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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Fri, 24 Feb 2017 08:09:16 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r314195 - head/sys/vm
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Author: kib
Date: Fri Feb 24 08:09:16 2017
New Revision: 314195
URL: https://svnweb.freebsd.org/changeset/base/314195

Log:
  Properly handle possible underflow in vm_fault_prefault().
  
  In vm_fault_prefault(), if backward count causes underflow in
  calculation of
  	starta = addra - backward * PAGE_SIZE;
  then starta must be clipped to entry->start, instead of zero.
  Clipping to zero allowed mapping outside of the map entries address
  ranges, in particular, map at zero.
  
  Submitted by:	Yanko Yankulov <yanko.yankulov@gmail.com>
  Reviewed by:	alc
  MFC after:	1 week

Modified:
  head/sys/vm/vm_fault.c

Modified: head/sys/vm/vm_fault.c
==============================================================================
--- head/sys/vm/vm_fault.c	Fri Feb 24 07:53:56 2017	(r314194)
+++ head/sys/vm/vm_fault.c	Fri Feb 24 08:09:16 2017	(r314195)
@@ -1368,11 +1368,12 @@ vm_fault_prefault(const struct faultstat
 
 	entry = fs->entry;
 
-	starta = addra - backward * PAGE_SIZE;
-	if (starta < entry->start) {
+	if (addra < backward * PAGE_SIZE) {
 		starta = entry->start;
-	} else if (starta > addra) {
-		starta = 0;
+	} else {
+		starta = addra - backward * PAGE_SIZE;
+		if (starta < entry->start)
+			starta = entry->start;
 	}
 
 	/*