px5g-mbedtls: Fix permission of private key

Store the private key with read and write permission for the user only
and not with read permissions for everyone. This converts the
write_file() function from fopen() to open() because open allows to
specify the permission mask of the newly created file. It also adds and
fixes some existing error handling.

OpenSSL does this in the same way already.

With this change it looks like this:
root@OpenWrt:/# ls -al /etc/uhttpd.crt /etc/uhttpd.key
-rw-r--r--    1 root     root           519 Nov  6 22:58 /etc/uhttpd.crt
-rw-------    1 root     root           121 Nov  6 22:58 /etc/uhttpd.key

Signed-off-by: Hauke Mehrtens <[email protected]>

package/utils/px5g-mbedtls/Makefile

@@ -8,7 +8,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=px5g-mbedtls
-PKG_RELEASE:=9
+PKG_RELEASE:=10
 PKG_LICENSE:=LGPL-2.1
 
 PKG_BUILD_FLAGS:=no-mips16

package/utils/px5g-mbedtls/px5g-mbedtls.c

@@ -30,6 +30,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <stdbool.h>
+#include <errno.h>
 
 #include <mbedtls/bignum.h>
 #include <mbedtls/entropy.h>
@@ -55,10 +56,13 @@ static int _urandom(void *ctx, unsigned char *out, size_t len)
 	return 0;
 }
 
-static void write_file(const char *path, int len, bool pem)
+static void write_file(const char *path, size_t len, bool pem, bool cert)
 {
-	FILE *f = stdout;
+	mode_t mode = S_IRUSR | S_IWUSR;
 	const char *buf_start = buf;
+	int fd = STDERR_FILENO;
+	ssize_t written;
+	int err;
 
 	if (!pem)
 		buf_start += sizeof(buf) - len;
@@ -67,17 +71,30 @@ static void write_file(const char *path, int len, bool pem)
 		fprintf(stderr, "No data to write\n");
 		exit(1);
 	}
+	
+	if (cert)
+		mode |= S_IRGRP | S_IROTH;
 
-	if (!f) {
+	if (path)
+		fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, mode);
+
+	if (fd < 0) {
 		fprintf(stderr, "error: I/O error\n");
 		exit(1);
 	}
 
+	written = write(fd, buf_start, len);
+	if (written != len) {
+		fprintf(stderr, "writing key failed with: %s\n", strerror(errno));
+		exit(1);
+	}
+	err = fsync(fd);
+	if (err < 0) {
+		fprintf(stderr, "syncing key failed with: %s\n", strerror(errno));
+		exit(1);
+	}
 	if (path)
-		f = fopen(path, "w");
-
-	fwrite(buf_start, 1, len, f);
-	fclose(f);
+		close(fd);
 }
 
 static mbedtls_ecp_group_id ecp_curve(const char *name)
@@ -110,7 +127,7 @@ static void write_key(mbedtls_pk_context *key, const char *path, bool pem)
 			len = 0;
 	}
 
-	write_file(path, len, pem);
+	write_file(path, len, pem, false);
 }
 
 static void gen_key(mbedtls_pk_context *key, bool rsa, int ksize, int exp,
@@ -301,7 +318,7 @@ int selfsigned(char **arg)
 			return 1;
 		}
 	}
-	write_file(certpath, len, pem);
+	write_file(certpath, len, pem, true);
 
 	mbedtls_x509write_crt_free(&cert);
 	mbedtls_mpi_free(&serial);