Patch "hwmon: (dell-smm) Restrict fan control and serial number to CAP_SYS_ADMIN by default" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

hwmon: (dell-smm) Restrict fan control and serial number to CAP_SYS_ADMIN by default

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
hwmon-dell-smm-restrict-fan-control-and-serial-number-to-cap_sys_admin-by-default.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: =?UTF-8?q?Pali=20Roh=C3=A1r?= <[email protected]>
Date: Sat, 18 Jun 2016 00:54:45 +0200
Subject: hwmon: (dell-smm) Restrict fan control and serial number to CAP_SYS_ADMIN by default
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From: Pali Rohár <[email protected]>

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.

---
 drivers/hwmon/dell-smm-hwmon.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

@@ -66,6 +66,7 @@
 
 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 uint i8k_fan_mult = I8K_FAN_MULT;

@@ -94,13 +95,13 @@ module_param(ignore_dmi, bool, 0);
 MODULE_PARM_DESC(ignore_dmi, "Continue probing hardware even if DMI data does not match");
 
 #if IS_ENABLED(CONFIG_I8K)
-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)");
 #endif
 
 static uint fan_mult;

@@ -392,9 +393,11 @@ 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:

@@ -511,7 +514,7 @@ static int i8k_proc_show(struct seq_file
 	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);

@@ -980,6 +983,8 @@ 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

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/hwmon-dell-smm-disallow-fan_type-calls-on-broken-machines.patch queue-4.4/hwmon-dell-smm-restrict-fan-control-and-serial-number-to-cap_sys_admin-by-default.patch queue-4.4/hwmon-dell-smm-cache-fan_type-calls-and-change-fan-detection.patch