Enforce maximum crontab line count of 1000

As any user can create a crontab that is read by the cron daemon, it is
possible for a user to cause a DoS via memory exhaustion by creating an
excessivly large crontab.

As a measure to prevent this, limit the size of individual crontab files
to 1000 lines.

While it is still technically possible for a user to create a larger crontab
(for example, by creating a single, very long comment), this should not affect
the daemon, as it simply skips over comments.

For crontab entries (for which the daemon allocates memory), the maximum
command length is already limited to 998 characters, so these allocations are
already kept in check.

cron.h

@@ -82,6 +82,7 @@
 #define	MAX_COMMAND	1000	/* max length of internally generated cmd */
 #define	MAX_TEMPSTR	1000	/* max length of envvar=value\0 strings */
 #define	MAX_ENVSTR	MAX_TEMPSTR	/* DO NOT change - buffer overruns otherwise */
+#define MAX_TAB_LINES	1000	/* max length of crontabs */
 #define	MAX_UNAME	20	/* max length of username, should be overkill */
 #define	ROOT_UID	0	/* don't change this, it really must be root */
 #define	ROOT_USER	"root"	/* ditto */
@@ -101,6 +102,7 @@
 #define	CRON_TAB(u)	"%s/%s", SPOOL_DIR, u
 #define	REG		register
 #define	PPC_NULL	((char **)NULL)
+#define NHEADER_LINES   3
 
 #ifndef MAXHOSTNAMELEN
 #define MAXHOSTNAMELEN 64
@@ -126,6 +128,8 @@
 			;
 #endif /* DEBUGGING */
 
+#define Stringify_(x)	#x
+#define Stringify(x)	Stringify_(x)
 #define	MkLower(ch)	(isupper(ch) ? tolower(ch) : ch)
 #define	MkUpper(ch)	(islower(ch) ? toupper(ch) : ch)
 #define	Set_LineNum(ln)	{Debug(DPARS|DEXT,("linenum=%d\n",ln)); \

crontab.c

@@ -46,9 +46,6 @@ static char rcsid[] = "$Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $";
 #endif
 
 
-#define NHEADER_LINES 3
-
-
 enum opt_t	{ opt_unknown, opt_list, opt_delete, opt_edit, opt_replace };
 
 #if DEBUGGING
@@ -879,7 +876,7 @@ replace_cmd() {
 	 */
 	Set_LineNum(1 - NHEADER_LINES)
 	CheckErrorCount = 0;  eof = FALSE;
-	while (!CheckErrorCount && !eof) {
+	while (!CheckErrorCount && !eof && LineNumber < MAX_TAB_LINES + 2) {
 		switch (load_env(envstr, tmp)) {
 		case ERR:
 			eof = TRUE;
@@ -896,6 +893,13 @@ replace_cmd() {
 		}
 	}
 
+	if (LineNumber >= MAX_TAB_LINES + 2) {
+		fprintf(stderr, "crontab is too long; maximum number of lines "
+				"is %d.\n", MAX_TAB_LINES);
+		fclose(tmp);  unlink(tn);
+		return (-1);
+	}
+
 	if (CheckErrorCount != 0) {
 		fprintf(stderr, "errors in crontab file, can't install.\n");
 		fclose(tmp);  unlink(tn);

user.c

@@ -242,6 +242,7 @@ load_user(crontab_fd, pw, uname, fname, tabname)
 	/*
 	 * load the crontab
 	 */
+	Set_LineNum(1)
 	do {
 		status = load_env(envstr, file);
 		switch (status) {
@@ -287,7 +288,19 @@ load_user(crontab_fd, pw, uname, fname, tabname)
 			}
 			break;
 		}
-	} while (status >= OK);
+	/* When counting lines, ignore the user-hidden header part, and account
+	 * for idiosyncrasies of LineNumber manipulation
+	 */
+	} while (status >= OK && LineNumber < MAX_TAB_LINES + NHEADER_LINES + 2);
+
+	if (LineNumber >= MAX_TAB_LINES + NHEADER_LINES + 2) {
+		log_it(fname, getpid(), "ERROR", "crontab must not be longer "
+				"than " Stringify(MAX_TAB_LINES) " lines, "
+				"this crontab file will be ignored");
+		free_user(u);
+		u = NULL;
+		goto done;
+	}
 
  done:
 	env_free(envp);