tail: Fix heap overflow in -F case.

The number of events we track can vary over time, but we only allocate
enough space for the exact number of events we are tracking when we
first begin, resulting in a trivially reproducable heap overflow.  Fix
this by allocating enough space for the greatest possible number of
events (two per file) and clean up the code a bit.

Also add a test case which triggers the aforementioned heap overflow,
although we don't currently have a way to detect it.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	allanjude, markj
Differential Revision:	https://reviews.freebsd.org/D42839

2 files changed, 38 insertions, 22 deletions

diff --git a/usr.bin/tail/forward.c b/usr.bin/tail/forward.c
index e8e2cb89ab01..a8f52ed376c4 100644
--- a/usr.bin/tail/forward.c
+++ b/usr.bin/tail/forward.c
@@ -272,7 +272,7 @@ set_events(file_info_t *files)
 
 	action = USE_KQUEUE;
 	for (i = 0, file = files; i < no_files; i++, file++) {
-		if (! file->fp)
+		if (!file->fp)
 			continue;
 
 		if (fstatfs(fileno(file->fp), &sf) == 0 &&
@@ -304,27 +304,21 @@ set_events(file_info_t *files)
 void
 follow(file_info_t *files, enum STYLE style, off_t off)
 {
-	int active, ev_change, i, n = -1;
+	int active, ev_change, i, n;
 	struct stat sb2;
 	file_info_t *file;
 	FILE *ftmp;
 	struct timespec ts;
 
 	/* Position each of the files */
-
-	file = files;
 	active = 0;
-	n = 0;
-	for (i = 0; i < no_files; i++, file++) {
-		if (file->fp) {
-			active = 1;
-			n++;
-			if (vflag || (qflag == 0 && no_files > 1))
-				printfn(file->file_name, 1);
-			forward(file->fp, file->file_name, style, off, &file->st);
-			if (Fflag && fileno(file->fp) != STDIN_FILENO)
-				n++;
-		}
+	for (i = 0, file = files; i < no_files; i++, file++) {
+		if (!file->fp)
+			continue;
+		active = 1;
+		if (vflag || (qflag == 0 && no_files > 1))
+			printfn(file->file_name, 1);
+		forward(file->fp, file->file_name, style, off, &file->st);
 	}
 	if (!Fflag && !active)
 		return;
@@ -334,9 +328,14 @@ follow(file_info_t *files, enum STYLE style, off_t off)
 	kq = kqueue();
 	if (kq < 0)
 		err(1, "kqueue");
-	ev = malloc(n * sizeof(struct kevent));
-	if (! ev)
-	    err(1, "Couldn't allocate memory for kevents.");
+	/*
+	 * The number of kqueue events we track may vary over time and may
+	 * even grow past its initial value in the -F case, but it will
+	 * never exceed two per file, so just preallocate that.
+	 */
+	ev = malloc(no_files * 2 * sizeof(struct kevent));
+	if (ev == NULL)
+		err(1, "Couldn't allocate memory for kevents.");
 	set_events(files);
 
 	for (;;) {
@@ -410,9 +409,7 @@ follow(file_info_t *files, enum STYLE style, off_t off)
 			 */
 			do {
 				n = kevent(kq, NULL, 0, ev, 1, Fflag ? &ts : NULL);
-				if (n < 0 && errno == EINTR)
-					continue;
-				if (n < 0)
+				if (n < 0 && errno != EINTR)
 					err(1, "kevent");
 			} while (n < 0);
 			if (n == 0) {
diff --git a/usr.bin/tail/tests/tail_test.sh b/usr.bin/tail/tests/tail_test.sh
index 8123a310fe67..9c941f8a2c2f 100755
--- a/usr.bin/tail/tests/tail_test.sh
+++ b/usr.bin/tail/tests/tail_test.sh
@@ -329,10 +329,28 @@ follow_stdin_body()
 	atf_check kill $pid
 }
 
+atf_test_case follow_create
+follow_create_head()
+{
+	atf_set "descr" "Verify that -F works when a file is created"
+}
+follow_create_body()
+{
+	local pid
+
+	rm -f infile
+	tail -F infile > outfile &
+	pid=$!
+	seq 1 5 >infile
+	sleep 2
+	atf_check cmp infile outfile
+	atf_check kill $pid
+}
+
 atf_test_case follow_rename
 follow_rename_head()
 {
-	atf_set "descr" "Verify that -F works"
+	atf_set "descr" "Verify that -F works when a file is replaced"
 }
 follow_rename_body()
 {
@@ -424,6 +442,7 @@ atf_init_test_cases()
 	atf_add_test_case stdin
 	atf_add_test_case follow
 	atf_add_test_case follow_stdin
+	atf_add_test_case follow_create
 	atf_add_test_case follow_rename
 	atf_add_test_case silent_header
 	atf_add_test_case verbose_header