From 3ec3ae80b430368969d293ecfeb97697a66af69b Mon Sep 17 00:00:00 2001
From: Lee Hinman <57081003+leehinman@users.noreply.github.com>
Date: Tue, 16 Mar 2021 09:53:50 -0500
Subject: [PATCH] [Auditbeat] btmp offset check (#24515) (#24551)

* auditbeat btmp offset check

Add check that saved offset is not larger than the current file size
to prevent seeking past the end of file

(cherry picked from commit b03a935282bec78b3160e74e66dfe29e2df8ee69)
---
 CHANGELOG.next.asciidoc                      |  1 +
 x-pack/auditbeat/module/system/login/utmp.go | 10 +++++-----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/CHANGELOG.next.asciidoc b/CHANGELOG.next.asciidoc
index 2f43375bf7b..57b2b61af2b 100644
--- a/CHANGELOG.next.asciidoc
+++ b/CHANGELOG.next.asciidoc
@@ -121,6 +121,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - system/socket: Fix dataset using 100% CPU and becoming unresponsive in some scenarios. {pull}19033[19033] {pull}19764[19764]
 - system/socket: Fixed tracking of long-running connections. {pull}19033[19033]
 - system/login: Fixed offset reset on inode reuse. {pull}24414[24414]
+- system/login: Add additional offset check for utmp files. {pull}24515[24515]
 
 *Filebeat*
 
diff --git a/x-pack/auditbeat/module/system/login/utmp.go b/x-pack/auditbeat/module/system/login/utmp.go
index 24263d210cd..273229f051a 100644
--- a/x-pack/auditbeat/module/system/login/utmp.go
+++ b/x-pack/auditbeat/module/system/login/utmp.go
@@ -181,14 +181,14 @@ func (r *UtmpFileReader) readNewInFile(loginRecordC chan<- LoginRecord, errorC c
 
 	size := utmpFile.Size
 	oldSize := savedUtmpFile.Size
-	if size < oldSize {
+	if size < oldSize || utmpFile.Offset > size {
 		// UTMP files are append-only and so this is weird. It might be a sign of
 		// a highly unlikely inode reuse - or of something more nefarious.
 		// Setting isKnownFile to false so we read the whole file from the beginning.
 		isKnownFile = false
 
-		r.log.Warnf("Unexpectedly, the file %v is smaller than before (new: %v, old: %v) - reading whole file.",
-			utmpFile.Path, size, oldSize)
+		r.log.Warnf("saved size or offset illogical (new=%+v, saved=%+v) - reading whole file.",
+			utmpFile, savedUtmpFile)
 	}
 
 	if !isKnownFile && size == 0 {
@@ -221,7 +221,7 @@ func (r *UtmpFileReader) readNewInFile(loginRecordC chan<- LoginRecord, errorC c
 
 		// This will be the usual case, but we do not want to seek with the stored offset
 		// if the saved size is smaller than the current one.
-		if size >= oldSize {
+		if size >= oldSize && utmpFile.Offset <= size {
 			_, err = f.Seek(utmpFile.Offset, 0)
 			if err != nil {
 				errorC <- errors.Wrapf(err, "error setting offset %d for file %v", utmpFile.Offset, utmpFile.Path)
@@ -230,7 +230,7 @@ func (r *UtmpFileReader) readNewInFile(loginRecordC chan<- LoginRecord, errorC c
 
 		// If the saved size is smaller than the current one, or the previous Seek failed,
 		// we retry one more time, this time resetting to the beginning of the file.
-		if size < oldSize || err != nil {
+		if size < oldSize || utmpFile.Offset > size || err != nil {
 			_, err = f.Seek(0, 0)
 			if err != nil {
 				errorC <- errors.Wrapf(err, "error setting offset 0 for file %v", utmpFile.Path)