A case study of sandboxing base systems with Capsicum. BSDTW Taipei 2017
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1 A case study of sandboxing base systems with Capsicum BSDTW Taipei
2 Mariusz Zaborski BSDTW Taipei
3 Outline Capsicum Is Capsicumizing hard? Debugging infrastructure Casper Future 3
4 Capsicum 4
5 Capsicum kernel infrastructure that provides: tight sandboxing 5
6 Capsicum vs. namespace Process IDs Jails File paths CPU sets NFS file handle Protocol address Filesystems IDs Routing tables Sysctl MIB System V IPC POSIX IPC System clocks 6
7 Capsicum kernel infrastructure that provides: tight sandboxing capability rights 7
8 Capsicum rights CAP_READ CAP_IOCTL CAP_WRITE CAP_RECV CAP_APPEND CAP_LISTEN CAP_ACCEPT... CAP_FCHMOD CAP_CREATE CAP_UNLINKAT 8
9 Capsicum Two ways to obtain more capabilities: the initialization phase delegation 9
10 Is Capsicumizing hard? No for new code. What about existing one? 10
11 Capsicum 2015 dhclient(8) tcpdump(8) hastd(8), hastctl(8) kdump(1) rhowd(8), rwho(1) ping(8) uniq(1) auditdistd(8) sshd(8) 11
12 Capsicum 2016 echo(1) last(1) tee(1) cmp(1) factor(6) locate(1) traceroute(1) col(1) fold(1) logname(1) bhyve(1) elfdump(1) getopt(1) md5(1) decryptcore(1) dc(1) hexdump(1) ministat(1) ktrdump(8) indent(1) printenv(1) procstat(1) jot(1) sleep(1) pkg ktrdump(8) soelim(1) irssi basename(1) dd(1) dirname(1) dma-mbox-create 12
13 bspatch(1) FreeBSD-SA-16:25 The implementation of bspatch does not check for a negative value on numbers of bytes read from the diff and extra streams, allowing an attacker who can control the patch file to write at arbitrary locations in the heap. FreeBSD-SA-16:29 The implementation of bspatch is susceptible to integer overflows with carefully crafted input, potentially allowing an attacker who can control the patch file to write at arbitrary locations in the heap. This issue was partially addressed in FreeBSD-SA-16:25.bspatch, but some possible integer overflows remained. 13
14 bspatch(1) - Step 0: read the code 14
15 bspatch(1) - Step 0: read the code 15
16 bspatch(1) - Step 1: code reorganization 16
17 bspatch(1) - Capsicumize??? cap_enter() 17
18 bspatch(1) - Step 2: read more code 18
19 bspatch(1) - Step 2: read more code 19
20 bspatch(1) - Step 3: Capsicumize 20
21 bspatch(1) - Step 3: Capsicumize 21
22 cmp(1) - deduplicate code 22
23 cmp(1) - deduplicate code 23
24 cmp(1) - deduplicate code 24
25 Capsicum helpers capsicum_helpers.h Inline functions: caph_limit_stream() caph_limit_stdout() caph_limit_stdin() caph_limit_stderr() 25
26 libc is not your friend err(3) localtime(3) syslog Modify virtual dynamic shared object (vdso) to not open device More capsicum helpers: caph_cache_catpages() caph_cache_tzdata() 26
27 cmp(1) - deduplicate code 27
28 Debugging infrastructure 28
29 Debugging - ktrace ktrace/kdump Getting only trace Very easy to miss something Hard to cover all paths 29
30 Debugging - ktrace 802 random CALL cap_enter 802 random RET cap_enter random CALL openat(at_fdcwd,0x400877,0<o_rdonly>) 802 random CAP restricted VFS lookup 802 random RET openat -1 errno 94 Not permitted in capability mode 802 random CALL sigprocmask(sig_block,0x c8,0x7fffffffe640) 802 random RET sigprocmask random CALL sigprocmask(sig_setmask,0x dc,0) 802 random RET sigprocmask random CALL sigprocmask(sig_block,0x c8,0x7fffffffe1b0) 30
31 Debugging - enotcap kern.trap_enotcap procctl(proc_trapcap_ctl) Getting core dump Hard to miss something Hard to cover all paths 31
32 Debugging - enotcap Program received signal SIGTRAP, Trace/breakpoint trap. 0x b34a in _openat () from /lib/libc.so.7 Current language: auto; currently minimal Breakpoint 1 at 0x80090b34a (gdb) bt #0 0x b34a in _openat () from /lib/libc.so.7 #1 0x e457 in open (path=<value optimized out>, flags=<value optimized out>) at /usr/src/lib/libc/sys/open.c:57 #2 0x a18 in main () at a.c:24 32
33 libcasper 33
34 Casper Provides functionality not available in capability mode through convenient APIs making Capsicum more practical Make easier to separate process Done before entering Capability mode Creating zygote Set of dynamic libraries 34
35 Casper - how its works? 35
36 Casper - how its works? cap_init() cap_service_open() cap_close() 36
37 Casper - how its works? cap_init() cap_service_open() cap_close() 37
38 Casper - how its works? cap_init() cap_service_open() cap_close() 38
39 Casper - how its works? cap_init() cap_service_open() cap_close() 39
40 Casper - how its works? cap_init() cap_service_open() cap_close() 40
41 Casper system.dns system.grp system.pwd system.random system.sysctl 41
42 Traceroute - Capsicumize with Casper 42
43 Traceroute - Capsicumize with Casper 43
44 Traceroute - Capsicumize with Casper 44
45 Casper - mocks Reduce amount of ifdefs in code Hide ifdefs in library itself Use inline/defines to create mocks. 45
46 Future! 46
47 Casper - next next generation!? Integration with libc? Make libc more plugable Start casper in _start Sandbox services Services run with user privileges Reduce TCB 47
48 Casper services system.dns system.tls system.grp system.socket system.pwd system.configuration system.random system.sysctl system.filesystem system.syslog system.login 48
49 Casper - dhclient(8) Starting devd. Starting dhclient. pid 336 (dhclient), uid (65): Path `/var/crash/dhclient.65.0.core' failed on initial open test, error = 2 pid 336 (dhclient), uid 65: exited on signal 5 Trace/BPT trap /etc/rc.d/dhclient: WARNING: failed to start dhclient add host : gateway lo0 fib 0: route already in table Script /etc/rc.d/defaultroute interrupted Creating and/or trimming log files. Starting syslogd. 49
50 Casper - dhclient(8) Starting devd. Starting dhclient. pid 336 (dhclient), uid (65): Path `/var/crash/dhclient.65.0.core' failed on initial open test, error = 2 pid 336 (dhclient), uid 65: exited on signal 5 Trace/BPT trap /etc/rc.d/dhclient: WARNING: failed to start dhclient add host : gateway lo0 fib 0: route already in table Script /etc/rc.d/defaultroute interrupted Creating and/or trimming log files. Starting syslogd. 50
51 Casper - dhclient(8) Starting program: /sbin/dhclient vtnet1 Program received signal SIGTRAP, Trace/breakpoint trap. 0x bbdd1a in _connect () from /lib/libc.so.7 Current language: auto; currently minimal (gdb) bt #0 0x bbdd1a in _connect () from /lib/libc.so.7 #1 0x bb0499 in connectlog () at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:379 #2 0x bb0090 in vsyslog (pri=<value optimized out>, fmt=<value optimized out>, ap=0x7fffffffe9c0) at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:254 #3 0x bafcdd in syslog (pri=<value optimized out>, fmt=<value optimized out>) at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:128 #4 0x cf7b in note (fmt=0x41056d "") at /usr/home/oshogbo/git/freebsd/sbin/dhclient/errwarn.c:132 #5 0x in send_discover (ipp=0x80066a000) at /usr/home/oshogbo/git/freebsd/sbin/dhclient/dhclient.c:1285 #6 0x a2 in main (argc=<value optimized out>, argv=<value optimized out>) 51
52 Casper - dhclient(8) Starting program: /sbin/dhclient vtnet1 Program received signal SIGTRAP, Trace/breakpoint trap. 0x bbdd1a in _connect () from /lib/libc.so.7 Current language: auto; currently minimal (gdb) bt #0 0x bbdd1a in _connect () from /lib/libc.so.7 #1 0x bb0499 in connectlog () at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:379 #2 0x bb0090 in vsyslog (pri=<value optimized out>, fmt=<value optimized out>, ap=0x7fffffffe9c0) at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:254 #3 0x bafcdd in syslog (pri=<value optimized out>, fmt=<value optimized out>) at /usr/home/oshogbo/git/freebsd/lib/libc/gen/syslog.c:128 #4 0x cf7b in note (fmt=0x41056d "") at /usr/home/oshogbo/git/freebsd/sbin/dhclient/errwarn.c:132 #5 0x in send_discover (ipp=0x80066a000) at /usr/home/oshogbo/git/freebsd/sbin/dhclient/dhclient.c:1285 #6 0x a2 in main (argc=<value optimized out>, argv=<value optimized out>) 52
53 Casper - dhclient(8) 53
54 Casper - dhclient(8) The routines closelog(), openlog(), syslog() and vsyslog() return no value. 54
55 Casper - dhclient(8) Starting devd. Starting dhclient. pid 336 (dhclient), uid (65): Path `/var/crash/dhclient.65.0.core' failed on initial open test, error = 2 pid 336 (dhclient), uid 65: exited on signal 5 Trace/BPT trap /etc/rc.d/dhclient: WARNING: failed to start dhclient add host : gateway lo0 fib 0: route already in table Script /etc/rc.d/defaultroute interrupted Creating and/or trimming log files. Starting syslogd. 55
56 Casper - dhclient(8) Starting devd. Starting dhclient. pid 336 (dhclient), uid (65): Path `/var/crash/dhclient.65.0.core' failed on initial open test, error = 2 pid 336 (dhclient), uid 65: exited on signal 5 Trace/BPT trap /etc/rc.d/dhclient: WARNING: failed to start dhclient add host : gateway lo0 fib 0: route already in table Script /etc/rc.d/defaultroute interrupted Creating and/or trimming log files. Starting syslogd. 56
57 Casper - system.syslog Change the order? Casper service: Fixed version of dhclient 57
58 Casper - sshd(8) $ ssh Connection to closed by remote host. Connection to closed. # dmesg pid (sshd), uid (22): Path `/var/crash/sshd.22.0.core' failed on initial open test, error = 2 pid (sshd), uid 22: exited on signal 5 58
59 Casper - sshd(8) 59
60 Casper - sshd(8) 60
61 Casper - sshd(8) 61
62 Casper - sshd(8) This code exists only in FreeBSD Opens two files $HOME/login.conf and /etc/login.conf system.login 62
63 Thanks! Allan Jude Baptiste Daroussin Conrad Mayer Ed Maste Konstantin Belousov 63
64 Thank you! BSDTW Taipei
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