[PATCH v3 2/3] soundwire: debugfs: replace writable string helper

debugfs_create_str() is being made read-only because its generic write
path is hard to make safe without adding more locking to the helper.

Convert the SoundWire firmware_file entry to a local write-only file
operation before removing writable string support from
debugfs_create_str().

Copy the firmware name before request_firmware() so later debugfs writes
cannot replace the string while it is being used.

Signed-off-by: Yichong Chen <[email protected]>
---
 drivers/soundwire/debugfs.c | 63 ++++++++++++++++++++++++++++++++-----
 1 file changed, 55 insertions(+), 8 deletions(-)

diff --git a/drivers/soundwire/debugfs.c b/drivers/soundwire/debugfs.c
index 099eb84a548e..a8a6db164fe7 100644
--- a/drivers/soundwire/debugfs.c
+++ b/drivers/soundwire/debugfs.c
@@ -145,6 +145,38 @@ static u32 start_addr;
 static size_t num_bytes;
 static u8 read_buffer[MAX_CMD_BYTES];
 static char *firmware_file;
+static DEFINE_MUTEX(firmware_file_lock);
+
+static ssize_t firmware_file_write(struct file *file,
+				   const char __user *user_buf,
+				   size_t count, loff_t *ppos)
+{
+	char *old, *new;
+
+	if (*ppos)
+		return -EINVAL;
+	if (count + 1 > PAGE_SIZE)
+		return -E2BIG;
+
+	new = memdup_user_nul(user_buf, count);
+	if (IS_ERR(new))
+		return PTR_ERR(new);
+	strim(new);
+
+	scoped_guard(mutex, &firmware_file_lock) {
+		old = firmware_file;
+		firmware_file = new;
+	}
+
+	kfree(old);
+	return count;
+}
+
+static const struct file_operations firmware_file_fops = {
+	.open = simple_open,
+	.write = firmware_file_write,
+	.llseek = default_llseek,
+};
 
 static int set_command(void *data, u64 value)
 {
@@ -246,6 +278,7 @@ static int cmd_go(void *data, u64 value)
 {
 	const struct firmware *fw = NULL;
 	struct sdw_slave *slave = data;
+	char *fw_name = NULL;
 	ktime_t start_t;
 	ktime_t finish_t;
 	int ret;
@@ -265,15 +298,24 @@ static int cmd_go(void *data, u64 value)
 	}
 
 	if (cmd == 0) {
-		ret = request_firmware(&fw, firmware_file, &slave->dev);
+		scoped_guard(mutex, &firmware_file_lock) {
+			if (firmware_file)
+				fw_name = kstrdup(firmware_file, GFP_KERNEL);
+		}
+		if (!fw_name) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		ret = request_firmware(&fw, fw_name, &slave->dev);
 		if (ret < 0) {
-			dev_err(&slave->dev, "firmware %s not found\n", firmware_file);
+			dev_err(&slave->dev, "firmware %s not found\n", fw_name);
 			goto out;
 		}
 		if (fw->size < num_bytes) {
 			dev_err(&slave->dev,
 				"firmware %s: firmware size %zd, desired %zd\n",
-				firmware_file, fw->size, num_bytes);
+				fw_name, fw->size, num_bytes);
 			goto out;
 		}
 	}
@@ -306,6 +348,7 @@ static int cmd_go(void *data, u64 value)
 out:
 	if (fw)
 		release_firmware(fw);
+	kfree(fw_name);
 
 	pm_runtime_mark_last_busy(&slave->dev);
 	pm_runtime_put(&slave->dev);
@@ -358,7 +401,8 @@ void sdw_slave_debugfs_init(struct sdw_slave *slave)
 
 	debugfs_create_file("read_buffer", 0400, d, slave, &read_buffer_fops);
 	if (firmware_file)
-		debugfs_create_str("firmware_file", 0200, d, &firmware_file);
+		debugfs_create_file("firmware_file", 0200, d, NULL,
+				    &firmware_file_fops);
 
 	slave->debugfs = d;
 }
@@ -370,8 +414,10 @@ void sdw_slave_debugfs_exit(struct sdw_slave *slave)
 
 void sdw_debugfs_init(void)
 {
-	if (!firmware_file)
-		firmware_file = kstrdup("", GFP_KERNEL);
+	scoped_guard(mutex, &firmware_file_lock) {
+		if (!firmware_file)
+			firmware_file = kstrdup("", GFP_KERNEL);
+	}
 
 	sdw_debugfs_root = debugfs_create_dir("soundwire", NULL);
 }
@@ -379,6 +425,7 @@ void sdw_debugfs_init(void)
 void sdw_debugfs_exit(void)
 {
 	debugfs_remove_recursive(sdw_debugfs_root);
-	kfree(firmware_file);
-	firmware_file = NULL;
+	scoped_guard(mutex, &firmware_file_lock) {
+		kfree(firmware_file);
+	}
 }
-- 
2.51.0