linux-kernel - [PATCH] tty: Permit some TIOCL_SETSEL modes without CAP_SYS

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Message-ID: <20241216150722.4079213-2-gnoack@google.com>
Date: Mon, 16 Dec 2024 15:07:23 +0000
From: "Günther Noack" <gnoack@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jann Horn <jannh@...gle.com>, "Hanno Böck" <hanno@...eck.de>, Jiri Slaby <jirislaby@...nel.org>, 
	linux-hardening@...r.kernel.org, regressions@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, 
	"Günther Noack" <gnoack@...gle.com>
Subject: [PATCH] tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN

With this, processes without CAP_SYS_ADMIN are able to use TIOCLINUX with
subcode TIOCL_SETSEL, in the selection modes TIOCL_SETPOINTER,
TIOCL_SELCLEAR and TIOCL_SELMOUSEREPORT.

TIOCL_SETSEL was previously changed to require CAP_SYS_ADMIN, as this IOCTL
let callers change the selection buffer and could be used to simulate
keypresses.  These three TIOCL_SETSEL selection modes, however, are safe to
use, as they do not modify the selection buffer.

This fixes a mouse support regression that affected Emacs (invisible mouse
cursor).

Link: https://lore.kernel.org/r/ee3ec63269b43b34e1c90dd8c9743bf8@finder.org
Fixes: 8d1b43f6a6df ("tty: Restrict access to TIOCLINUX' copy-and-paste subcommands")
Signed-off-by: Günther Noack <gnoack@...gle.com>
---
 drivers/tty/vt/selection.c | 14 ++++++++++++++
 drivers/tty/vt/vt.c        |  3 +--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 564341f1a74f..0bd6544e30a6 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -192,6 +192,20 @@ int set_selection_user(const struct tiocl_selection __user *sel,
 	if (copy_from_user(&v, sel, sizeof(*sel)))
 		return -EFAULT;
 
+	/*
+	 * TIOCL_SELCLEAR, TIOCL_SELPOINTER and TIOCL_SELMOUSEREPORT are OK to
+	 * use without CAP_SYS_ADMIN as they do not modify the selection.
+	 */
+	switch (v.sel_mode) {
+	case TIOCL_SELCLEAR:
+	case TIOCL_SELPOINTER:
+	case TIOCL_SELMOUSEREPORT:
+		break;
+	default:
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
+	}
+
 	return set_selection_kernel(&v, tty);
 }
 
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 96842ce817af..ed65b3b80fbd 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3345,8 +3345,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 
 	switch (type) {
 	case TIOCL_SETSEL:
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
+		/* CAP_SYS_ADMIN check happens in set_selection_user(). */
 		return set_selection_user(param, tty);
 	case TIOCL_PASTESEL:
 		if (!capable(CAP_SYS_ADMIN))
-- 
2.47.1.613.gc27f4b7a9f-goog