linux-kernel - [PATCH V33 19/30] Lock down TIOCSSERIAL

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Date:   Thu, 20 Jun 2019 18:19:30 -0700
From:   Matthew Garrett <matthewgarrett@...gle.com>
To:     jmorris@...ei.org
Cc:     linux-security@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org
Subject: [PATCH V33 19/30] Lock down TIOCSSERIAL

From: David Howells <dhowells@...hat.com>

Lock down TIOCSSERIAL as that can be used to change the ioport and irq
settings on a serial port.  This only appears to be an issue for the serial
drivers that use the core serial code.  All other drivers seem to either
ignore attempts to change port/irq or give an error.

Reported-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Matthew Garrett <mjg59@...gle.com>
cc: Jiri Slaby <jslaby@...e.com>
Cc: linux-serial@...r.kernel.org
---
 drivers/tty/serial/serial_core.c | 7 +++++++
 include/linux/security.h         | 1 +
 security/lockdown/lockdown.c     | 1 +
 3 files changed, 9 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 351843f847c0..2dbef7dc23f6 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -22,6 +22,7 @@
 #include <linux/serial_core.h>
 #include <linux/delay.h>
 #include <linux/mutex.h>
+#include <linux/security.h>
 
 #include <linux/irq.h>
 #include <linux/uaccess.h>
@@ -852,6 +853,12 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 	new_flags = (__force upf_t)new_info->flags;
 	old_custom_divisor = uport->custom_divisor;
 
+	if ((change_port || change_irq) &&
+	    security_is_locked_down(LOCKDOWN_TIOCSSERIAL)) {
+		retval = -EPERM;
+		goto exit;
+	}
+
 	if (!capable(CAP_SYS_ADMIN)) {
 		retval = -EPERM;
 		if (change_irq || change_port ||
diff --git a/include/linux/security.h b/include/linux/security.h
index 87c433f1e7db..cb5d74f9b9ff 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -91,6 +91,7 @@ enum lockdown_reason {
 	LOCKDOWN_MSR,
 	LOCKDOWN_ACPI_TABLES,
 	LOCKDOWN_PCMCIA_CIS,
+	LOCKDOWN_TIOCSSERIAL,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index ced4ddbb36b4..c6456f300220 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -27,6 +27,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_MSR] = "raw MSR access",
 	[LOCKDOWN_ACPI_TABLES] = "modified ACPI tables",
 	[LOCKDOWN_PCMCIA_CIS] = "direct PCMCIA CIS storage",
+	[LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };
-- 
2.22.0.410.gd8fdbe21b5-goog