The problem arises because our read() function grabs a lock of the
circular buffer, finds something of interest, then invokes copy_to_user()
straight from the buffer, which in turn takes mm->mmap_sem. In the same
time, the callback mon_bin_vma_fault() is invoked under mm->mmap_sem.
It attempts to take the fetch lock and deadlocks.

This patch does away with protecting of our page list with any
semaphores, and instead relies on the kernel not close the device
while mmap is active in a process.

In addition, we prohibit re-sizing of a buffer while mmap is active.
This way, when (now unlocked) fault is processed, it works with the
page that is intended to be mapped-in, and not some other random page.
Note that this may have an ABI impact, but hopefully no legitimate
program is this wrong.
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Reported-by: syzbot+56f9673bb4cdcbeb0e92@syzkaller.appspotmail.com
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 46eb14a6 ("USB: fix usbmon BUG trigger")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191204203941.3503452b@suzdal.zaitcev.lanSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>