Re: [PATCH] Avoid null-pointer access in w1/slaves/w1_therm

[David Fries <david@fries.net>]

On Mon, Mar 09, 2015 at 11:47:24PM +0100, Thorsten Bschorr wrote:
> > Here's a different strategy, add a w1_therm family_data specific mutex
> > so w1_therm_remove_slave won't return while another thread is in
> > w1_slave_show.  Thoughts?
> >
> > I included three patches, the first is the proposed fix, the second
> > makes it more reproducible (and since my testing doesn't have external
> > power I had to ignore that check), the third is just some debugging
> > messages.  For testing I'm starting a read from w1_slave then manually
> > remove the sensor, which seems to do the trick.
> 
> I'll test your patch, but keeping the original sleep-time tm.

Oops, sorry, it got lost in the shuffle, here's the first patch again
(the others were for debugging and increase that time and so wouldn't
go upstream anyway).

>From 777f5fd75f5f99a3352863e83d226c7b65ebdaa4 Mon Sep 17 00:00:00 2001
From: David Fries <David@Fries.net>
Date: Sat, 7 Mar 2015 22:25:37 -0600
Subject: [PATCH] w1_therm, don't let the slave go away while in w1_slave_show

A temperature conversion can take 750 ms to complete, if the sensor is
externally powered it releases the bus_mutex while it waits, but if
the slave device is removed sl becomes a dangling pointer.
The race condition window can be 10 * 750 ms = 7.5 seconds if the crc
check fails.

Reported-By: Thorsten Bschorr <thorsten@bschorr.de>
Signed-off-by: David Fries <David@Fries.net>
---
 drivers/w1/slaves/w1_therm.c | 51 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 13 deletions(-)

diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
index 1f11a20..39a9e6a 100644
--- a/drivers/w1/slaves/w1_therm.c
+++ b/drivers/w1/slaves/w1_therm.c
@@ -59,9 +59,20 @@ MODULE_ALIAS("w1-family-" __stringify(W1_THERM_DS28EA00));
 static int w1_strong_pullup = 1;
 module_param_named(strong_pullup, w1_strong_pullup, int, 0);
 
+struct w1_therm_family_data {
+	uint8_t rom[9];
+	struct mutex lock;
+};
+
+/* return the address of the lock in the family data */
+#define THERM_LOCK(sl) \
+	(&((struct w1_therm_family_data*)sl->family_data)->lock)
+
 static int w1_therm_add_slave(struct w1_slave *sl)
 {
-	sl->family_data = kzalloc(9, GFP_KERNEL);
+	sl->family_data = kzalloc(sizeof(struct w1_therm_family_data),
+		GFP_KERNEL);
+	mutex_init(THERM_LOCK(sl));
 	if (!sl->family_data)
 		return -ENOMEM;
 	return 0;
@@ -69,6 +80,11 @@ static int w1_therm_add_slave(struct w1_slave *sl)
 
 static void w1_therm_remove_slave(struct w1_slave *sl)
 {
+	/* Getting the lock means w1_slave_show isn't sleeping and the
+	 * family_data can be freed.
+	 */
+	mutex_lock(THERM_LOCK(sl));
+	mutex_unlock(THERM_LOCK(sl));
 	kfree(sl->family_data);
 	sl->family_data = NULL;
 }
@@ -194,13 +210,15 @@ static ssize_t w1_slave_show(struct device *device,
 	struct w1_slave *sl = dev_to_w1_slave(device);
 	struct w1_master *dev = sl->master;
 	u8 rom[9], crc, verdict, external_power;
-	int i, max_trying = 10;
+	int i, ret, max_trying = 10;
 	ssize_t c = PAGE_SIZE;
 
-	i = mutex_lock_interruptible(&dev->bus_mutex);
-	if (i != 0)
-		return i;
+	ret = mutex_lock_interruptible(&dev->bus_mutex);
+	if (ret != 0)
+		goto post_unlock;
 
+	/* prevent the slave from going away in sleep */
+	mutex_lock(THERM_LOCK(sl));
 	memset(rom, 0, sizeof(rom));
 
 	while (max_trying--) {
@@ -230,17 +248,19 @@ static ssize_t w1_slave_show(struct device *device,
 				mutex_unlock(&dev->bus_mutex);
 
 				sleep_rem = msleep_interruptible(tm);
-				if (sleep_rem != 0)
-					return -EINTR;
+				if (sleep_rem != 0) {
+					ret = -EINTR;
+					goto post_unlock;
+				}
 
-				i = mutex_lock_interruptible(&dev->bus_mutex);
-				if (i != 0)
-					return i;
+				ret = mutex_lock_interruptible(&dev->bus_mutex);
+				if (ret != 0)
+					goto post_unlock;
 			} else if (!w1_strong_pullup) {
 				sleep_rem = msleep_interruptible(tm);
 				if (sleep_rem != 0) {
-					mutex_unlock(&dev->bus_mutex);
-					return -EINTR;
+					ret = -EINTR;
+					goto pre_unlock;
 				}
 			}
 
@@ -279,9 +299,14 @@ static ssize_t w1_slave_show(struct device *device,
 
 	c -= snprintf(buf + PAGE_SIZE - c, c, "t=%d\n",
 		w1_convert_temp(rom, sl->family->fid));
+	ret = PAGE_SIZE - c;
+
+pre_unlock:
 	mutex_unlock(&dev->bus_mutex);
 
-	return PAGE_SIZE - c;
+post_unlock:
+	mutex_unlock(THERM_LOCK(sl));
+	return ret;
 }
 
 static int __init w1_therm_init(void)
-- 
2.1.4