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@ens-lyon.org>
Acked-by: Kees Cook <keescook@chromium.org>
Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
Cc: stable@vger.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))

On 10. 07. 23, 2:26, Samuel Thibault wrote:
> 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@ens-lyon.org>
> Acked-by: Kees Cook <keescook@chromium.org>
> Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
> Cc: stable@vger.kernel.org

Acked-by: Jiri Slaby <jirislaby@kernel.org>

thanks,
-- 
js
suse labs