
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.TH PRCTL 2 2013-05-21 "Linux" "Linux Programmer's Manual"
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@@ -771,6 +772,77 @@ option.
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.\" symbolic-link transitions over all process running in a system.
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.\" ========== END FIXME
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.RE
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+.TP
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+.BR PR_MPX_ENABLE_MANAGEMENT ", " PR_MPX_DISABLE_MANAGEMENT " (since Linux 3.19) "
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+.\" commit fe3d197f84319d3bce379a9c0dc17b1f48ad358c
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+.\" See also http://lwn.net/Articles/582712/
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+.\" See also https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler
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+Enable or disable kernel management of Memory Protection eXtensions (MPX)
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+bounds tables.
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+The
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+.IR arg2 ,
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+.IR arg3 ,
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+.IR arg4 ,
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+and
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+.IR arg5
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+.\" commit e9d1b4f3c60997fe197bf0243cb4a41a44387a88
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+arguments must be zero.
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+
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+MPX is a hardware-assisted mechanism for performing bounds checking on
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+pointers.
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+It consists of a set of registers storing bounds information
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+and a set of special instruction prefixes that tell the CPU on which
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+instructions it should do bounds enforcement.
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+There is a limited number of these registers and
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+when there are more pointers than registers,
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+their contents must be "spilled" into a set of tables.
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+These tables are called "bounds tables" and the MPX
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+.BR prctl ()
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+operations control
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+whether the kernel manages their allocation and freeing.
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+
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+When management is enabled, the kernel will take over allocation
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+and freeing of the bounds tables.
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+It does this by trapping the #BR exceptions that result
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+at first use of missing bounds tables and
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+instead of delivering the exception to user space,
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+it allocates the table and populates the bounds directory
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+with the location of the new table.
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+For freeing, the kernel checks to see if bounds tables are
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+present for memory which is not allocated, and frees them if so.
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+
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+Before enabling MPX management using
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+.BR PR_MPX_ENABLE_MANAGEMENT ,
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+the application must first have allocated a user-space buffer for
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+the bounds directory and placed the location of that directory in the
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+.I bndcfgu
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+register.
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+
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+These calls will fail if the CPU or kernel does not support MPX.
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+Kernel support for MPX is enabled via the
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+.BR CONFIG_X86_INTEL_MPX
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+configuration option.
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+You can check whether the CPU supports MPX by looking for the 'mpx'
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+CPUID bit, like with the following command:
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+
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+ cat /proc/cpuinfo | grep ' mpx '
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+
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+A thread may not switch in or out of long (64-bit) mode while MPX is
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+enabled.
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+
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+All threads in a process are affected by these calls.
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+
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+The child of a
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+.BR fork (2)
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+inherits the state of MPX management.
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+During
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+.BR execve (2),
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+MPX management is reset to a state as if
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+.BR PR_MPX_DISABLE_MANAGEMENT
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+had been called.
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+
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+For further information on Intel MPX, see the kernel source file
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+.IR Documentation/x86/intel_mpx.txt .
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.\"
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.SH RETURN VALUE
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On success,
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@@ -957,6 +1029,15 @@ capability.
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.\" is
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.\" .BR PR_SET_SECCOMP ,
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.\" and secure computing mode is already 1.
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+.TP
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+.B ENXIO
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+.I option
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+was
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+.BR PR_MPX_ENABLE_MANAGEMENT
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+or
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+.BR PR_MPX_DISABLE_MANAGEMENT
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+and the kernel or the CPU does not support MPX management.
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+Check that the kernel and processor have MPX support.
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.SH VERSIONS
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The
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.BR prctl ()
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--
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2.7.4
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