Re: [PATCH v8 6/7] platform/x86/dell-laptop: Protect kbd_state against races

["Pali Rohár" <pali.rohar@gmail.com>]

On Tuesday 21 February 2017 15:42:02 Hans de Goede wrote:
> Hi,
> 
> On 21-02-17 15:25, Pali Rohár wrote:
> >On Tuesday 21 February 2017 15:18:14 Hans de Goede wrote:
> >>Hi,
> >>
> >>On 21-02-17 15:06, Pali Rohár wrote:
> >>>On Thursday 09 February 2017 16:44:16 Hans de Goede wrote:
> >>>>The kbd led code has multiple entry points each of which modifies the
> >>>>kbd_state by reading it, modifying a copy, writing the copy and on
> >>>>error setting the modified copy writing back the original state.
> >>>>
> >>>>This is racy, so add a mutex protection the read-modify-write cycle
> >>>>on each of the entry points.
> >>>
> >>>Is this mutex really needed? kbd_get_state and kbd_set_state are already
> >>>locked by mutex. Which situation is trying this patch fix?
> >>
> >>Yes this is really necessary, between getting the state
> >>and storing it writing to another sysfs attribute
> >>may cause a change to the state which will then get
> >>overwritten by the write of the earlier gotten state.
> >>
> >>This is a classic read-modify-write race and as such
> >>needs protection.
> >
> >Right, to preserve all changes (by all concurrent modifications) it is
> >needed to do that kbd_get_state() + modify + kbd_set_state() atomically.
> >
> >So this patch fix all concurrent modifications by kernel.
> >
> >But does not fix race condition when both userspace and kernel want to
> >change keyboard brightness settings.
> 
> Yes using libsmbios is ALWAYS racy, as said in my previous mails,
> people really should use the proper kernel interfaces rather then
> directly poking hw from userspace, but just because one path is
> racy is not a good reason to not fix the races in another path,
> esp, when that other path is the preferred way to do things
> and is actually the path which all modern desktop environments use.

Yes, I'm not against this kernel fix. I just pointed that it does not
fix race condition between userspace and kernel...

Anyway, patch seems good so add my Reviewed-by.

> Regards,
> 
> Hans
> 
> 
> 
> >
> >>Regards,
> >>
> >>Hans
> >>
> >>
> >>>
> >>>>Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> >>>>---
> >>>>Changes in v8:
> >>>>-New patch in v8 of this patch-set
> >>>>---
> >>>>drivers/platform/x86/dell-laptop.c | 112 +++++++++++++++++++++++++------------
> >>>>1 file changed, 76 insertions(+), 36 deletions(-)
> >>>>
> >>>>diff --git a/drivers/platform/x86/dell-laptop.c b/drivers/platform/x86/dell-laptop.c
> >>>>index a2913a5..70951f3 100644
> >>>>--- a/drivers/platform/x86/dell-laptop.c
> >>>>+++ b/drivers/platform/x86/dell-laptop.c
> >>>>@@ -1133,6 +1133,7 @@ static u8 kbd_previous_level;
> >>>>static u8 kbd_previous_mode_bit;
> >>>>
> >>>>static bool kbd_led_present;
> >>>>+static DEFINE_MUTEX(kbd_led_mutex);
> >>>>
> >>>>/*
> >>>> * NOTE: there are three ways to set the keyboard backlight level.
> >>>>@@ -1562,9 +1563,11 @@ static ssize_t kbd_led_timeout_store(struct device *dev,
> >>>>		}
> >>>>	}
> >>>>
> >>>>+	mutex_lock(&kbd_led_mutex);
> >>>>+
> >>>>	ret = kbd_get_state(&state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>	new_state = state;
> >>>>	new_state.timeout_value = value;
> >>>>@@ -1572,9 +1575,12 @@ static ssize_t kbd_led_timeout_store(struct device *dev,
> >>>>
> >>>>	ret = kbd_set_state_safe(&new_state, &state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>-	return count;
> >>>>+	ret = count;
> >>>>+out:
> >>>>+	mutex_unlock(&kbd_led_mutex);
> >>>>+	return ret;
> >>>>}
> >>>>
> >>>>static ssize_t kbd_led_timeout_show(struct device *dev,
> >>>>@@ -1634,9 +1640,11 @@ static ssize_t kbd_led_triggers_store(struct device *dev,
> >>>>	if (trigger[0] != '+' && trigger[0] != '-')
> >>>>		return -EINVAL;
> >>>>
> >>>>+	mutex_lock(&kbd_led_mutex);
> >>>>+
> >>>>	ret = kbd_get_state(&state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>	if (kbd_triggers_supported)
> >>>>		triggers_enabled = kbd_is_trigger_mode_bit(state.mode_bit);
> >>>>@@ -1650,18 +1658,24 @@ static ssize_t kbd_led_triggers_store(struct device *dev,
> >>>>			if (strcmp(trigger+1, kbd_led_triggers[i]) != 0)
> >>>>				continue;
> >>>>			if (trigger[0] == '+' &&
> >>>>-			    triggers_enabled && (state.triggers & BIT(i)))
> >>>>-				return count;
> >>>>+			    triggers_enabled && (state.triggers & BIT(i))) {
> >>>>+				ret = count;
> >>>>+				goto out;
> >>>>+			}
> >>>>			if (trigger[0] == '-' &&
> >>>>-			    (!triggers_enabled || !(state.triggers & BIT(i))))
> >>>>-				return count;
> >>>>+			    (!triggers_enabled || !(state.triggers & BIT(i)))) {
> >>>>+				ret = count;
> >>>>+				goto out;
> >>>>+			}
> >>>>			trigger_bit = i;
> >>>>			break;
> >>>>		}
> >>>>	}
> >>>>
> >>>>-	if (trigger_bit == -1)
> >>>>-		return -EINVAL;
> >>>>+	if (trigger_bit == -1) {
> >>>>+		ret = -EINVAL;
> >>>>+		goto out;
> >>>>+	}
> >>>>
> >>>>	new_state = state;
> >>>>	if (trigger[0] == '+')
> >>>>@@ -1677,22 +1691,29 @@ static ssize_t kbd_led_triggers_store(struct device *dev,
> >>>>			new_state.triggers &= ~BIT(2);
> >>>>	}
> >>>>	if ((kbd_info.triggers & new_state.triggers) !=
> >>>>-	    new_state.triggers)
> >>>>-		return -EINVAL;
> >>>>+	    new_state.triggers) {
> >>>>+		ret = -EINVAL;
> >>>>+		goto out;
> >>>>+	}
> >>>>	if (new_state.triggers && !triggers_enabled) {
> >>>>		new_state.mode_bit = KBD_MODE_BIT_TRIGGER;
> >>>>		kbd_set_level(&new_state, kbd_previous_level);
> >>>>	} else if (new_state.triggers == 0) {
> >>>>		kbd_set_level(&new_state, 0);
> >>>>	}
> >>>>-	if (!(kbd_info.modes & BIT(new_state.mode_bit)))
> >>>>-		return -EINVAL;
> >>>>+	if (!(kbd_info.modes & BIT(new_state.mode_bit))) {
> >>>>+		ret = -EINVAL;
> >>>>+		goto out;
> >>>>+	}
> >>>>	ret = kbd_set_state_safe(&new_state, &state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>	if (new_state.mode_bit != KBD_MODE_BIT_OFF)
> >>>>		kbd_previous_mode_bit = new_state.mode_bit;
> >>>>-	return count;
> >>>>+	ret = count;
> >>>>+out:
> >>>>+	mutex_unlock(&kbd_led_mutex);
> >>>>+	return ret;
> >>>>}
> >>>>
> >>>>static ssize_t kbd_led_triggers_show(struct device *dev,
> >>>>@@ -1749,12 +1770,16 @@ static ssize_t kbd_led_als_enabled_store(struct device *dev,
> >>>>	if (ret)
> >>>>		return ret;
> >>>>
> >>>>+	mutex_lock(&kbd_led_mutex);
> >>>>+
> >>>>	ret = kbd_get_state(&state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>-	if (enable == kbd_is_als_mode_bit(state.mode_bit))
> >>>>-		return count;
> >>>>+	if (enable == kbd_is_als_mode_bit(state.mode_bit)) {
> >>>>+		ret = count;
> >>>>+		goto out;
> >>>>+	}
> >>>>
> >>>>	new_state = state;
> >>>>
> >>>>@@ -1774,15 +1799,20 @@ static ssize_t kbd_led_als_enabled_store(struct device *dev,
> >>>>			new_state.mode_bit = KBD_MODE_BIT_ON;
> >>>>		}
> >>>>	}
> >>>>-	if (!(kbd_info.modes & BIT(new_state.mode_bit)))
> >>>>-		return -EINVAL;
> >>>>+	if (!(kbd_info.modes & BIT(new_state.mode_bit)))  {
> >>>>+		ret = -EINVAL;
> >>>>+		goto out;
> >>>>+	}
> >>>>
> >>>>	ret = kbd_set_state_safe(&new_state, &state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>	kbd_previous_mode_bit = new_state.mode_bit;
> >>>>
> >>>>-	return count;
> >>>>+	ret = count;
> >>>>+out:
> >>>>+	mutex_unlock(&kbd_led_mutex);
> >>>>+	return ret;
> >>>>}
> >>>>
> >>>>static ssize_t kbd_led_als_enabled_show(struct device *dev,
> >>>>@@ -1817,18 +1847,23 @@ static ssize_t kbd_led_als_setting_store(struct device *dev,
> >>>>	if (ret)
> >>>>		return ret;
> >>>>
> >>>>+	mutex_lock(&kbd_led_mutex);
> >>>>+
> >>>>	ret = kbd_get_state(&state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>	new_state = state;
> >>>>	new_state.als_setting = setting;
> >>>>
> >>>>	ret = kbd_set_state_safe(&new_state, &state);
> >>>>	if (ret)
> >>>>-		return ret;
> >>>>+		goto out;
> >>>>
> >>>>-	return count;
> >>>>+	ret = count;
> >>>>+out:
> >>>>+	mutex_unlock(&kbd_led_mutex);
> >>>>+	return ret;
> >>>>}
> >>>>
> >>>>static ssize_t kbd_led_als_setting_show(struct device *dev,
> >>>>@@ -1913,27 +1948,32 @@ static int kbd_led_level_set(struct led_classdev *led_cdev,
> >>>>	u16 num;
> >>>>	int ret;
> >>>>
> >>>>+	mutex_lock(&kbd_led_mutex);
> >>>>+
> >>>>	if (kbd_get_max_level()) {
> >>>>		ret = kbd_get_state(&state);
> >>>>		if (ret)
> >>>>-			return ret;
> >>>>+			goto out;
> >>>>		new_state = state;
> >>>>		ret = kbd_set_level(&new_state, value);
> >>>>		if (ret)
> >>>>-			return ret;
> >>>>-		return kbd_set_state_safe(&new_state, &state);
> >>>>-	}
> >>>>-
> >>>>-	if (kbd_get_valid_token_counts()) {
> >>>>+			goto out;
> >>>>+		ret = kbd_set_state_safe(&new_state, &state);
> >>>>+	} else if (kbd_get_valid_token_counts()) {
> >>>>		for (num = kbd_token_bits; num != 0 && value > 0; --value)
> >>>>			num &= num - 1; /* clear the first bit set */
> >>>>		if (num == 0)
> >>>>-			return 0;
> >>>>-		return kbd_set_token_bit(ffs(num) - 1);
> >>>>+			ret = 0;
> >>>>+		else
> >>>>+			ret = kbd_set_token_bit(ffs(num) - 1);
> >>>>+	} else {
> >>>>+		pr_warn("Keyboard brightness level control not supported\n");
> >>>>+		ret = -ENXIO;
> >>>>	}
> >>>>
> >>>>-	pr_warn("Keyboard brightness level control not supported\n");
> >>>>-	return -ENXIO;
> >>>>+out:
> >>>>+	mutex_unlock(&kbd_led_mutex);
> >>>>+	return ret;
> >>>>}
> >>>>
> >>>>static struct led_classdev kbd_led = {
> >>>
> >

-- 
Pali Rohár
pali.rohar@gmail.com