Analyzing the Performance Effects of Meltdown + Spectre on Apache Spark Workloads with Chris Stevens
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Meltdown and Spectre are two security vulnerabilities disclosed in early 2018 that expose systems to cross-VM and cross-process attacks. They were the first of their kind and opened up a new class of exploits that allow one program to scan another program’s memory. The kernel and VM patches released to address these vulnerabilities have shown to degrade the performance of Apache Spark workloads in the cloud by 2-5%. This talk will dive deep into the exploits and their patches in order to help explain the origin of this decline in performance.
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Analyzing the Performance Effects of Meltdown + Spectre on Apache Spark Workloads with Chris Stevens
- 1. Meltdown, Spectre and Apache Spark™ Performance Chris Stevens June 5, 2018 1
- 2. Databricks Performance on AWS 2 3-5% performance degradation
- 3. Overview Goal: Understand the 3-5% degradation Steps: - TPC-DS Benchmarks - System Analysis - Breakdown the exploits and patches - Meltdown - Spectre V1 - Bounds Check Bypass - Specter V2 - Branch Target Injection 3
- 4. TPC-DS: q1-v2.4 4 WITH customer_total_return AS (SELECT sr_customer_sk AS ctr_customer_sk, sr_store_sk AS ctr_store_sk, sum(sr_return_amt) AS ctr_total_return FROM store_returns, date_dim WHERE sr_returned_date_sk = d_date_sk AND d_year = 2000 GROUP BY sr_customer_sk, sr_store_sk) SELECT c_customer_id FROM customer_total_return ctr1, store, customer WHERE ctr1.ctr_total_return > (SELECT avg(ctr_total_return)*1.2 FROM customer_total_return ctr2 WHERE ctr1.ctr_store_sk = ctr2.ctr_store_sk) AND s_store_sk = ctr1.ctr_store_sk AND s_state = 'TN' AND ctr1.ctr_customer_sk = c_customer_sk ORDER BY c_customer_id LIMIT 100
- 7. q1-v2.4 CPU Utilization 7mpstat -P ALL
- 8. TPC-DS: q1-v2.4 8strace -f -c -p PID
- 9. Exploits Background Out-of-Order Execution + Side Channel Attacks 9
- 10. In-order Execution x = a + b y = c + d z = x + y Add A + B Store X Decode Add Retire Add C + B Store Y Decode Add Retire Add X + Y Store Z Decode Add Retire Time 10
- 11. Out-of-order Execution x = a + b y = c + d z = x + y Add A + B Store X Decode Add Retire Add C + D Store Y Decode Add Retire Add X+Y Store Z Decode Add Retire Time 11
- 12. Side-Channel Attacks FLUSH +RELOAD 0 0xABAB 1 0xCDCD 2 0xEFEF 3 0x1212 0 INVALID 1 INVALID 2 INVALID 3 INVALID 0 INVALID 1 INVALID 2 0xEFEF 3 INVALID Speculative execution fills cache entry 2 0 0xABAB 1 0xCDCD 2 0xEFEF 3 0x1212 ATTACK clflush instruction invalidates the cache Reload measures the time to read each cache entry 12
- 14. Meltdown 14
- 15. Meltdown kernelByte = *kernelAddr; probeArray[kernelByte * 4096]; 15
- 16. Meltdown kernelByte = *kernelAddr; probeArray[kernelByte * 4096]; Get value of kernelAddr Get value of probeArray Read 1 Byte at kernelAddr Retire Multiply kernel byte by 4096 Read probeArray at offset Time Decode Multiply Decode Array Access Decode Memory Read Retire Retire Permission Check Here Side Channel Attack 16
- 17. Side-Channel Attack FLUSH +RELOAD 0*4096 INVALID 1*4096 INVALID 2*4096 INVALID 3*4096 INVALID Speculative execution reads probeArray at kernelByte * 4096 ATTACK Flush each page in probeArray Reload measures the time to read each page in probeArray 0*4096 INVALID 1*4096 INVALID 2*4096 0xEFEF 3*4096 INVALID 0*4096 0xABAB 1*4096 0xCDCD 2*4096 0xEFEF 3*4096 0x1212 Sees that page 2 was the fastest => kernelByte = 0x2 17
- 18. Kernel Page-Table Isolation Process Page Tables without KPTI Combined Page Tables Kernel Virtual Memory User Virtual Memory Sensitive Data Meltdown Code Process Page Tables with KPTI Kernel Page Tables Kernel Virtual Memory User Virtual Memory Sensitive Data Meltdown Code Meltdown Code User Mode Page Tables Same Physical Memory 18
- 19. Meltdown kernelByte = *kernelAddr; probeArray[kernelByte * 4096]; Get value of kernelAddr Get value of probeArray Read 1 Byte at kernelAddr Retire Multiply kernel byte by 4096 Read probeArray at offset Time Decode Multiply Decode Array Access Decode Memory Read Retire Retire 19
- 20. Kernel Page-Table Isolation Process Page Tables without KPTI Combined Page Tables Kernel Virtual Memory User Virtual Memory Sensitive Data Meltdown Code Process Page Tables with KPTI Kernel Page Tables Kernel Virtual Memory User Virtual Memory Sensitive Data Meltdown Code Meltdown Code User Mode Page Tables Same Physical Memory 20
- 21. TLB before KPTI Virtual Address Physical Address - - - - while True: print *UserAddress sys_clock_gettime Virtual Address Physical Address UserAddress Page 1 - - Virtual Address Physical Address UserAddress Page 1 KernelTime Page 2 print *UserAddress print *UserAddresssys_clock_gettime TLB MISS TLB MISS TLB HIT 21
- 22. TLB with KPTI Virtual Address Physical Address - - - - while True: print *UserAddress sys_clock_gettime Virtual Address Physical Address - - - - Virtual Address Physical Address - - - - print *UserAddress print *UserAddresssys_clock_gettime TLB MISS TLB MISS TLB MISS 22
- 23. Meltdown TLB Misses 23perf stat -e dtlb-load-misses
- 24. TLB with KPTI and PCID Virtual Address PCID Physical Address - - - - - - while True: print *UserAddress sys_clock_gettime print *UserAddress print *UserAddresssys_clock_gettime TLB MISS TLB MISS TLB HIT Virtual Address PCID Physical Address UserAddress 1 Page 1 - - - Virtual Address PCID Physical Address UserAddress 1 Page 1 KernelTime 0 Page 2
- 26. Spectre V1 Bounds Check Bypass 26
- 27. Bounds Check Bypass if (index < size) { val = array[index]; probeArray[val]; } 27
- 28. Bounds Check Bypass 28 if (index < size) { val = array[index]; probeArray[val]; } Read size Is (index < size)? Retire Read array at index Time Was (index < size)? Decode If Is (index < size>)? Yes Read probeArray Retire Retire Is (index < size>)? Yes Yes Side Channel Attack
- 29. Bounds Check Bypass 29 if (index < size) { val = array[index]; probeArray[val]; } ATTACKTRAIN Was (index < size)? - Was (index < size)? yes Code size = 10 index = 1000 Code size = 10 index = 5
- 30. Observable Speculation Barrier 30 - Protects against Spectre V1 - Bounds Check Bypass - ~14 in the Ubuntu kernel and drivers - Stops speculative array access with the LFENCE barrier if (index < size) { val = array[index]; probeArray[val]; } if (index < size) { osb(); val = array[index]; probeArray[val]; } Before After
- 31. Observable Speculation Barrier 31 if (index < size) { osb() val = array[index]; probeArray[val]; } Read size Is (index < size)? Retire LFENCE Time Was (index < size)? Decode If Is (index < size>)? Yes Retire Yes Read array at index
- 32. Spectre V2 Branch Target Injection 32
- 33. Branch Target Injection 33 def call_func(func, arg): func(arg)
- 34. Branch Target Injection 34 def call_func(func, arg): func(arg) Read func from memory Call func Retire Call previous func Time What was func last time? Decode call func == previous func Retire func == previous func No Yes
- 35. Branch Target Injection 35 def call_func(func, arg): func(arg) ATTACKTRAIN What was func? - What was func? attack_widget call_func(attack_widget, 1000) innocent, 10 call_func(innocent, 10) attack_widget, 1000
- 36. Intel Microcode Updates 36 - Protect against Spectre V2 - Branch Target Injection - Indirect Branch Restricted Speculation (IBRS) - Stops attacks from code running at lower privilege levels - Stops attacks from code running on the sibling hyperthread (STIBP) - Indirect Branch Prediction Barrier (IBPB) - Stops attacks from code running at the same privilege level - Inserted at User-to-User and Guest-to-Guest transitions
- 37. IBPB and IBRS Patches 37 def call_func(func, arg): func(arg) ATTACKTRAIN What was func? - What was func? - call_func(attack_widget, 1000) innocent, 10 call_func(innocent, 10) attack_widget, 1000 IBPB, IBRS
- 38. Branch Target Injection 38 def call_func(func, arg): func(arg) Read func from memory Call func Retire Cache Miss Time What was func last time? Decode call func == previous func No
- 39. Spectre V2 Branch Misses 39perf stat -e branch-misses
- 41. Retpoline - Protects indirect branches/calls from speculative execution - Uses an infinite loop to do it 41 jmp rax call set_up_target speculative_loop: pause lfence jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Return Stack Buffer . . . speculative_loop Stack . . . speculative_loop
- 42. Retpoline - Protects indirect branches/calls from speculative execution - Uses an infinite loop to do it 42 jmp rax call set_up_target speculative_loop: pause lfence jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Return Stack Buffer . . . speculative_loop Stack . . . rax
- 43. Retpoline - Protects indirect branches/calls from speculative execution - Uses an infinite loop to do it 43 jmp rax call set_up_target speculative_loop: pause lfence jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Return Stack Buffer . . . speculative_loop Stack . . . rax
- 44. Retpoline - Protects indirect branches/calls from speculative execution - Uses an infinite loop to do it 44 jmp rax call set_up_target speculative_loop: pause lfence jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Return Stack Buffer . . . speculative_loop Stack . . . rax
- 45. Retpoline - Protects indirect branches/calls from speculative execution - Uses an infinite loop to do it 45 jmp rax call set_up_target speculative_loop: pause lfence jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Return Stack Buffer . . . speculative_loop Stack . . . rax
- 46. Retpoline (Improved) - Check against a known target and direct branch 46 jmp rax cmp rax, known_target jne retpoline jmp known_target retpoline: call set_up_target speculative_loop: pause jmp speculative_loop set_up_target: mov rax, [rsp] ret Before After Direct Branch (fast)
- 48. Retpoline CPU Utilization 48mpstat -P ALL
- 49. Retpoline System Calls 49strace -f -c -p PID
- 50. A Tale of Two Queries 50 q1-v2.4 ss_max-v2.4 SELECT count(*) AS total, count(ss_sold_date_sk) AS not_null_total, count(DISTINCT ss_sold_date_sk) AS unique_days, max(ss_sold_date_sk) AS max_ss_sold_date_sk, max(ss_sold_time_sk) AS max_ss_sold_time_sk, max(ss_item_sk) AS max_ss_item_sk, max(ss_customer_sk) AS max_ss_customer_sk, max(ss_cdemo_sk) AS max_ss_cdemo_sk, max(ss_hdemo_sk) AS max_ss_hdemo_sk, max(ss_addr_sk) AS max_ss_addr_sk, max(ss_store_sk) AS max_ss_store_sk, max(ss_promo_sk) AS max_ss_promo_sk FROM store_sales WITH customer_total_return AS (SELECT sr_customer_sk AS ctr_customer_sk, sr_store_sk AS ctr_store_sk, sum(sr_return_amt) AS ctr_total_return FROM store_returns, date_dim WHERE sr_returned_date_sk = d_date_sk AND d_year = 2000 GROUP BY sr_customer_sk, sr_store_sk) SELECT c_customer_id FROM customer_total_return ctr1, store, customer WHERE ctr1.ctr_total_return > (SELECT avg(ctr_total_return)*1.2 FROM customer_total_return ctr2 WHERE ctr1.ctr_store_sk = ctr2.ctr_store_sk) AND s_store_sk = ctr1.ctr_store_sk AND s_state = 'TN' AND ctr1.ctr_customer_sk = c_customer_sk ORDER BY c_customer_id LIMIT 100
- 51. q1-v2.4 vs ss_max-v2.4 51perf stat -e raw_syscalls:sys_enter -a -I 1000
- 53. Backup Slides 53
- 54. array_index_nospec() 54 - Protects against Spectre V1 - Bounds Check Bypass - ~50 in the Linux kernel and drivers (not in Ubuntu) - Stops speculative array access by clamping the index if (index < size) { val = array[index]; probeArray[val]; } if (index < size) { index = array_index_nospec(index, size); val = array[index]; probeArray[val]; } Before After
- 55. Speculative Store Bypass 55 - CVE-2018-3639 - “Variant 4” - Public Date: May 21, 2018 - Details: - https://access.redhat.com/security/vulnerabilities/ssbd - Ubuntu Command Line Parameter: - spec_store_bypass_disable=on
- 57. TPC-DS: q1-v2.4 57 Baselines Pre-Skylake 2% Skylake+ 11%
- 58. TPC-DS: ss_max-v2.4 58 Baselines Pre-Skylake 0% Skylake+ 1%
- 59. q1-v2.4 Runtime Box Plots 59
- 60. ss_max-v2.4 Runtime Box Plots 60
- 61. Single Node TPC-DS Setup • Intel Core i7-4820k @ 3.70GHz (Ivy Bridge - ca. 2012) • 8 CPUs • 8GB of RAM • Disk: Western Digital Blue – WDC10EZEX-08M2NA0 – 1TB – 7200 rpm – 146MB/s sequential read (http://hdd.userbenchmark.com/) • Ubuntu 16.04.4 LTS (Xenial Xerus) – 64-bit server image – 4.4.0-116-generic Linux kernel 61
- 62. Repositories - https://github.com/apache/spark.git - https://github.com/databricks/spark-sql-perf.git - https://github.com/databricks/tpcds-kit - https://github.com/speed47/spectre-meltdown-checker - https://github.com/brendangregg/pmc-cloud-tools - https://github.com/brendangregg/perf-tools 62