Attacking the Linux PRNG on Android. David Kaplan, Sagi Kedmi, Roee Hay & Avi Dayan IBM Security Systems

Transcription

1 Attacking the Linux PRNG on Android David Kaplan, Sagi Kedmi, Roee Hay & Avi Dayan IBM Security Systems

2 MOTIVATION

3 motivation_keystore_buffer_overflow We discovered CVE , a stack-based Buffer Overflow in keystore Service responsible of securely storing crypto related data We had privately reported to Google and they provided a patch available in KITKAT. Exploit must overcome various defense mechanisms, including Stack Canaries. /* KeyStore is a secured storage for key-value pairs. In this implementation, * each file stores one key-value pair. Keys are encoded in file names, and * values are encrypted with checksums. The encryption key is protected by a * user-defined password. To keep things simple, buffers are always larger than * the maximum space we needed, so boundary checks on buffers are omitted. */

4 motivation_keystore_buffer_overflow Attacks on the Stack-Smashing Protection: Stack Guard initialization Linux PRNG Naive Online Bruteforce of the Canary Value Impractical: 2^32 attempts on average. AUXV(AT_RANDOM) 128 bits stack_chk_guard 32 bits Stack layout LR Saved Registers Canary (32 bits) Buffer

5 motivation_keystore_buffer_overflow Attacks on the Stack-Smashing Protection: Naive Online Bruteforce of the Canary Value Impractical: 2^32 attempts on average. Online Learning of the Canary Value By another info leak issue Re-forking server: Very efficient: 514 attempts until success on average Stack Guard initialization Linux PRNG AUXV(AT_RANDOM) 128 bits stack_chk_guard 32 bits Stack layout LR Saved Registers Canary (32 bits) Buffer

6 motivation_keystore_buffer_overflow Attacks on the Stack-Smashing Protection: Naive Online Bruteforce of the Canary Value Impractical: 2^32 attempts on average. Online Learning of the Canary Value By another info leak issue Re-forking server: Very efficient: 514 attempts until success on average Overwrite stack_chk_guard By overwriting some pointer Stack Guard initialization AUXV(AT_RANDOM) stack_chk_guard 32 bits Stack layout Linux PRNG 128 bits LR Saved Registers Canary (32 bits) Buffer

7 motivation_keystore_buffer_overflow Attacks on the Stack-Smashing Protection: Naive Online Bruteforce of the Canary Value Impractical: 2^32 attempts on average. Online Learning of the Canary Value By another info leak issue Re-forking server: Very efficient: 514 attempts until success on average Overwrite stack_chk_guard By overwriting some pointer Our attack: Offline reconstruction of the PRNG s internal state Stack Guard initialization AUXV(AT_RANDOM) stack_chk_guard 32 bits Stack layout Linux PRNG 128 bits LR Saved Registers Canary (32 bits) Buffer

8 LINUX PRNG

9 entropy_sources time seconds nanoseconds INPUT ktime_t ktime_t DISK T I M E R INPUT POOL NON-BLOCKING-POOL INTERRUPT if KEC >= 192 bits EXTRACTION (PULL) *KEC = Kernel Entropy Count

10 OUR WORK

11 contribution Prior art on weakness in early boot * Present practical run-time attack Formalize attack Demonstrate PoC against current mobile platforms * Heninger et al. 2012, Becherer et al. 2009, Ding et al. 2014

12 attack Given a LEAK of a value extracted from the non-blocking pool and LOW ENTROPY AT BOOT, the STATE of the PRNG can be determined prior to time of leak until external entropy is indeterminate

13 attack_flow SIMULATE

14 attack_flow CANDIDATE POOL SEEDS input non-blocking SIMULATE

15 attack_flow CANDIDATE POOL SEEDS input non-blocking CANDIDATE EXTERNAL ENTROPY SIMULATE

16 attack_flow CANDIDATE POOL SEEDS input non-blocking CANDIDATE EXTERNAL ENTROPY SIMULATE ONLINE LEAK COMPARE

17 attack_flow CANDIDATE POOL SEEDS input non-blocking CANDIDATE EXTERNAL ENTROPY SIMULATE ONLINE LEAK COMPARE SEEDS + ENTROPY PATH

18 EXPERIMENT

19 s4_non-blocking_seed CANDIDATE POOL SEEDS SIMULATE LEAK COMPARE SEEDS + ENTROPY PATH H (s nb )=23.5 bits

20 prng_value_leak ANDROID ZYGOTE (app_process) Apps fork()ed from Zygote process Same AUX VECTOR AT_RANDOM 16 bytes extracted from PRNG LEAK: Malicious app extracts AT_RANDOM from mem space Linux PRNG LEAK CANDIDATE POOL SEEDS SIMULATE COMPARE SEEDS + ENTROPY PATH AUXV(AT_RANDOM) app_process stack_chk_guard APP-1 fork() stack_chk_guard APP-2 fork() stack_chk_guard fork() MALWARE stack_chk_guard

21 simulate_prng Issue 1: order of extraction non-deterministic due to concurrent execution LEAK CANDIDATE POOL SEEDS SIMULATE COMPARE POOL STATE SEEDS + ENTROPY PATH Process A Process B Process C Which process is Zygote? 4 Process D Solution: linear search in set of extracted blocks

22 simulate_prng Issue 2: pool state corruption due to concurrent execution LEAK CANDIDATE POOL SEEDS SIMULATE COMPARE POOL STATE SEEDS + ENTROPY PATH 1 Yellow Path Green Path 2 - Process A: extract from pool - Process A: mix into pool - Process A: extract from pool - Process B: extract from pool Process B: extract from pool - Process B: mix into pool - Process A: mix into pool - Process B: mix into pool 4 4 Most likely path in 20% of cases ~uniform distribution otherwise extract POOL mix

23 s4_demo CANDIDATE POOL SEEDS SIMULATE LEAK COMPARE SEEDS + ENTROPY PATH DEMO

24 wrap_up Demonstrated practical attack against S4 derandomization KeyStore canary Another remote leak can be found in the paper Affects majority of devices in the field Mitigations in newer kernels + devices

25 ? Thank you