Detecting past and present intrusions through vulnerability- specific predicates Ashlesha Joshi, Sam King, George Dunlap, and Peter Chen

Index Authors Motivation & Introduction Goals Challenges & Solutions Evaluation Related work Conclusion 2

Author group EECS Department University of Michigan Peter M. Chen is the leader of the group, and the other 3 authors are his students. The group is investigating how to add security services through virtual machines. Paper: Operating System Support for Virtual Machines USENIX

Authors Ashlesha Joshi Ph.D in Computer Science, University of Michigan 4

Samuel T. King 2006 Ph.D University of Michigan Now in University of Illinois at Urbanna-Champain Research areas: Operating System, Security and VM. important papers: 1.Virtualization and Security: Back to the Future IEEE S&P SubVirt: Implementing malware with virtual machines IEEE S&P Capo: a software-hardware interface for practical deterministic multiprocessor replay ASPLOS '09 5

George Washington Dunlap Ph.D University of Michigan Research areas: Operating System, VM. important papers: 1.Execution replay of multiprocessor virtual machines VEE’08 2.Debugging operating systems with time- traveling virtual machines ATEC '05 6

Peter M. Chen 1992, Ph.D. in Computer Science from the University of California at Berkeley, Research areas: Operating Systems, Databases, Distributed Systems. important papers: 1.Tolerating latency in replicated state machines through client speculation NSDI’09 2.Execution replay of multiprocessor virtual machines VEE,08 3.Rethink the sync OSDI’06 4.Backtracking intrusions SOSP’03 7

8 Motivation Software contains bugs, including flaws that may be exploited by an attacker Some time passes before vendor becomes aware of bug Software vendors try to release patches quickly time patch released patch applied vulnerability introduced Vulnerability discovered

9 Motivation Was this vulnerability triggered on my machine in the past? Can I somehow protect my system before I install the patch? time patch released patch applied vulnerability introduced time patch released patch applied vulnerability introduced Vulnerability discovered

10 Predicates Patch writer knows exactly what conditions during program execution indicate triggering of vulnerability Use this knowledge to write vulnerability- specific predicates that check these conditions –No false positives or false negatives

11 An example 1 char *str = some_string; 2 int length = strlen (str); 3 char buf [BUFSIZE]; 4 strcpy(buf,str); // D’oh! Predicate: (length >= BUFSIZE)

12 Approach vulnerability introduced “past”“present” time patch released patch applied Using replay, detect if vulnerability was triggered in past Monitor ongoing execution to detect and respond to attempts to trigger vulnerability

13 Goals The system must… 1.Not perturb the target software 2.Work for both OS and application-level vulnerabilities 3.Allow predicates to be installed dynamically 4.Allow predicates to be written easily 5.Have low overhead

14 Challenge #1: Where do predicates execute? On a normal computer, software runs either as a user-level application or in the operating system kernel. Neither of these locations is suitable for executing predicates because predicates should run outside the target system to avoid perturbing its state.

15 control IntroVirt structure hardware host OS guest OS application predicate engine state predicates intrusions detected VMM application

16 Challenge #2: Semantic gap Problem: VMM exposes guest state at the wrong level of abstraction –It gives us registers, memory locations, disk blocks, … –We want program variables, files, … 1 uid = getuid(); 2 // forget to check group membership 3 perform privileged action Predicate –Perform missing authentication, e.g., read /etc/group

17 Bridging the semantic gap How could the programmer write this predicate? –Determine memory location where uid is stored; if page not resident, read from disk; read value of uid; traverse guest OS file system structures to see if /etc/group in file cache, if so, read from memory; if not, traverse FS structures to see which disk blocks contain it, then read blocks from disk; … –i.e., emulate guest functionality Our solution: call guest code –Leverages existing guest code that does what we want –Here, we cause the guest itself to read the file and check group membership

18 Challenge #3: Avoiding perturbations to target state Calling guest functions perturbs target Solution: use checkpoint and restore –Take a checkpoint before changing guest state –Restore to checkpoint after predicate execution Also protects from (buggy) predicates that modify guest state incorrectly

19 the state checked by the predicate can change after the predicate executes but before the state is used by the vulnerable code. Challenge #4: Preemptions between the predicate and the bug

20 Predicate refresh Detect and respond to race –“Predicate refresh” –Observation: in uniprocessors, a scheduling event must occur before any other process can run –Re-execute predicate on scheduling events to detect relevant changes in state

Evaluation The system has 5 goals. Goal 1,2,3 are met by design. Goal 4:Allow predicates to be written easily and goal 5:low overhead, are the main evaluation objectives. 21

Example Predicates CAN : This bug involves a missing bounds check in the Linux kernel’s do_brk function The function neglects to check for integer overflow and to check if the process is trying to expand its heap above the address TASK SIZE. The patch consists of the following code, inserted before line 1044 of mmap.c 22

23 Predicate for CAN Actual Patch: if((addr + len) > TASK_SIZE || (addr + len) < addr) return –EINVAL; Predicate: registerBreak(“mmap.c:1044:begin”, brkEventHandler); void brkEventHandler() { unsigned long addr = readVar(“addr”); unsigned long len = readVar(“len”); if((addr+len) > TASK_SIZE || (addr+len) < addr) { cout << “brk bug triggered” << endl; } }

CAN Vulnerability: static int get_client_master_key(SSL *s) {... s->session->key_arg_length=i; // line 419 s->state=SSL2_ST_GET_CLIENT_MASTER_KEY_B;...} Patch: if(i > SSL_MAX_KEY_ARG_LENGTH) { SSLerr(SSL_F_GET_CLIENT_MASTER_KEY, SSL_R_KEY_ARG_TOO_LONG); return -1; } 24

Predicate: void sslEventHandler() { unsigned long i = readVar("i"); if(i > SSL_MAX_KEY_ARG_LENGTH) // "kill process" response strategy introvirt.killCurrentProcess(); } 25

26 Experience Wrote predicates for 20 real vulnerabilities (Linux kernel, bind, emacs, gv, imapd, OpenSSL, php, smbd, squid, wu-ftpd, xpdf) –Easy to write once vulnerability is understood –Length and complexity comparable to patch –Most are simple, e.g., just read a few variables Overhead for most predicates is less than 10% –Many predicates are on infrequently executed code paths –Frequently executed predicates are simple and fast –Checkpoint/restore adds 5ms

Predicates they have written 27

28 Related work VM introspection [Rosenblum97] VM introspection for intrusion detection [Garfinkel03] Shield [Wang04] Vigilante [Costa05]

29 Conclusions Vulnerability-specific predicates detect triggering of software vulnerabilities IntroVirt predicate engine –Simple to write general-purpose predicates –No perturbations in state Alert users about past attacks Detect and respond to attacks in the present