w1: therm: Fix off-by-one buffer overflow in alarms_store

The sysfs buffer passed to alarms_store() is allocated with 'size + 1'
bytes and a NUL terminator is appended. However, the 'size' argument
does not account for this extra byte. The original code then allocated
'size' bytes and used strcpy() to copy 'buf', which always writes one
byte past the allocated buffer since strcpy() copies until the NUL
terminator at index 'size'.

Fix this by parsing the 'buf' parameter directly using simple_strtoll()
without allocating any intermediate memory or string copying. This
removes the overflow while simplifying the code.

Cc: [e-mail]
Fixes: e2c94d6f5720 ("w1_therm: adding alarm sysfs entry")
Signed-off-by: Thorsten Blum <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Krzysztof Kozlowski <[e-mail]>

drivers/w1/slaves/w1_therm.c

···
 	struct w1_slave *sl = dev_to_w1_slave(device);
 	struct therm_info info;
 	u8 new_config_register[3];	/* array of data to be written */
-	int temp, ret;
-	char *token = NULL;
+	long long temp;
+	int ret = 0;
 	s8 tl, th;	/* 1 byte per value + temp ring order */
-	char *p_args, *orig;
+	const char *p = buf;
+	char *endp;
 
-	p_args = orig = kmalloc(size, GFP_KERNEL);
-	/* Safe string copys as buf is const */
-	if (!p_args) {
-		dev_warn(device,
-			"%s: error unable to allocate memory %d\n",
-			__func__, -ENOMEM);
-		return size;
-	}
-	strcpy(p_args, buf);
-
-	/* Split string using space char */
-	token = strsep(&p_args, " ");
-
-	if (!token)	{
-		dev_info(device,
-			"%s: error parsing args %d\n", __func__, -EINVAL);
-		goto free_m;
-	}
-
-	/* Convert 1st entry to int */
-	ret = kstrtoint (token, 10, &temp);
+	temp = simple_strtoll(p, &endp, 10);
+	if (p == endp || *endp != ' ')
+		ret = -EINVAL;
+	else if (temp < INT_MIN || temp > INT_MAX)
+		ret = -ERANGE;
 	if (ret) {
 		dev_info(device,
 			"%s: error parsing args %d\n", __func__, ret);
-		goto free_m;
+		return size;
 	}
 
 	tl = int_to_short(temp);
 
-	/* Split string using space char */
-	token = strsep(&p_args, " ");
-	if (!token)	{
-		dev_info(device,
-			"%s: error parsing args %d\n", __func__, -EINVAL);
-		goto free_m;
-	}
-	/* Convert 2nd entry to int */
-	ret = kstrtoint (token, 10, &temp);
+	p = endp + 1;
+	temp = simple_strtoll(p, &endp, 10);
+	if (p == endp)
+		ret = -EINVAL;
+	else if (temp < INT_MIN || temp > INT_MAX)
+		ret = -ERANGE;
 	if (ret) {
 		dev_info(device,
 			"%s: error parsing args %d\n", __func__, ret);
-		goto free_m;
+		return size;
 	}
 
 	/* Prepare to cast to short by eliminating out of range values */
···
 		dev_info(device,
 			"%s: error reading from the slave device %d\n",
 			__func__, ret);
-		goto free_m;
+		return size;
 	}
 
 	/* Write data in the device RAM */
···
 		dev_info(device,
 			"%s: Device not supported by the driver %d\n",
 			__func__, -ENODEV);
-		goto free_m;
+		return size;
 	}
 
 	ret = SLAVE_SPECIFIC_FUNC(sl)->write_data(sl, new_config_register);
···
 		dev_info(device,
 			"%s: error writing to the slave device %d\n",
 			__func__, ret);
-
-free_m:
-	/* free allocated memory */
-	kfree(orig);
 
 	return size;
 }