um: Do not set unsecure permission for temporary file

Remove the insecure 0777 mode for temporary file to prohibit other users
to change the executable mapped code.

An attacker could gain access to the mapped file descriptor from the
temporary file (before it is unlinked) in a read-only mode but it should
not be accessible in write mode to avoid arbitrary code execution.

To not change the hostfs behavior, the temporary file creation
permission now depends on the current umask(2) and the implementation of
mkstemp(3).
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Acked-by: Tristan Schmelcher <tschmelcher@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/mem.c

@@ -142,12 +142,6 @@ static int __init create_tmp_file(unsigned long long len)
 	if (fd < 0)
 		exit(1);
 
-	err = fchmod(fd, 0777);
-	if (err < 0) {
-		perror("fchmod");
-		exit(1);
-	}
-
 	/*
 	 * Seek to len - 1 because writing a character there will
 	 * increase the file size by one byte, to the desired length.