linux-kernel - [PATCH] TIOCSTI: always enable for CAP_SYS

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Date:   Mon, 10 Jul 2023 02:26:45 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Simon Brand <simon.brand@...tadigitale.de>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Dave@...lke.cc
Subject: [PATCH] TIOCSTI: always enable for CAP_SYS_ADMIN

83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled") broke BRLTTY's
ability to simulate keypresses on the console, thus effectively breaking
braille keyboards of blind users.

This restores the TIOCSTI feature for CAP_SYS_ADMIN processes, which
BRLTTY is, thus fixing braille keyboards without re-opening the security
issue.

Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
Acked-by: Kees Cook <keescook@...omium.org>
Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
Cc: stable@...r.kernel.org

Index: linux-6.4/drivers/tty/tty_io.c
===================================================================
--- linux-6.4.orig/drivers/tty/tty_io.c
+++ linux-6.4/drivers/tty/tty_io.c
@@ -2276,7 +2276,7 @@ static int tiocsti(struct tty_struct *tt
 	char ch, mbz = 0;
 	struct tty_ldisc *ld;
 
-	if (!tty_legacy_tiocsti)
+	if (!tty_legacy_tiocsti && !capable(CAP_SYS_ADMIN))
 		return -EIO;
 
 	if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))