Avoid falling prey to tiocsti#1660
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We've long known (see https://www.halfdog.net/Security/2012/TtyPushbackPrivilegeEscalation/ and https://jdebp.uk/FGA/dont-abuse-su-for-dropping-privileges.html) that su should be used to gain but not drop privilege. Much more recently, linux added the ability to prevent TIOCSTI through a configurable /proc/sys/dev/tty/legacy_tiocsti setting. If /proc/sys/dev/tty/legacy_tiocsti is set to 0, then we are protected from the callee injecting commands on caller's tty through TIOCSTI. If it's 1, or doesn't exist, then we are not. That can be dangerous if caller is root. We currently give up the controlling terminal for non-interactive uses of su (-c). Let's do that for interactive calls as well, only in the dangerous case.
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Jul 8, 2026
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Reviewed-by: Alejandro Colomar <alx@kernel.org>
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We've long known (see
https://www.halfdog.net/Security/2012/TtyPushbackPrivilegeEscalation/ and https://jdebp.uk/FGA/dont-abuse-su-for-dropping-privileges.html) that su should be used to gain but not drop privilege. Much more recently, linux added the ability to prevent TIOCSTI through a configurable /proc/sys/dev/tty/legacy_tiocsti setting.
If /proc/sys/dev/tty/legacy_tiocsti is set to 0, then we are protected from the callee injecting commands on caller's tty through TIOCSTI. If it's 1, or doesn't exist, then we are not. That can be dangerous if caller is root. We currently give up the controlling terminal for non-interactive uses of su (-c). Let's do that for interactive calls as well, only in the dangerous case.