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Message-Id: <20230828122109.3529221-2-gnoack@google.com>
Date: Mon, 28 Aug 2023 14:21:09 +0200
From: "Günther Noack" <gnoack@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "Hanno Böck" <hanno@...eck.de>, kernel-hardening@...ts.openwall.com,
Kees Cook <keescook@...omium.org>, Jiri Slaby <jirislaby@...nel.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>, Paul Moore <paul@...l-moore.com>,
Samuel Thibault <samuel.thibault@...-lyon.org>, David Laight <David.Laight@...lab.com>,
Simon Brand <simon.brand@...tadigitale.de>, Dave Mielke <Dave@...lke.cc>,
"Mickaël Salaün" <mic@...ikod.net>, KP Singh <kpsingh@...gle.com>,
Nico Schottelius <nico-gpm2008@...ottelius.org>,
"Günther Noack" <gnoack@...gle.com>
Subject: [PATCH v2 1/1] tty: Restrict access to TIOCLINUX' copy-and-paste subcommands
From: Hanno Böck <hanno@...eck.de>
TIOCLINUX can be used for privilege escalation on virtual terminals when
code is executed via tools like su/sudo and sandboxing tools.
By abusing the selection features, a lower-privileged application can
write content to the console, select and copy/paste that content and
thereby executing code on the privileged account. See also the poc
here:
https://www.openwall.com/lists/oss-security/2023/03/14/3
Selection is usually used by tools like gpm that provide mouse features
on the virtual console. gpm already runs as root (due to earlier
changes that restrict access to a user on the current TTY), therefore
it will still work with this change.
With this change, the following TIOCLINUX subcommands require
CAP_SYS_ADMIN:
* TIOCL_SETSEL - setting the selected region on the terminal
* TIOCL_PASTESEL - pasting the contents of the selected region into
the input buffer
* TIOCL_SELLOADLUT - changing word-by-word selection behaviour
The security problem mitigated is similar to the security risks caused
by TIOCSTI, which, since kernel 6.2, can be disabled with
CONFIG_LEGACY_TIOCSTI=n.
Signed-off-by: Hanno Böck <hanno@...eck.de>
Tested-by: Günther Noack <gnoack@...gle.com>
---
drivers/tty/vt/vt.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 1e8e57b45688..1eb30ed1118d 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3156,9 +3156,13 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
switch (type) {
case TIOCL_SETSEL:
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
return set_selection_user((struct tiocl_selection
__user *)(p+1), tty);
case TIOCL_PASTESEL:
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
return paste_selection(tty);
case TIOCL_UNBLANKSCREEN:
console_lock();
@@ -3166,6 +3170,8 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
console_unlock();
break;
case TIOCL_SELLOADLUT:
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
console_lock();
ret = sel_loadlut(p);
console_unlock();
--
2.42.0.rc2.253.gd59a3bf2b4-goog
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