Re: Watch Performance

Hello, Over the last day or two, I re-worked the user space audit code to be able to control the new file system audit subsystem. As I was doing the work, I became concerned about the performance impact since it appears to be using the syscall exit filter. The syscall exit filter (and entry filter) is expensive to use except in cases where you need to use it. This is because each rule in it must be examined during each syscall to see if the current syscall is of interest. The current lspp configuration has 10 syscall audit rules. I became curious what the measured impact would be with the current file system audit implementation. I decide to run the same performance test that I tested the audit system with a couple weeks ago when inode and IPC problems were noticed. I used the lspp.16 kernel with profile=2 boot param. The following table shows the results: rules seconds 0 49 10 56 25 75 50 115 75 143 90 185

0 rules had this for function usage: 1284 __d_lookup 4.7380 1170 __link_path_walk 0.3098 1065 avc_has_perm_noaudit 1.2144 706 _atomic_dec_and_lock 8.4048 612 do_path_lookup 0.8204 561 dput 1.2986 509 _raw_spin_lock 2.1477 10 rules had this: 1295 __d_lookup 4.7786 1089 audit_filter_syscall 6.3684 1081 __link_path_walk 0.2862 889 avc_has_perm_noaudit 1.0137 676 audit_getname 2.6000 658 do_path_lookup 0.8820 596 _atomic_dec_and_lock 7.0952 25 rules had this: 3193 audit_filter_rules 3.0009 2178 audit_filter_syscall 12.7368 1280 __d_lookup 4.7232 1131 __link_path_walk 0.2994 956 avc_has_perm_noaudit 1.0901 652 _atomic_dec_and_lock 7.7619 530 dput 1.2269 50 rules had this: 11213 audit_filter_rules 10.5385 4654 audit_filter_syscall 27.2164 4100 selinux_task_ctxid 141.3793 1212 __d_lookup 4.4723 1103 __link_path_walk 0.2920 1012 avc_has_perm_noaudit 1.1539 788 _atomic_dec_and_lock 9.3810 75 had this: 15351 audit_filter_rules 14.4276 6032 audit_filter_syscall 35.2749 2066 selinux_task_ctxid 71.2414 1237 __d_lookup 4.5646 1184 __link_path_walk 0.3135 1014 avc_has_perm_noaudit 1.1562 592 _atomic_dec_and_lock 7.0476 and 90 rules had this: 18287 audit_filter_rules 17.1870 9173 audit_filter_syscall 53.6433 4346 selinux_task_ctxid 149.8621 1314 __link_path_walk 0.3479 1218 __d_lookup 4.4945 1070 avc_has_perm_noaudit 1.2201 682 _atomic_dec_and_lock 8.1190 As you can see, the audit_filter_rules and audit_filter_syscall overwhelmed the profile quickly. It would not be unreasonable for a system to have 40 watches. The lspp rules have 56 of them. With 10 syscall rules added, the performance of a correctly configured lspp machine will be similar to the 75 rules test. This represents a 186% performance hit compared to no audit rules. I do not believe optimizing the audit_filter_rules function will solve the problem. I think the file system audit algorithm needs to be re-thought. It simply cannot penalize every syscall. There are several ways to solve the problem. Maybe what we need to do is use the watch list to store watches on and add a new field to the context. If a watch is triggered it sets the flag in the context. When syscall exit is done, it checks the flag and if set, does both the watch list and the exit list. Otherwise, it skips the watch list. I don't know if this is feasible, or a preferred solution, but we need to start looking at how to decouple the exit list and watches. -Steve -- Linux-audit mailing list Linux-audit(a)redhat.com https://www.redhat.com/mailman/listinfo/linux-audit