Change in osmo-bsc[master]: abis_rsl: prioritize emergency calls over regular calls

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laforge gerrit-no-reply at lists.osmocom.org
Thu Sep 3 10:31:20 UTC 2020


laforge has posted comments on this change. ( https://gerrit.osmocom.org/c/osmo-bsc/+/19793 )

Change subject: abis_rsl: prioritize emergency calls over regular calls
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Patch Set 2:

(1 comment)

https://gerrit.osmocom.org/c/osmo-bsc/+/19793/2/src/osmo-bsc/abis_rsl.c 
File src/osmo-bsc/abis_rsl.c:

https://gerrit.osmocom.org/c/osmo-bsc/+/19793/2/src/osmo-bsc/abis_rsl.c@1406 
PS2, Line 1406: 	l
how do we protect against Denial of Service here?  I think we unconditioally append every channel request to the queue, growing it to extreme lengths.  Think of somebody running a RACH DoS attack.

If I read the code correctly:

With the previous code in master (without a queue), I think there is stil la somewhat fair chance that a valid RACH request from a user gets a channel, as valid and invalid requests compete.

With an indefinitely long queue, all of the hundreds to thousands of fake RACH requests make a very long queue, and until that queue has been fully processed, no new real RACH request has any chance of succeeding.

So I think this patch would create a "DoS amplification", as it makes the situation even worse than it already is under DoS.

If the queue grows beyond a certain length, the phone will have given up waiting for an IMMEDIATE ASSIGN (or IMMEDIATE ASSIGN REJECT) by the time we process the entry in the queue.  So I think the entries must have a timeout that correlates with the timeout on the MS side, which in turn I believe is configured by the RACH control parameters in the system information.

What about: When we dequeue the entries, we first check if the timeout already has expired.  If it has, discard that queue entry and move to the next?



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Gerrit-Project: osmo-bsc
Gerrit-Branch: master
Gerrit-Change-Id: If8651265928797dbda9f528b544931dcfa4a0b36
Gerrit-Change-Number: 19793
Gerrit-PatchSet: 2
Gerrit-Owner: dexter <pmaier at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: neels <nhofmeyr at sysmocom.de>
Gerrit-CC: laforge <laforge at osmocom.org>
Gerrit-Comment-Date: Thu, 03 Sep 2020 10:31:20 +0000
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