The remove_dir_all crate is a Rust library that offers additional features over the Rust
standard library fs::remove_dir_all function.
It was possible to trick a privileged process doing a recursive delete in an
attacker controlled directory into deleting privileged files, on all operating systems.
For instance, consider deleting a tree called 'etc' in a parent directory
called 'p'. Between calling remove_dir_all("a") and remove_dir_all("a")
actually starting its work, the attacker can move 'p' to 'p-prime', and
replace 'p' with a symlink to '/'. Then the privileged process deletes 'p/etc'
which is actually /etc, and now your system is broken. There are some
mitigations for this exact scenario, such as CWD relative file lookup, but
they are not guaranteed - any code using absolute paths will not have that
protection in place.
The same attack could be performed at any point in the directory tree being
deleted: if 'a' contains a child directory called 'etc', attacking the
deletion by replacing 'a' with a link is possible.
The new code in this release mitigates the attack within the directory tree
being deleted by using file-handle relative operations: to open 'a/etc', the
path 'etc' relative to 'a' is opened, where 'a' is represented by a file
descriptor (Unix) or handle (Windows). With the exception of the entry points
into the directory deletion logic, this is robust against manipulation of the
directory hierarchy, and remove_dir_all will only delete files and directories
contained in the tree it is deleting.
The entry path however is a challenge - as described above, there are some
potential mitigations, but since using them must be done by the calling code,
it is hard to be confident about the security properties of the path based
interface.
The new extension trait RemoveDir provides an interface where it is much
harder to get it wrong.
somedir.remove_dir_contents("name-of-child").
Callers can then make their own security evaluation about how to securely get
a directory handle. That is still not particularly obvious, and we're going to
follow up with a helper of some sort (probably in the fs_at crate). Once
that is available, the path based entry points will get deprecated.
In the interim, processes that might run with elevated privileges should
figure out how to securely identify the directory they are going to delete, to
avoid the initial race. Pragmatically, other processes should be fine with the
path based entry points : this is the same interface std::fs::remove_dir_all
offers, and an unprivileged process running in an attacker controlled
directory can't do anything that the attacker can't already do.
