char: xillybus: Prevent use-after-free due to race condition

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>

diff --git a/drivers/char/xillybus/xillyusb.c b/drivers/char/xillybus/xillyusb.c
index 39bcbfd..5a5afa1 100644 (file)
--- a/drivers/char/xillybus/xillyusb.c
+++ b/drivers/char/xillybus/xillyusb.c
@@ -184,6 +184,14 @@ 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,
@@ -1237,9 +1245,16 @@ 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;
@@ -1275,8 +1290,6 @@ 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;
 
@@ -1413,6 +1426,7 @@ unopen:
        return rc;
 
 unmutex_fail:
+       kref_put(&xdev->kref, cleanup_dev);
        mutex_unlock(&chan->lock);
        return rc;
 }
@@ -2227,7 +2241,9 @@ 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 = {