Re: races in drivers/usb/gadget/inode.c, leak fix in the same file

From: David Brownell
Date: Mon May 22 2006 - 15:31:56 EST

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() 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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