Re: Oops/Warning report for the week of March 28th 2008

[Dmitry Torokhov]

On Fri, Mar 28, 2008 at 01:51:38PM -0700, Linus Torvalds wrote:
> 
> 
> On Fri, 28 Mar 2008, Linus Torvalds wrote:
> > 
> > Is there something obvious that I'm missing? I'd really like to see the 
> > whole posting that the oops came from. Do you save the originals or even 
> > just message ID's from the ones you pick from emails?
> 
> Hmm. Definitely not from the kernel mailing list. I'm intrigued, where did 
> that oops #5814 come from (picked a recent one at random)?
> 
> The thing is recent, and oopses on "mutex_lock(dev->mutex)" in 
> input_release_device. In particular, the path *seems* to be this one:
> 
>   evdev_release ->
>     evdev_ungrab ->
>       input_release_device ->
>         mutex_lock ->
>           mutex_lock_nested ->
>             __mutex_lock_common ->
>               list_add_tail(&waiter.list, &lock->wait_list)
> 
> where "lock->wait_list.prev" seems to be 0x6b6b6b6b6b6b6b6b, which is the 
> use-after-free poison pattern.
> 
> (In fact, I think the access that actually oopses is when the 
> debug version of __list_add() does
> 
> 	if (unlikely(prev->next != next)) {
> 
> because that "prev" pointer is crap).
> 
> So it seems that when input_release_device() does:
> 
> 	struct input_dev *dev = handle->dev;
> 
> 	mutex_lock(&dev->mutex);
> 
> the "dev" it uses has already been released. And this only shows up as a 
> problem when you have slab debugging turned on (like the Fedora kernels 
> do, thank you all Fedora guys).
> 
> The odd thing is that I don't think any of this code has really changed 
> recently. 
> 

There is a patch from Pete that works around the problem by not
calling input_release_device() on devices that are gone. But what
I don't understand is why the parent input device is gone since
sysfs/driver core should be keeping a reference to it since it is
a parent of evdev. input_dev shoudl only be released after
evdev_free() is called.

-- 
Dmitry
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