Messages in this thread | | | From | David Brownell <> | Subject | Re: races in drivers/usb/gadget/inode.c, leak fix in the same file | Date | Mon, 22 May 2006 12:28:56 -0700 |
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On Tuesday 25 April 2006 11:20 pm, Al Viro wrote:
Thanks for the review/feedback. The AIO interfaces have evidently had some incompatible changes since that code was first written (as turned up by Alan Stern's recent investigations), and it's always had a few glitchey (but uncommon) corner cases.
> activate_ep_files() allocates a bunch of ep_data, creates inodes and > dentries for them and sticks them into a cyclic list. inode->u.generic_ip > is set to created ep_data, refcount is 1. > > destroy_ep_files() goes through that cyclic list, gets dentry from each > ep_data on it and drops ep_data. Then it kills dentry and inode by > /* break link to dcache */ > mutex_lock (&parent->i_mutex); > d_delete (dentry); > dput (dentry); > mutex_unlock (&parent->i_mutex); > Tries to kill, anyway. The thing is, ep_open() (->open() on that sucker) > does
Hmm ...
> static int > ep_open (struct inode *inode, struct file *fd) > { > struct ep_data *data = inode->u.generic_ip; > int value = -EBUSY; > > if (down_interruptible (&data->lock) != 0) > return -EINTR; > followed eventually by pinning data (i.e. ep_data for that inode) down. > > It's way too late - by the time we enter ep_open(), inode->u.generic_ip > might be already pointing to freed memory. > > AFAICS, the cheapest way to deal with that would be to set ->u.generic_ip > to NULL before dropping ep_data in ep_destroy_files(), have that done > under global spinlock and have ep_open() et.al. start with checking if > the pointer is NULL under the same spinlock and grabbing a reference if > it isn't.
Sounds right. I'll look at it some more, and test.
> There's another question: opened files are opened; what protects them > from gadgetfs_unbind() freeing ep->req while ep is still pinned down > by an opened file? We do not hold dev->lock or ep->lock there...
Controller drivers should have canceled all pending requests before they call the unbind() method ... and must have completed them all by the time ep_disable() returnes. Two key invariants: the controller driver holds no more usb_request objects, and it won't accept any more until after the unbind() completes and some new driver is bound.
Now, there are two I/O paths. The AIO path dynamically allocates the usb_request objects, one per kiocb, and deallocates them as soon as the request completes. So, no issue there. And for synchronous I/O paths, there's a task in ep_io() which holds the ep->lock ... and the return paths there don't reference ep->req.
> One more: what protects dev->state? I don't see any lock that would > be held over all places that modify it.
It should be the spinlock. You're right, there are a few dodgy references there. Those should get fixed ... fortunately changing that is reasonably infrequent.
> Oh, and a trivial leak fix, while we are at it: > > Subject: [PATCH] fix leak in activate_ep_files()
I'd prefer the slightly more comprehensive one from Alan Stern. Even if they _are_ rare we might as well handle all of them! :)
- Dave
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