px5g-wolfssl: 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.*
-rw-r--r--    1 root     root           749 Nov  6 23:14 /etc/uhttpd.crt
-rw-------    1 root     root           121 Nov  6 23:14 /etc/uhttpd.key

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/utils/px5g-wolfssl/Makefile

@@ -5,7 +5,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=px5g-wolfssl
-PKG_RELEASE:=8.2
+PKG_RELEASE:=9
 PKG_LICENSE:=GPL-2.0-or-later
 
 PKG_BUILD_FLAGS:=no-mips16

package/utils/px5g-wolfssl/px5g-wolfssl.c

@@ -7,6 +7,8 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
 #include <wolfssl/options.h>
 #include <wolfssl/wolfcrypt/asn.h>
 #include <wolfssl/wolfcrypt/asn_public.h>
@@ -24,27 +26,38 @@ enum {
   RSA_KEY_TYPE = 1,
 };
 
-int write_file(byte *buf, int bufSz, char *path) {
-  int ret;
-  FILE *file;
+int write_file(byte *buf, int bufSz, char *path, bool cert) {
+  mode_t mode = S_IRUSR | S_IWUSR;
+  ssize_t written;
+  int err;
+  int fd;
+
+  if (cert)
+    mode |= S_IRGRP | S_IROTH;
+
   if (path) {
-    file = fopen(path, "wb");
-    if (file == NULL) {
+    fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, mode);
+    if (fd < 0) {
       perror("Error opening file");
       exit(1);
     }
   } else {
-    file = stdout;
+    fd = STDERR_FILENO;
+  }
+  written = write(fd, buf, bufSz);
+  if (written != bufSz) {
+    perror("Error write file");
+    exit(1);
+  }
+  err = fsync(fd);
+  if (err < 0) {
+    perror("Error fsync file");
+    exit(1);
   }
-  ret = (int)fwrite(buf, 1, bufSz, file);
   if (path) {
-    fclose(file);
+    close(fd);
   }
-  if (ret > 0) {
-    /* ret > 0 indicates a successful file write, set to zero for return */
-    ret = 0;
-  }
-  return ret;
+  return 0;
 }
 
 int write_key(ecc_key *ecKey, RsaKey *rsaKey, int type, int keySz, char *fName,
@@ -73,9 +86,9 @@ int write_key(ecc_key *ecKey, RsaKey *rsaKey, int type, int keySz, char *fName,
       fprintf(stderr, "DER to PEM failed: %d\n", ret);
     }
     pemSz = ret;
-    ret = write_file(pem, pemSz, fName);
+    ret = write_file(pem, pemSz, fName, false);
   } else {
-    ret = write_file(der, derSz, fName);
+    ret = write_file(der, derSz, fName, false);
   }
   return ret;
 }
@@ -281,7 +294,7 @@ int selfsigned(WC_RNG *rng, char **arg) {
   }
   pemSz = ret;
 
-  ret = write_file(pemBuf, pemSz, certpath);
+  ret = write_file(pemBuf, pemSz, certpath, true);
   if (ret != 0) {
     fprintf(stderr, "Write Cert failed: %d\n", ret);
     return ret;