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Reduces attack surface on kernel devs opening the links for MITM
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Signed-off-by: Alexander A. Klimov <grandmaster@al2klimov.de>
Link: https://lore.kernel.org/r/20200709182742.24724-1-grandmaster@al2klimov.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
213 lines
8.4 KiB
ReStructuredText
213 lines
8.4 KiB
ReStructuredText
=================================================
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Linux API for read access to z/VM Monitor Records
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=================================================
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Date : 2004-Nov-26
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Author: Gerald Schaefer (geraldsc@de.ibm.com)
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Description
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===========
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This item delivers a new Linux API in the form of a misc char device that is
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usable from user space and allows read access to the z/VM Monitor Records
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collected by the `*MONITOR` System Service of z/VM.
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User Requirements
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=================
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The z/VM guest on which you want to access this API needs to be configured in
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order to allow IUCV connections to the `*MONITOR` service, i.e. it needs the
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IUCV `*MONITOR` statement in its user entry. If the monitor DCSS to be used is
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restricted (likely), you also need the NAMESAVE <DCSS NAME> statement.
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This item will use the IUCV device driver to access the z/VM services, so you
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need a kernel with IUCV support. You also need z/VM version 4.4 or 5.1.
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There are two options for being able to load the monitor DCSS (examples assume
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that the monitor DCSS begins at 144 MB and ends at 152 MB). You can query the
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location of the monitor DCSS with the Class E privileged CP command Q NSS MAP
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(the values BEGPAG and ENDPAG are given in units of 4K pages).
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See also "CP Command and Utility Reference" (SC24-6081-00) for more information
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on the DEF STOR and Q NSS MAP commands, as well as "Saved Segments Planning
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and Administration" (SC24-6116-00) for more information on DCSSes.
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1st option:
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-----------
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You can use the CP command DEF STOR CONFIG to define a "memory hole" in your
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guest virtual storage around the address range of the DCSS.
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Example: DEF STOR CONFIG 0.140M 200M.200M
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This defines two blocks of storage, the first is 140MB in size an begins at
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address 0MB, the second is 200MB in size and begins at address 200MB,
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resulting in a total storage of 340MB. Note that the first block should
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always start at 0 and be at least 64MB in size.
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2nd option:
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-----------
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Your guest virtual storage has to end below the starting address of the DCSS
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and you have to specify the "mem=" kernel parameter in your parmfile with a
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value greater than the ending address of the DCSS.
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Example::
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DEF STOR 140M
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This defines 140MB storage size for your guest, the parameter "mem=160M" is
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added to the parmfile.
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User Interface
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==============
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The char device is implemented as a kernel module named "monreader",
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which can be loaded via the modprobe command, or it can be compiled into the
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kernel instead. There is one optional module (or kernel) parameter, "mondcss",
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to specify the name of the monitor DCSS. If the module is compiled into the
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kernel, the kernel parameter "monreader.mondcss=<DCSS NAME>" can be specified
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in the parmfile.
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The default name for the DCSS is "MONDCSS" if none is specified. In case that
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there are other users already connected to the `*MONITOR` service (e.g.
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Performance Toolkit), the monitor DCSS is already defined and you have to use
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the same DCSS. The CP command Q MONITOR (Class E privileged) shows the name
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of the monitor DCSS, if already defined, and the users connected to the
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`*MONITOR` service.
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Refer to the "z/VM Performance" book (SC24-6109-00) on how to create a monitor
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DCSS if your z/VM doesn't have one already, you need Class E privileges to
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define and save a DCSS.
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Example:
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--------
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::
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modprobe monreader mondcss=MYDCSS
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This loads the module and sets the DCSS name to "MYDCSS".
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NOTE:
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-----
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This API provides no interface to control the `*MONITOR` service, e.g. specify
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which data should be collected. This can be done by the CP command MONITOR
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(Class E privileged), see "CP Command and Utility Reference".
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Device nodes with udev:
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-----------------------
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After loading the module, a char device will be created along with the device
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node /<udev directory>/monreader.
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Device nodes without udev:
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--------------------------
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If your distribution does not support udev, a device node will not be created
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automatically and you have to create it manually after loading the module.
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Therefore you need to know the major and minor numbers of the device. These
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numbers can be found in /sys/class/misc/monreader/dev.
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Typing cat /sys/class/misc/monreader/dev will give an output of the form
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<major>:<minor>. The device node can be created via the mknod command, enter
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mknod <name> c <major> <minor>, where <name> is the name of the device node
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to be created.
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Example:
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--------
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::
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# modprobe monreader
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# cat /sys/class/misc/monreader/dev
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10:63
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# mknod /dev/monreader c 10 63
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This loads the module with the default monitor DCSS (MONDCSS) and creates a
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device node.
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File operations:
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----------------
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The following file operations are supported: open, release, read, poll.
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There are two alternative methods for reading: either non-blocking read in
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conjunction with polling, or blocking read without polling. IOCTLs are not
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supported.
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Read:
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-----
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Reading from the device provides a 12 Byte monitor control element (MCE),
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followed by a set of one or more contiguous monitor records (similar to the
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output of the CMS utility MONWRITE without the 4K control blocks). The MCE
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contains information on the type of the following record set (sample/event
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data), the monitor domains contained within it and the start and end address
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of the record set in the monitor DCSS. The start and end address can be used
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to determine the size of the record set, the end address is the address of the
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last byte of data. The start address is needed to handle "end-of-frame" records
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correctly (domain 1, record 13), i.e. it can be used to determine the record
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start offset relative to a 4K page (frame) boundary.
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See "Appendix A: `*MONITOR`" in the "z/VM Performance" document for a description
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of the monitor control element layout. The layout of the monitor records can
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be found here (z/VM 5.1): https://www.vm.ibm.com/pubs/mon510/index.html
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The layout of the data stream provided by the monreader device is as follows::
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...
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<0 byte read>
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<first MCE> \
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<first set of records> |
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... |- data set
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<last MCE> |
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<last set of records> /
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<0 byte read>
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...
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There may be more than one combination of MCE and corresponding record set
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within one data set and the end of each data set is indicated by a successful
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read with a return value of 0 (0 byte read).
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Any received data must be considered invalid until a complete set was
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read successfully, including the closing 0 byte read. Therefore you should
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always read the complete set into a buffer before processing the data.
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The maximum size of a data set can be as large as the size of the
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monitor DCSS, so design the buffer adequately or use dynamic memory allocation.
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The size of the monitor DCSS will be printed into syslog after loading the
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module. You can also use the (Class E privileged) CP command Q NSS MAP to
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list all available segments and information about them.
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As with most char devices, error conditions are indicated by returning a
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negative value for the number of bytes read. In this case, the errno variable
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indicates the error condition:
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EIO:
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reply failed, read data is invalid and the application
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should discard the data read since the last successful read with 0 size.
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EFAULT:
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copy_to_user failed, read data is invalid and the application should
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discard the data read since the last successful read with 0 size.
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EAGAIN:
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occurs on a non-blocking read if there is no data available at the
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moment. There is no data missing or corrupted, just try again or rather
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use polling for non-blocking reads.
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EOVERFLOW:
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message limit reached, the data read since the last successful
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read with 0 size is valid but subsequent records may be missing.
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In the last case (EOVERFLOW) there may be missing data, in the first two cases
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(EIO, EFAULT) there will be missing data. It's up to the application if it will
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continue reading subsequent data or rather exit.
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Open:
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-----
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Only one user is allowed to open the char device. If it is already in use, the
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open function will fail (return a negative value) and set errno to EBUSY.
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The open function may also fail if an IUCV connection to the `*MONITOR` service
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cannot be established. In this case errno will be set to EIO and an error
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message with an IPUSER SEVER code will be printed into syslog. The IPUSER SEVER
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codes are described in the "z/VM Performance" book, Appendix A.
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NOTE:
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-----
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As soon as the device is opened, incoming messages will be accepted and they
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will account for the message limit, i.e. opening the device without reading
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from it will provoke the "message limit reached" error (EOVERFLOW error code)
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eventually.
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