linux-kernel - [PATCH 3.16 201/305] hwmon: (dell-smm) Restrict fan control and serial number to CAP_SYS

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Message-ID: <lsq.1471110171.10861326@decadent.org.uk>
Date:	Sat, 13 Aug 2016 18:42:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Mario Limonciello" <Mario_Limonciello@...l.com>,
	"Guenter Roeck" <linux@...ck-us.net>,
	"Pali Rohár" <pali.rohar@...il.com>
Subject: [PATCH 3.16 201/305] hwmon: (dell-smm) Restrict fan control and
 serial number to CAP_SYS_ADMIN by default

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Pali Rohár <pali.rohar@...il.com>

commit 7613663cc186f8f3c50279390ddc60286758001c upstream.

For security reasons ordinary user must not be able to control fan speed
via /proc/i8k by default. Some malicious software running under "nobody"
user could be able to turn fan off and cause HW problems. So this patch
changes default value of "restricted" parameter to 1.

Also restrict reading of DMI_PRODUCT_SERIAL from /proc/i8k via "restricted"
parameter. It is because non root user cannot read DMI_PRODUCT_SERIAL from
sysfs file /sys/class/dmi/id/product_serial.

Old non secure behaviour of file /proc/i8k can be achieved by loading this
module with "restricted" parameter set to 0.

Note that this patch has effects only for kernels compiled with CONFIG_I8K
and only for file /proc/i8k. Hwmon interface provided by this driver was
not changed and root access for setting fan speed was needed also before.

Reported-by: Mario Limonciello <Mario_Limonciello@...l.com>
Signed-off-by: Pali Rohár <pali.rohar@...il.com>
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/char/i8k.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

--- a/drivers/char/i8k.c
+++ b/drivers/char/i8k.c
@@ -62,6 +62,7 @@
 
 static DEFINE_MUTEX(i8k_mutex);
 static char bios_version[4];
+static char bios_machineid[16];
 static struct device *i8k_hwmon_dev;
 static u32 i8k_hwmon_flags;
 static int i8k_fan_mult;
@@ -85,13 +86,13 @@ static bool ignore_dmi;
 module_param(ignore_dmi, bool, 0);
 MODULE_PARM_DESC(ignore_dmi, "Continue probing hardware even if DMI data does not match");
 
-static bool restricted;
+static bool restricted = true;
 module_param(restricted, bool, 0);
-MODULE_PARM_DESC(restricted, "Allow fan control if SYS_ADMIN capability set");
+MODULE_PARM_DESC(restricted, "Restrict fan control and serial number to CAP_SYS_ADMIN (default: 1)");
 
 static bool power_status;
 module_param(power_status, bool, 0600);
-MODULE_PARM_DESC(power_status, "Report power status in /proc/i8k");
+MODULE_PARM_DESC(power_status, "Report power status in /proc/i8k (default: 0)");
 
 static int fan_mult = I8K_FAN_MULT;
 module_param(fan_mult, int, 0);
@@ -350,9 +351,11 @@ i8k_ioctl_unlocked(struct file *fp, unsi
 		break;
 
 	case I8K_MACHINE_ID:
-		memset(buff, 0, 16);
-		strlcpy(buff, i8k_get_dmi_data(DMI_PRODUCT_SERIAL),
-			sizeof(buff));
+		if (restricted && !capable(CAP_SYS_ADMIN))
+			return -EPERM;
+
+		memset(buff, 0, sizeof(buff));
+		strlcpy(buff, bios_machineid, sizeof(buff));
 		break;
 
 	case I8K_FN_STATUS:
@@ -469,7 +472,7 @@ static int i8k_proc_show(struct seq_file
 	return seq_printf(seq, "%s %s %s %d %d %d %d %d %d %d\n",
 			  I8K_PROC_FMT,
 			  bios_version,
-			  i8k_get_dmi_data(DMI_PRODUCT_SERIAL),
+			  (restricted && !capable(CAP_SYS_ADMIN)) ? "-1" : bios_machineid,
 			  cpu_temp,
 			  left_fan, right_fan, left_speed, right_speed,
 			  ac_power, fn_key);
@@ -765,6 +768,8 @@ static int __init i8k_probe(void)
 
 	strlcpy(bios_version, i8k_get_dmi_data(DMI_BIOS_VERSION),
 		sizeof(bios_version));
+	strlcpy(bios_machineid, i8k_get_dmi_data(DMI_PRODUCT_SERIAL),
+		sizeof(bios_machineid));
 
 	/*
 	 * Get SMM Dell signature