Re: libpq minor TOCTOU violation

Hi,

> I was playing with a static analyzer security scanner and it flagged a
> time-of-check-time-of-use violation in libpq. I was going to propose a
> fix for this on -hackers, since you probably can't do anything
> interesting with this, but then I figured I'd better check here first.
>
> libpq checks the permissions of the password file before opening it and
> rejects it if the permissions are more permissive than 0600. It does
> this in two separate steps, first stat(), then fopen(), which is what
> triggers the static analyzer. The standard fix for this kind of thing
> is to open the file first and then use fstat() on the file handle for
> the permission check.
>
> Note that libpq doesn't check who the owner of the file is or what
> directory it is in. The location of the password file can be changed
> from its default via libpq connection settings. So it seems to me that
> if the location of the password file were a world-writable directory, an
> "attacker" could supply a dummy file with 0600 permissions for the
> stat() call and then swap it out for a world-readable file with
> passwords for the fopen() call, both files owned by the attacker. And
> so they could have the libpq application try out passwords on their
> behalf. Obviously, this requires a number of unlikely circumstances,
> including the ability to repeatedly exploit the gap between the stat()
> and fopen() call. But it seems formally incorrect, so it seems good to
> fix it, at least to make the code a better example.

Not entirely sure about the presence of a serious security issue but
silencing a static analyzer sounds like a good idea, especially since
the fix is simple.

--
Best regards,
Aleksander Alekseev