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>
