lpd: fix OOM vulnerability (was eating arbitrarily large commands)

include/libbb.h

@@ -529,7 +529,7 @@ extern char *reads(int fd, char *buf, size_t count);
 // Read one line a-la fgets. Reads byte-by-byte.
 // Useful when it is important to not read ahead.
 // Bytes are appended to pfx (which must be malloced, or NULL).
-extern char *xmalloc_reads(int fd, char *pfx);
+extern char *xmalloc_reads(int fd, char *pfx, size_t *maxsz_p);
 extern ssize_t read_close(int fd, void *buf, size_t count);
 extern ssize_t open_read_close(const char *filename, void *buf, size_t count);
 extern void *xmalloc_open_read_close(const char *filename, size_t *sizep);

libbb/read.c

@@ -152,13 +152,14 @@ char *reads(int fd, char *buffer, size_t size)
 // Read one line a-la fgets. Reads byte-by-byte.
 // Useful when it is important to not read ahead.
 // Bytes are appended to pfx (which must be malloced, or NULL).
-char *xmalloc_reads(int fd, char *buf)
+char *xmalloc_reads(int fd, char *buf, size_t *maxsz_p)
 {
 	char *p;
-	int sz = buf ? strlen(buf) : 0;
+	size_t sz = buf ? strlen(buf) : 0;
+	size_t maxsz = maxsz_p ? *maxsz_p : MAXINT(size_t);
 
 	goto jump_in;
-	while (1) {
+	while (sz < maxsz) {
 		if (p - buf == sz) {
  jump_in:
 			buf = xrealloc(buf, sz + 128);
@@ -178,6 +179,8 @@ char *xmalloc_reads(int fd, char *buf)
 		p++;
 	}
 	*p++ = '\0';
+	if (maxsz_p)
+		*maxsz_p  = p - buf - 1;
 	return xrealloc(buf, p - buf);
 }
 

printutils/lpd.c

@@ -58,8 +58,6 @@
  */
 #include "libbb.h"
 
-// TODO: xmalloc_reads is vulnerable to remote OOM attack!
-
 // strip argument of bad chars
 static char *sane(char *str)
 {
@@ -75,6 +73,21 @@ static char *sane(char *str)
 	return str;
 }
 
+/* vfork() disables some optimizations. Moving its use
+ * to minimal, non-inlined function saves bytes */
+static NOINLINE void vfork_close_stdio_and_exec(char **argv)
+{
+	if (vfork() == 0) {
+		// CHILD
+		// we are the helper. we wanna be silent.
+		// this call reopens stdio fds to "/dev/null"
+		// (no daemonization is done)
+		bb_daemonize_or_rexec(DAEMON_DEVNULL_STDIO | DAEMON_ONLY_SANITIZE, NULL);
+		BB_EXECVP(*argv, argv);
+		_exit(127);
+	}
+}
+
 static void exec_helper(const char *fname, char **argv)
 {
 	char *p, *q, *file;
@@ -103,26 +116,24 @@ static void exec_helper(const char *fname, char **argv)
 		// next line, plz!
 		q = p;
 	}
+	free(file);
 
-	if (vfork() == 0) {
-		// CHILD
-		// we are the helper. we wanna be silent
-		// this call reopens stdio fds to "/dev/null"
-		// (no daemonization is done)
-		bb_daemonize_or_rexec(DAEMON_DEVNULL_STDIO | DAEMON_ONLY_SANITIZE, NULL);
-		BB_EXECVP(*argv, argv);
-		_exit(127);
-	}
+	vfork_close_stdio_and_exec(argv);
 
 	// PARENT (or vfork error)
 	// clean up...
-	free(file);
 	while (--env_idx >= 0) {
 		*strchrnul(our_env[env_idx], '=') = '\0';
 		unsetenv(our_env[env_idx]);
 	}
 }
 
+static char *xmalloc_read_stdin(void)
+{
+	size_t max = 4 * 1024; /* more than enough for commands! */
+	return xmalloc_reads(STDIN_FILENO, NULL, &max);
+}
+
 int lpd_main(int argc, char *argv[]) MAIN_EXTERNALLY_VISIBLE;
 int lpd_main(int argc ATTRIBUTE_UNUSED, char *argv[])
 {
@@ -130,7 +141,7 @@ int lpd_main(int argc ATTRIBUTE_UNUSED, char *argv[])
 	char *s, *queue;
 
 	// read command
-	s = xmalloc_reads(STDIN_FILENO, NULL);
+	s = xmalloc_read_stdin();
 
 	// we understand only "receive job" command
 	if (2 != *s) {
@@ -168,7 +179,7 @@ int lpd_main(int argc ATTRIBUTE_UNUSED, char *argv[])
 		write(STDOUT_FILENO, "", 1);
 
 		// get subcommand
-		s = xmalloc_reads(STDIN_FILENO, NULL);
+		s = xmalloc_read_stdin();
 		if (!s)
 			return EXIT_SUCCESS; // probably EOF
 		// we understand only "control file" or "data file" cmds

shell/hush.c

@@ -1061,7 +1061,7 @@ static int builtin_read(char **argv)
 	char *string;
 	const char *name = argv[1] ? argv[1] : "REPLY";
 
-	string = xmalloc_reads(STDIN_FILENO, xasprintf("%s=", name));
+	string = xmalloc_reads(STDIN_FILENO, xasprintf("%s=", name), NULL);
 	return set_local_var(string, 0);
 }