git: c7cec6fa6e4a - stable/15 - tty: Revalidate after dropping the tty lock in ioctl handlers

The branch stable/15 has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=c7cec6fa6e4aeca7488130e17f4fd1ad2c476fa0

commit c7cec6fa6e4aeca7488130e17f4fd1ad2c476fa0
Author:     Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2026-08-24 18:14:18 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2026-08-25 15:47:13 +0000

    tty: Revalidate after dropping the tty lock in ioctl handlers
    
    The TIOCSCTTY ioctl handler drops the tty lock in order to acquire the
    proctree relock.  After relocking the tty, it did not revalidate the
    tty state, and it could end up linking a doomed tty to the calling
    process' session.  This race can be exploited to escalate privileges.
    
    TIOCSPGRP has a similar race, fix that too.
    
    Approved by:    so
    Security:       FreeBSD-SA-26:62.tty
    Security:       CVE-2026-58093
    Reported by:    tsune of GMO Cybersecurity by Ierae, Inc. working with TrendAI Zero Day Initiative
    Reviewed by:    kib
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D59126
---
 sys/kern/tty.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 09ed1c6d0c5d..f412205a3d2b 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -1890,7 +1890,12 @@ tty_generic_ioctl(struct tty *tp, u_long cmd, void *data, int fflag,
 		/* XXX: This looks awful. */
 		tty_unlock(tp);
 		sx_xlock(&proctree_lock);
-		tty_lock(tp);
+		error = ttydev_enter(tp);
+		if (error != 0) {
+			sx_xunlock(&proctree_lock);
+			tty_lock(tp);
+			return (error);
+		}
 
 		if (!SESS_LEADER(p)) {
 			/* Only the session leader may do this. */
@@ -1954,7 +1959,12 @@ tty_generic_ioctl(struct tty *tp, u_long cmd, void *data, int fflag,
 			tty_lock(tp);
 			return (EPERM);
 		}
-		tty_lock(tp);
+		error = ttydev_enter(tp);
+		if (error != 0) {
+			sx_sunlock(&proctree_lock);
+			tty_lock(tp);
+			return (error);
+		}
 
 		/*
 		 * Determine if this TTY is the controlling TTY after