Harden the code that does permission checks to protect against
re-opening of stdin.
Code that runs FFI is vulnerable to an attack where fd 0 is closed
during a permission check and re-opened with a file that contains a
positive response (ie: `y` or `A`). While FFI code is dangerous in
general, we can make it more difficult for FFI-enabled code to bypass
additional permission checks.
- Checks to see if the underlying file for stdin has changed from the
start to the end of the permission check (detects races)
- Checks to see if the message is excessively long (lowering the window
for races)
- Checks to see if stdin and stderr are still terminals at the end of
the function (making races more difficult)
5d7ebea99f
