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tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)
ioctl(TIOCGETD) retrieves the line discipline id directly from the ldisc because the line discipline id (c_line) in termios is untrustworthy; userspace may have set termios via ioctl(TCSETS*) without actually changing the line discipline via ioctl(TIOCSETD). However, directly accessing the current ldisc via tty->ldisc is unsafe; the ldisc ptr dereferenced may be stale if the line discipline is changing via ioctl(TIOCSETD) or hangup. Wait for the line discipline reference (just like read() or write()) to retrieve the "current" line discipline id. Cc: <email@example.com> Signed-off-by: Peter Hurley <firstname.lastname@example.org> Signed-off-by: Greg Kroah-Hartman <email@example.com>
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