1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<head>
<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type">
<title>skas mode</title>
</head>
<body alink="#FF0000" vlink="#55188A" link="#0000EF" bgcolor="#FFFFFF" text="#000099">
<table border="0">
<tr align="left">
<td valign="top">
<table border="0">
<tr align="left"><td valign="top" >
<img src="http://user-mode-linux.sourceforge.net/uml-small.png" height="171" width="120"/>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/index.html">Site Home Page</a></font>
<br>
<font size="-1"><a href="http://uml.harlowhill.com">The UML Wiki</a></font>
<br>
<font size="-1"><a href="http://usermodelinux.org">UML Community Site</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/roadmap.html">The UML roadmap</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/uses.html">What it's good for</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/case-studies.html">Case Studies</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/kernel.html">Kernel Capabilities</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/dl-sf.html">Downloading it</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/run.html">Running it</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/compile.html">Compiling</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/install.html">Installation</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/skas.html">Skas Mode</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/patches.html">Incremental Patches</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/tests.html">Test Suite</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/devanon.html">Host memory use</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/fs_making.html">Building filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/faq.html">Troubles</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/contrib.html">User Contributions</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/links.html">Related Links</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/todo.html">The ToDo list</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/projects.html">Projects</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/diary.html">Diary</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/thanks.html">Thanks</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/contacts.html">Contacts</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Tutorials<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO.html">The HOWTO (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO.txt">The HOWTO (text)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/hostfs.html">Host file access</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/input.html">Device inputs</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/shared_fs.html">Sharing filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/fs.html">Creating filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/resize.html">Resizing filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/networking.html">Virtual Networking</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/mconsole.html">Management Console</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/debugging.html">Kernel Debugging</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/honeypots.html">UML Honeypots</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/gprof.html">gprof and gcov</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/xtut.html">Running X</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/trouble.html">Diagnosing problems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/config.html">Configuration</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slack_readme.html">Installing Slackware</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/arch-port.html">Porting UML</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/iomem.html">IO memory emulation</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/2G-2G.html">UML on 2G/2G hosts</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/lksct/index.html">Adding a UML system call</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/nesting.html">Running nested UMLs</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">How you can help<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-gen.html">Overview</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-doc.html">Documentation</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-userspace.html">Utilities</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-kernel-v1.html">Kernel bugs</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/projects.html">Kernel projects</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Screenshots<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/net.html">A virtual network</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/x.html">An X session</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Transcripts<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/login.html">A login session</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/debug-session.html">A debugging session</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slackinst.html">Slackware installation</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Reference<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/switches.html">Kernel switches</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slack_readme.html">Slackware README</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Papers<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2000/index.html">ALS 2000 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2000.tex">ALS 2000 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/als2000/slides.html">ALS 2000 slides</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/lca2001/lca.html">LCA 2001 slides</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ols2001/index.html">OLS 2001 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/ols2001.tex">OLS 2001 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2001/index.html">ALS 2001 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2001.tex">ALS 2001 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ists2002/umlsec.htm">UML security (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/lca2002/lca2002.htm">LCA 2002 (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/wvu2002/wvu2002.htm">WVU 2002 (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ists_rt/ists_rt.htm">Security Roundtable (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ols2002/ols2002.html">OLS 2002 slides</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/lwe2005/LWE2005.html">LWE 2005 slides</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Fun and Games<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/cgi-bin/hangman">Kernel Hangman</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/sdotm.html">Disaster of the Month</a></font>
</td></tr>
</table>
</td>
<td valign="top" align="left">
<center>
<h3>skas mode</h3>
</center>
<a name="Introduction"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Introduction</font>
</b>
</td>
</tr>
</table>
<blockquote head="Introduction">
Traditionally, UML has had the following design
<ul>
<li>
each UML process gets a process on the host
</li>
<li>
there is a special thread, the tracing thread, which does system call
tracing on the UML processes
</li>
<li>
the tracing thread nullified system calls, and caused the process to
enter the UML kernel, which is mapped into the upper part of its
address space
</li>
</ul>
This was forced by the limited support for UML in Linux and has a
number of problems
<ul>
<li>
The UML kernel is present in the address space of each of its
processes, and, by default, is writeable. This is obviously a
security problem, since, with write access to kernel data, a process
can break out to the host. UML's 'jail' mode fixes this problem by
making UML data readonly while a process is running, but this imposes
a huge performance penalty. Also, the kernel is still there, and can
be read, so this isn't acceptable for honeypots, since a bad guy can
easily tell that the system is a UML.
</li>
<li>
UML uses signals to force control to the UML kernel during a system
call or interrupt. Signal delivery and return are slow, and impose a
noticable performance hit.
</li>
</ul>
During the fall of 2002, I started fixing these problems by adding
support in the host Linux for features that would solve these
problems. The result is now stable, and is being used by a number of
people with good results.
<p>
In short, the changes cause the UML kernel to run in an entirely
different host address space from its processes. This solves the
security and honeypot fingerprinting problems by making the UML kernel
totally inaccessible to UML processes. Their address spaces are
identical to what they would be on the host. This also provides a
noticable speedup by eliminating the signal delivery that used to
happen for every UML system call.
<p>
The old mode is now called 'tt mode', for Tracing Thread, and the new
mode is called 'skas mode', for Separate Kernel Address Space.
</blockquote>
<a name="Using skas mode"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Using skas mode</font>
</b>
</td>
</tr>
</table>
<blockquote head="Using skas mode">
Skas mode requires that a patch be applied to the host kernel. This
patch implements the address space support needed by skas mode, plus
some additions to ptrace which are necessary. The patch is available
from the <a href="dl-sf.html#Host patches">UML
download page</a>.
<p>
These patches are against 2.4.19, but are fairly non-intrusive, and
should apply to any nearby kernel. You should get the latest host skas
patch and run the latest UML on it. You should only use an older
host skas patch if you need to run an older UML for some reason.
<p>
With the patch applied to the host, all you need to do is make sure
that CONFIG_MODE_SKAS is enabled in UML, and run it. It will detect
the host support and use it. If the host support isn't there, it will
fall back to tt mode. It will also fall back to tt mode if it doesn't
detect a specific version of the host skas support. It will tell you
during the early boot which version it's looking for:
<tt>
<pre>
<font size="-1">
Checking for the skas3 patch in the host...found
Checking for /proc/mm...found
</font>
</pre>
</tt>
Once you see this, you're all set. UML is happily running in skas
mode.
</blockquote>
<a name="Skas effects on UML"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Skas effects on UML</font>
</b>
</td>
</tr>
</table>
<blockquote head="Skas effects on UML">
This is simple - you shouldn't really notice any difference, except
for performance. Inside UML, skas mode causes no changes in
functionality. For most uses, you should see a noticable improvement
in performance. My favorite benchmark, a kernel build, is almost
twice as fast with skas mode as with tt mode, and is within 30% of the
host's time. Bill Stearns saw a script's running time drop from ~50
seconds to ~14 seconds, almost a quadrupling in speed.
</blockquote>
<a name="Skas effects on the host"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Skas effects on the host</font>
</b>
</td>
</tr>
</table>
<blockquote head="Skas effects on the host">
You will notice some differences on the host. If you run ps, you will
notice only four processes per UML rather than the dozens that you see
in tt mode. They are
<ul>
<li>
The UML kernel thread, which runs in the separate kernel address
space, executes kernel code, and does system call interception on UML
processes
</li>
<li>
The UML userspace thread, which runs all UML process code and switches
between host address spaces on each UML context switch
</li>
<li>
The ubd driver asynchronous IO thread
</li>
<li>
The write SIGIO emulation thread
</li>
</ul>
If you are running many UMLs on the host, and you convert them over to
skas mode, you will also notice a large reduction in the host load.
The UML server that I am most familiar with, which runs 25-30 UMLs at
any given time, reduced its typical load average from 2 - 5 with tt
mode to .1 - .5 with skas mode.
</blockquote>
<a name="Skas present"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Skas present</font>
</b>
</td>
</tr>
</table>
<blockquote head="Skas present">
I know of only one bug in the current skas patch, which only affects
running nested UMLs as far as I know. I have been running the skas3
patch continuously since the fall with no problems. Other people
have, as well, and I've heard of no problems from them. It's
SMP-safe, and it's running stably on SMP hosts.
</blockquote>
<a name="Skas future"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Skas future</font>
</b>
</td>
</tr>
</table>
<blockquote head="Skas future">
There is going to be a skas4 patch at some point. There will be no
real functional changes. The bug mentioned above will be fixed, and a
performance tweak or two will be added.
<p>
The major change will be the interface to the new stuff. Currently,
new address spaces are created by opening /proc/mm, and they are
modified by writing to the returned file descriptor. Linus hates this
interface (and I wasn't very fond of it, either). After some
discussion, he proposed adding two new system calls, one which creates
new address spaces, and another which executes an arbitrary system
call in the context of one of these address spaces.
<p>
The skas4 patch will implement these new system calls, and UML will
use them at that point. After that, when the patch has had some
testing, I will think about how and when to get it into the main
kernel pools.
</blockquote>
</td>
</tr>
</table>
<center>
<font size="-1">Hosted at </font>
<a href="http://sourceforge.net">
<img src="http://sourceforge.net/sflogo.php?group_id=429" width="88" height="31" border="0" alt="SourceForge Logo">
</a>
</center>
</body>
</html>
|
