[PATCH] w1_therm reference count family data

[David Fries]

A temperature conversion can take 750 ms and when possible the
w1_therm slave driver drops the bus_mutex to allow other bus
operations, but that includes operations such as a periodic slave
search, which can remove this slave when it is no longer detected.
If that happens the sl->family_data will be freed and set to NULL
causing w1_slave_show to crash when it wakes up.

Signed-off-by: David Fries <David@xxxxxxxxx>
Reported-By: Thorsten Bschorr <thorsten@xxxxxxxxxx>
Tested-by: Thorsten Bschorr <thorsten@xxxxxxxxxx>
Acked-by: Evgeniy Polyakov <zbr@xxxxxxxxxxx>
---
This should be applied to the stable series as well.  In the name of
full disclosure, this just narrows the race window, from crashing in
normal operation on the reporters system to no longer crashing with
multiple readers and another process hammering on inserting/removing
the slave device.

This patch has been tested for some weeks by those who were affected,
Evgeniy Polyakov (maintainer) has approved this fix with the intention
of switching to sysfs device reference counting (as w1_slave_show is
called through sysfs).

 drivers/w1/slaves/w1_therm.c |   62 ++++++++++++++++++++++++++++++++----------
 1 file changed, 47 insertions(+), 15 deletions(-)

diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
index 1f11a20..55eb86c 100644
--- a/drivers/w1/slaves/w1_therm.c
+++ b/drivers/w1/slaves/w1_therm.c
@@ -59,16 +59,32 @@ 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];
+	atomic_t refcnt;
+};
+
+/* return the address of the refcnt in the family data */
+#define THERM_REFCNT(family_data) \
+	(&((struct w1_therm_family_data*)family_data)->refcnt)
+
 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);
 	if (!sl->family_data)
 		return -ENOMEM;
+	atomic_set(THERM_REFCNT(sl->family_data), 1);
 	return 0;
 }
 
 static void w1_therm_remove_slave(struct w1_slave *sl)
 {
+	int refcnt = atomic_sub_return(1, THERM_REFCNT(sl->family_data));
+	while(refcnt) {
+		msleep(1000);
+		refcnt = atomic_read(THERM_REFCNT(sl->family_data));
+	}
 	kfree(sl->family_data);
 	sl->family_data = NULL;
 }
@@ -194,13 +210,22 @@ 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;
+	u8 *family_data = sl->family_data;
+
+	ret = mutex_lock_interruptible(&dev->bus_mutex);
+	if (ret != 0)
+		goto post_unlock;
 
-	i = mutex_lock_interruptible(&dev->bus_mutex);
-	if (i != 0)
-		return i;
+	if(!sl->family_data)
+	{
+		ret = -ENODEV;
+		goto pre_unlock;
+	}
 
+	/* prevent the slave from going away in sleep */
+	atomic_inc(THERM_REFCNT(family_data));
 	memset(rom, 0, sizeof(rom));
 
 	while (max_trying--) {
@@ -230,17 +255,19 @@ 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;
 				}
 			}
 
@@ -269,19 +296,24 @@ static ssize_t w1_slave_show(struct device *device,
 	c -= snprintf(buf + PAGE_SIZE - c, c, ": crc=%02x %s\n",
 			   crc, (verdict) ? "YES" : "NO");
 	if (verdict)
-		memcpy(sl->family_data, rom, sizeof(rom));
+		memcpy(family_data, rom, sizeof(rom));
 	else
 		dev_warn(device, "Read failed CRC check\n");
 
 	for (i = 0; i < 9; ++i)
 		c -= snprintf(buf + PAGE_SIZE - c, c, "%02x ",
-			      ((u8 *)sl->family_data)[i]);
+			      ((u8 *)family_data)[i]);
 
 	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:
+	atomic_dec(THERM_REFCNT(family_data));
+	return ret;
 }
 
 static int __init w1_therm_init(void)
-- 
1.7.10.4

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/