tty: fix saved termios reset race

Overview

In the Linux kernel, the following vulnerability has been resolved:

tty: fix saved termios reset race

Resetting saved termios state on device registration is needed where a minor number can be reused for an entirely different device and where the old settings may prevent the port from even being opened (e.g. when CLOCAL is not set).

Not all TTY drivers guarantee that the minor number is no longer in use when registering devices however, something which can lead to a use-after-free when closing a TTY (and saving its termios) races with re-registration.

Add a new TTY_DRIVER_RESET_SAVED_TERMIOS flag to request that any saved termios state is reset on registration and only set it for drivers that make sure that the minor number is no longer in use.

Affected

• Linux >= 93857edd9829e144acb6c7e72d593f6e01aead66 < 1572d86bcf0bcb848d20fc5e5de7ed6963034c2b
• Linux >= 93857edd9829e144acb6c7e72d593f6e01aead66 < 08bf919f041af9de372428bb0c938157ea144164
• Linux >= 93857edd9829e144acb6c7e72d593f6e01aead66 < dcaf0251f847a45861e94c6617024a9455687d2f
• Linux >= 93857edd9829e144acb6c7e72d593f6e01aead66 < 8df07fe93573e9e548d6645273e9bcb6eb74059e
• Linux 4.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.