Messages in this thread | | | From | Paul Moore <> | Date | Thu, 18 Aug 2022 21:59:02 -0400 | Subject | Re: data-race in audit_log_start / audit_receive |
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On Thu, Aug 18, 2022 at 6:23 PM Abhishek Shah <abhishek.shah@columbia.edu> wrote: > Hi all, > > We found a data race involving the audit_cmd_mutex.owner variable. We think this bug is concerning because audit_ctl_owner_current is used at a location that controls the scheduling of tasks shown here. Please let us know what you think. > > Thanks! > > -----------------Report---------------------- > > write to 0xffffffff881d0710 of 8 bytes by task 6541 on cpu 0: > audit_ctl_lock kernel/audit.c:237 [inline]
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> read to 0xffffffff881d0710 of 8 bytes by task 6542 on cpu 1: > audit_ctl_owner_current kernel/audit.c:258 [inline]
Yes, technically there is a race condition if/when an auditd instance is registering itself the exact same time as another task is attempting to log an audit record via audit_log_start(). The risk being that a *very* limited number of audit records could be mis-handled with respect to their queue priority and that is it; no records would be lost or misplaced. Correcting this would likely involve a more complex locking scheme[1] or a rather severe performance penalty due to an additional lock in the audit_log_start() code path. There may be some value in modifying audit_ctl_owner_current() to use READ_ONCE(), but it isn't clear to me that this would significantly improve things or have no impact on performance.
Have you noticed any serious problems on your system due to this? If you have a reproducer which shows actual harm on the system could you please share that?
[1] The obvious choice would be to move to a RCU based scheme, but even that doesn't totally solve the problem as there would still be a window where some tasks would have an "old" value. It might actually end up extending the race window on large multi-core systems due to the time needed for all of the critical sections to complete.
-- paul-moore.com
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