[Auditbeat] btmp offset check (elastic#24515)

* 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 b03a935)

CHANGELOG.next.asciidoc

@@ -159,6 +159,7 @@ 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*
 

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)