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Date: Tue, 14 Mar 2023 16:57:25 -0400
From: Shawn Webb <>
Subject: Re: TTY pushback vulnerabilities / TIOCSTI

On Tue, Mar 14, 2023 at 09:51:03AM +0100, Hanno Böck wrote:
> Hi,
> This blogpost highlights TTY Pushback vulnerabilities enabled via the
> TIOCSTI kernel functionality available in the Linux kernel:
> This has been discussed here previously:
> Though I think there are some noteworthy updates. In the 2017 post
> solar designer mentioned that the Linux kernel developers have multiple
> times rejected changes in the kernel. However this has now changed:
> Starting with Kernel 6.2 it is possible to disable TIOCSTI
> (unset CONFIG_LEGACY_TIOCSTI). It also appears that very few (or no?)
> applications practically use TIOCSTI.
> This seems to be the only real mitigation for this issue. It appears
> su has a parameter, and in sudo one can configure the creation of a new
> pty in the sudoers file. I don't consider these as satisfying fixes, as
> they are optinal, and thus rely on the expectation that users are aware
> of this risk and manually use these mitigations. That does not seem
> realistic to me.
> This also affects such a large number of tools, not just
> su/sudo-like tools, but also sandboxing tools. E.g. bubblewrap [1] is
> affected by this by default.
> Thus I strongly recommend that people disable this in the kernel.
> [1]

With commit c7d6d4bb4874720d9dab1625df62c2ea6eeb9df5[0], I've added a
toggle in HardenedBSD to disable TIOCSTI. The toggle is set to
prohibit TIOCSTI by default. Now attempts to use TIOCSTI will be met
with EPERM.

I've verified the toggle in a real-world scenario with the doas issue
PoC found at [1].



Shawn Webb
Cofounder / Security Engineer

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