From: Eli Billauer eli.billauer@gmail.com

[ Upstream commit 282a4b71816b6076029017a7bab3a9dcee12a920 ]

The driver for XillyUSB devices maintains a kref reference count on each xillyusb_dev structure, which represents a physical device. This reference count reaches zero when the device has been disconnected and there are no open file descriptors that are related to the device. When this occurs, kref_put() calls cleanup_dev(), which clears up the device's data, including the structure itself.

However, when xillyusb_open() is called, this reference count becomes tricky: This function needs to obtain the xillyusb_dev structure that relates to the inode's major and minor (as there can be several such). xillybus_find_inode() (which is defined in xillybus_class.c) is called for this purpose. xillybus_find_inode() holds a mutex that is global in xillybus_class.c to protect the list of devices, and releases this mutex before returning. As a result, nothing protects the xillyusb_dev's reference counter from being decremented to zero before xillyusb_open() increments it on its own behalf. Hence the structure can be freed due to a rare race condition.

To solve this, a mutex is added. It is locked by xillyusb_open() before the call to xillybus_find_inode() and is released only after the kref counter has been incremented on behalf of the newly opened inode. This protects the kref reference counters of all xillyusb_dev structs from being decremented by xillyusb_disconnect() during this time segment, as the call to kref_put() in this function is done with the same lock held.

There is no need to hold the lock on other calls to kref_put(), because if xillybus_find_inode() finds a struct, xillyusb_disconnect() has not made the call to remove it, and hence not made its call to kref_put(), which takes place afterwards. Hence preventing xillyusb_disconnect's call to kref_put() is enough to ensure that the reference doesn't reach zero before it's incremented by xillyusb_open().

It would have been more natural to increment the reference count in xillybus_find_inode() of course, however this function is also called by Xillybus' driver for PCIe / OF, which registers a completely different structure. Therefore, xillybus_find_inode() treats these structures as void pointers, and accordingly can't make any changes.

Reported-by: Hyunwoo Kim imv4bel@gmail.com Suggested-by: Alan Stern stern@rowland.harvard.edu Signed-off-by: Eli Billauer eli.billauer@gmail.com Link: https://lore.kernel.org/r/20221030094209.65916-1-eli.billauer@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Bin Lan bin.lan.cn@windriver.com --- drivers/char/xillybus/xillyusb.c | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/char/xillybus/xillyusb.c b/drivers/char/xillybus/xillyusb.c index 3a2a0fb3d928..45771b1a3716 100644 --- a/drivers/char/xillybus/xillyusb.c +++ b/drivers/char/xillybus/xillyusb.c @@ -185,6 +185,14 @@ struct xillyusb_dev { struct mutex process_in_mutex; /* synchronize wakeup_all() */ };

+/* + * kref_mutex is used in xillyusb_open() to prevent the xillyusb_dev + * struct from being freed during the gap between being found by + * xillybus_find_inode() and having its reference count incremented. + */ + +static DEFINE_MUTEX(kref_mutex); + /* FPGA to host opcodes */ enum { OPCODE_DATA = 0, @@ -1234,9 +1242,16 @@ static int xillyusb_open(struct inode *inode, struct file *filp) int rc; int index;

+ mutex_lock(&kref_mutex); + rc = xillybus_find_inode(inode, (void **)&xdev, &index); - if (rc) + if (rc) { + mutex_unlock(&kref_mutex); return rc; + } + + kref_get(&xdev->kref); + mutex_unlock(&kref_mutex);

chan = &xdev->channels[index]; filp->private_data = chan; @@ -1272,8 +1287,6 @@ static int xillyusb_open(struct inode *inode, struct file *filp) ((filp->f_mode & FMODE_WRITE) && chan->open_for_write)) goto unmutex_fail;

- kref_get(&xdev->kref); - if (filp->f_mode & FMODE_READ) chan->open_for_read = 1;

@@ -1410,6 +1423,7 @@ static int xillyusb_open(struct inode *inode, struct file *filp) return rc;

unmutex_fail: + kref_put(&xdev->kref, cleanup_dev); mutex_unlock(&chan->lock); return rc; } @@ -2244,7 +2258,9 @@ static void xillyusb_disconnect(struct usb_interface *interface)

xdev->dev = NULL;

+ mutex_lock(&kref_mutex); kref_put(&xdev->kref, cleanup_dev); + mutex_unlock(&kref_mutex); }

static struct usb_driver xillyusb_driver = {