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From: Michael Tuexen <tuexen@FreeBSD.org>
Date: Tue, 5 May 2020 19:54:30 +0000 (UTC)
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Subject: svn commit: r360671 - head/sys/netinet
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Author: tuexen
Date: Tue May  5 19:54:30 2020
New Revision: 360671
URL: https://svnweb.freebsd.org/changeset/base/360671

Log:
  Avoid underflowing a variable, which would result in taking more
  data from the stream queues then needed.
  
  Thanks to Timo Voelker for finding this bug and providing a fix.
  
  MFC after:		3 days

Modified:
  head/sys/netinet/sctp_output.c

Modified: head/sys/netinet/sctp_output.c
==============================================================================
--- head/sys/netinet/sctp_output.c	Tue May  5 19:31:47 2020	(r360670)
+++ head/sys/netinet/sctp_output.c	Tue May  5 19:54:30 2020	(r360671)
@@ -7763,7 +7763,11 @@ sctp_fill_outqueue(struct sctp_tcb *stcb,
 		}
 		strq = stcb->asoc.ss_functions.sctp_ss_select_stream(stcb, net, asoc);
 		total_moved += moved;
-		space_left -= moved;
+		if (space_left >= moved) {
+			space_left -= moved;
+		} else {
+			space_left = 0;
+		}
 		if (space_left >= SCTP_DATA_CHUNK_OVERHEAD(stcb)) {
 			space_left -= SCTP_DATA_CHUNK_OVERHEAD(stcb);
 		} else {