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My speculation on the race condition fixed in the patch:

The while loop in `main` calls `play_beep` multiple times. Each call to `play_beep` opens the `--device` and sets the global `console_fd`, and then sets the global `console_type` based on the `ioctl(EVIOCGSND)` error, before calling `do_beep`.

This normally prevents the user from writing to arbitrary files with `--device`, because without the `ioctl(EVIOCGSND)` succeeding, `do_beep` with `BEEP_TYPE_CONSOLE` only does a (harmless?) `ioctl(KIOCSOUND)`, not a `write` with the `struct input_event`. However, the signal handler calls `do_beep` directly using the globals set by `play_beep`...

So I image that with something along the lines of `beep --device=./symlink-to-tty ... --new ...`, you can rewrite the symlink to point to an arbitrary file during the first `play_beep`, and then race the open/ioctl in the second `play_beep` with the signal handler such that `do_beep` gets called with `console_fd` pointing to your arbitrary file, and with `console_type` still set to `BEEP_TYPE_EVDEV`, resulting in a `write` to your arbitrary file.

Exploiting that for privesc would require control over the `struct input_event` for the `write`... `handle_signal` calls `do_beep` with a fixed `freq` of 0, so all of the initialized fields are set to fixed values... However, there's an unitialized `struct timeval` at the beginning of the `struct input_event`, and it's allocated on the stack...

Seems like a curious security vulnerability, I'll assume the debian security team must have a working PoC in order to actually call it out as a local privesc vulnerability... I'd love to see the actual PoC eventually :)

There's now a PoC exploiting this race, seemingly placing the 32-bit -l option value into the uninitialized part of the `struct input_event` to modify a shell script that runs as root: https://gist.github.com/fkt/5f8f9560ef54e11ff7df8bec09dc8f9a

Remains to be seen if the PoC actually works though, I've been unable to win the race so far, although the varying ioctl errors indicate it might be close?

Yeah, that makes sense. I took the liberty of pasting this comment to the upstream bug tracker:


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