[PATCH 4/4] appletlib: Fix file-system race in parse_config_file()

[Ryan Mallon]

There is a small filesystem race window between the stat checks for
the Busybox config file and opening of the file. Although this window
is very small, and the file must be called /etc/busybox, the code is
written to be paranoid. Be completely paranoid and eliminate the race
by opening the file and doing the stat checks on the file descriptor.

Signed-off-by: Ryan Mallon <rmallon at gmail.com>
---
 libbb/appletlib.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index 8f3a8a1..729be85 100644
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -286,12 +286,16 @@ static void parse_config_file(void)
 	if (ruid == 0) /* run by root - don't need to even read config file */
 		return;
 
-	if ((stat(config_file, &st) != 0)       /* No config file? */
+	f = fopen_for_read(config_file);
+	if (!f)
+		return;
+
+	if ((fstat(fileno(f), &st) != 0)        /* Cannot stat? */
 	 || !S_ISREG(st.st_mode)                /* Not a regular file? */
 	 || (st.st_uid != 0)                    /* Not owned by root? */
 	 || (st.st_mode & (S_IWGRP | S_IWOTH))  /* Writable by non-root? */
-	 || !(f = fopen_for_read(config_file))  /* Cannot open? */
 	) {
+		fclose(f);
 		return;
 	}
 
-- 
1.7.1