Dealing with legacy code

Index
• Dealing with legacy (bad) application code
• Sandboxing
• Isolation

Running untrusted code
• We often need to run buggy/untrusted code:
• programs from untrusted Internet sites:
• toolbars, viewers, codecs for media player
• old or insecure applications: ghostview, outlook
• legacy daemons: sendmail, bind
• honeypots
• Goal: if application “misbehaves,” kill it

Approach: confinement
• Confinement: ensure application does not deviate from pre-approved
behavior
• Can be implemented at many levels:
• Hardware: run application on isolated hardware (air gap)
• difficult to manage
• Virtual machines: isolate OS’s on single hardware
• System call interposition:
• Isolates a process in a single operating system
• Isolating threads sharing same address space:
• Software Fault Isolation (SFI)
• Application specific: e.g. browser-based confinement

Implementing confinement
• Key component: reference monitor
• Mediates requests from applications
• Implements protection policy
• Enforces isolation and confinement
• Must always be invoked:
• Every application request must be mediated
• Tamperproof:
• Reference monitor cannot be killed
• … or if killed, then monitored process is killed too
• Small enough to be analyzed and validated

A simple example: chroot
• Often used for “guest” accounts on ftp sites
• To use do: (must be root)
chroot /tmp/guest root dir “/” is now “/tmp/guest”
su guest EUID set to “guest”
• Now “/tmp/guest” is added to file system accesses for applications in jail
open(“/etc/passwd”, “r”) 
open(“/tmp/guest/etc/passwd”, “r”)
 application cannot access files outside of jail

Jailkit
Problem: all utility progs (ls, ps, vi) must live inside jail
• jailkit project: auto builds files, libs, and dirs needed in jail
environment
• jk_init: creates jail environment
• jk_check: checks jail env for security problems
• checks for any modified programs,
• checks for world writable directories, etc.
• jk_lsh: restricted shell to be used inside jail
• note: simple chroot jail does not limit network access

Escaping from jails
• Early escapes: relative paths
open( “../../etc/passwd”, “r”) 
open(“/tmp/guest/../../etc/passwd”, “r”)
• chroot should only be executable by root
• otherwise jailed app can do:
• create dummy file “/aaa/etc/passwd”
• run chroot “/aaa”
• run su root to become root
(bug in Ultrix 4.0)

Many ways to escape jail as root
• Create device that lets you access raw disk
• Send signals to non chrooted process
• Reboot system
• Bind to privileged ports

Freebsd jail
• Stronger mechanism than simple chroot
• To run:
jail jail-path hostname IP-addr cmd
• calls hardened chroot (no “../../” escape)
• can only bind to sockets with specified IP address
and authorized ports
• can only communicate with process inside jail
• root is limited, e.g. cannot load kernel modules

Problems with chroot and jail
• Coarse policies:
• All or nothing access to file system
• Inappropriate for apps like web browser
• Needs read access to files outside jail
(e.g. for sending attachments in gmail)
• Do not prevent malicious apps from:
• Accessing network and messing with other machines
• Trying to crash host OS

System call interposition:
a better approach to confinement

Sys call interposition
• Observation: to damage host system (i.e. make persistent changes) app must
make system calls
• To delete/overwrite files: unlink, open, write
• To do network attacks: socket, bind, connect, send
• Idea:
• monitor app system calls and block unauthorized calls
• Implementation options:
• Completely kernel space (e.g. GSWTK)
• Completely user space (e.g. program shepherding)
• Hybrid (e.g. Systrace)

Initial implementation (Janus)
• Linux ptrace: process tracing
tracing process calls: ptrace (… , pid_t pid , …)
and wakes up when pid makes sys call.
• Monitor kills application if request is disallowed
OS Kernel
monitored
application
(outlook)
monitor
user space
open(“etc/passwd”, “r”)

Complications
• If app forks, monitor must also fork
• Forked monitor monitors forked app
• If monitor crashes, app must be killed
• Monitor must maintain all OS state associated with app
• current-working-dir (CWD), UID, EUID, GID
• Whenever app does “cd path” monitor must also update its CWD
• otherwise: relative path requests interpreted incorrectly

Problems with ptrace
• Ptrace too coarse for this application
• Trace all system calls or none
• e.g. no need to trace “close” system call
• Monitor cannot abort sys-call without killing app
• Security problems: race conditions
• Example: symlink: me -> mydata.dat
proc 1: open(“me”)
monitor checks and authorizes
proc 2: me -> /etc/passwd
OS executes open(“me”)
• Classic TOCTOU bug: time-of-check / time-of-use
time
not atomic

Alternate design: systrace
• systrace only forwards monitored sys-calls to monitor (saves
context switches)
• systrace resolves sym-links and replaces sys-call path arguments
by full path to target
• When app calls execve, monitor loads new policy file
OS Kernel
monitored
application
(outlook)
monitor
user space
open(“etc/passwd”, “r”)
sys-call
gateway
systrace
permit/deny
policy file
for app

Policy
• Sample policy file:
path allow /tmp/*
path deny /etc/passwd
network deny all
• Specifying policy for an app is quite difficult
• Systrace can auto-gen policy by learning how app behaves
on “good” inputs
• If policy does not cover a specific sys-call, ask user
… but user has no way to decide
• Difficulty with choosing policy for specific apps (e.g. browser)
is main reason this approach is not widely used

Confinement using Virtual Machines

Virtual Machines
Virtual Machine Monitor (VMM)
Guest OS 2
Apps
Guest OS 1
Apps
Hardware
Host OS
VM2 VM1
Example: NSA NetTop
• single HW platform used for both classified and
unclassified data

Why so popular now?
• VMs in the 1960’s:
• Few computers, lots of users
• VMs allow many users to shares a single computer
• VMs 1970’s – 2000: non-existent
• VMs since 2000:
• Too many computers, too few users
• Print server, Mail server, Web server,
File server, Database server, …
• Wasteful to run each service on a different computer
• VMs save power while isolating services

VMM security assumption
• VMM Security assumption:
• Malware can infect guest OS and guest apps
• But malware cannot escape from the infected VM
• Cannot infect host OS
• Cannot infect other VMs on the same hardware
• Requires that VMM protect itself and is not buggy
• VMM is much simpler than full OS
• but device drivers run in Host OS

Problem: covert channels
• Covert channel: unintended communication channel between
isolated components
• Can be used to leak classified data from secure component to
public component
Classified VM Public VM
secret
doc
malware
listener
covert
channel
VMM

An example covert channel
• Both VMs use the same underlying hardware
• To send a bit b  {0,1} malware does:
• b= 1: at 1:30.00am do CPU intensive calculation
• b= 0: at 1:30.00am do nothing
• At 1:30.00am listener does a CPU intensive calculation and
measures completion time
• Now b = 1  completion-time > threshold
• Many covert channel exist in running system:
• File lock status, cache contents, interrupts, …
• Very difficult to eliminate

VMM Introspection: protecting the anti-virus system

Intrusion Detection / Anti-virus
• Runs as part of OS kernel and user space process
• Kernel root kit can shutdown protection system
• Common practice for modern malware
• Standard solution: run IDS system in the network
• Problem: insufficient visibility into user’s machine
• Better: run IDS as part of VMM (protected from malware)
• VMM can monitor virtual hardware for anomalies
• VMI: Virtual Machine Introspection
• Allows VMM to check Guest OS internals

Sample checks
Stealth malware:
• Creates processes that are invisible to “ps”
• Opens sockets that are invisible to “netstat”
1. Lie detector check
• Goal: detect stealth malware that hides processes
and network activity
• Method:
• VMM lists processes running in GuestOS
• VMM requests GuestOS to list processes (e.g. ps)
• If mismatch, kill VM

Sample checks
2. Application code integrity detector
• VMM computes hash of user app-code running in VM
• Compare to whitelist of hashes
• Kills VM if unknown program appears
3. Ensure GuestOS kernel integrity
• example: detect changes to sys_call_table
4. Virus signature detector
• Run virus signature detector on GuestOS memory
5. Detect if GuestOS puts NIC in promiscuous mode

Subvirt:
subvirting VMM confinement

Subvirt
• Virus idea:
• Once on the victim machine, install a malicious VMM
• Virus hides in VMM
• Invisible to virus detector running inside VM
HW
OS

HW
OS
VMM and virus
Anti-virus
Anti-virus

VM Based Malware (blue pill virus)
• VMBR: a virus that installs a malicious VMM (hypervisor)
• Microsoft Security Bulletin:
• Suggests disabling hardware virtualization features
by default for client-side systems
• But VMBRs are easy to defeat
• A guest OS can detect that it is running on top of VMM

VMM Detection
• Can an OS detect it is running on top of a VMM?
• Applications:
• Virus detector can detect VMBR
• Normal virus (non-VMBR) can detect VMM
• refuse to run to avoid reverse engineering
• Software that binds to hardware (e.g. MS Windows) can
refuse to run on top of VMM
• DRM systems may refuse to run on top of VMM

VMM detection (red pill techniques)
1. VM platforms often emulate simple hardware
• VMWare emulates an ancient i440bx chipset
… but report 8GB RAM, dual Opteron CPUs, etc.
2. VMM introduces time latency variances
• Memory cache behavior differs in presence of VMM
• Results in relative latency in time variations
for any two operations
3. VMM shares the TLB with GuestOS
• GuestOS can detect reduced TLB size
… and many more methods [GAWF’07]

VMM Detection
Bottom line: The perfect VMM does not exist
• VMMs today (e.g. VMWare) focus on:
Compatibility: ensure off the shelf software works
Performance: minimize virtualization overhead
• VMMs do not provide transparency
• Anomalies reveal existence of VMM

Software Fault Isolation
• Goal: confine apps running in same address space
• Codec code should not interfere with media player
• Device drivers should not corrupt kernel
• Simple solution: runs apps in separate address spaces
• Problem: slow if apps communicate frequently
• requires context switch per message

Software Fault Isolation
• SFI approach:
• Partition process memory into segments
• Locate unsafe instructions: jmp, load, store
• At compile time, add guards before unsafe instructions
• When loading code, ensure all guard are present
code
segment
data
segment
code
segment
data
segment
app #1 app #2

Segment matching technique
• Designed for MIPS processor. Many registers available.
• dr1, dr2: dedicated registers not used by binary
• Compiler pretends these registers don’t exist
• dr2 contains segment ID
• Indirect load instruction R12  [addr]
becomes:
dr1  addr
scratch-reg  (dr1 >> 20) : get segment ID
compare scratch-reg and dr2: validate seg. ID
trap if not equal
R12  [addr] : do load
Guard ensures code does not
load data from another segment

Address sandboxing technique
• dr2: holds segment ID
• Indirect load instruction R12  [addr]
becomes:
dr1  addr & segment-mask : zero out seg bits
dr1  dr1 | dr2 : set valid seg ID
R12  [dr1] : do load
• Fewer instructions than segment matching
… but does not catch offending instructions
• Lots of room for optimizations: reduce # of guards

Cross domain calls
caller
domain
callee
domain
call draw
stub draw:
return
br addr
br addr
br addr
stub
• Only stubs allowed to make croos-domain jumps
• Jump table contains allowed exit points from callee
• Addresses are hard coded, read-only segment

SFI: concluding remarks
• For shared memory: use virtual memory hardware
• Map same physical page to two segments in addr space
• Performance
• Usually good: mpeg_play, 4% slowdown
• Limitations of SFI: harder to implement on x86 :
• variable length instructions: unclear where to put guards
• few registers: can’t dedicate three to SFI
• many instructions affect memory: more guards needed

Summary
• Many sandboxing techniques:
• Physical air gap,
• Virtual air gap (VMMs),
• System call interposition
• Software Fault isolation
• Application specific (e.g. Javascript in browser)
• Often complete isolation is inappropriate
• Apps need to communicate through regulated interfaces
• Hardest aspect of sandboxing:
• Specifying policy: what can apps do and not do

Dealing with legacy code

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