[RFC] deadlock in "drm/exynos: fix wrong pointer access at vm close"

[Inki Dae]

Thanks for your comment.

> -----Original Message-----
> From: Al Viro [mailto:viro at ftp.linux.org.uk] On Behalf Of Al Viro
> Sent: Monday, September 23, 2013 10:07 PM
> To: Inki Dae
> Cc: 'YoungJun Cho'; dri-devel at lists.freedesktop.org
> Subject: Re: [RFC] deadlock in "drm/exynos: fix wrong pointer access at vm
> close"
> 
> On Mon, Sep 23, 2013 at 04:49:30PM +0900, Inki Dae wrote:
> > Hi,
> >
> > > -----Original Message-----
> > > From: Al Viro [mailto:viro at ftp.linux.org.uk] On Behalf Of Al Viro
> > > Sent: Monday, September 23, 2013 6:29 AM
> > > To: YoungJun Cho
> > > Cc: dri-devel at lists.freedesktop.org; Inki Dae
> > > Subject: [RFC] deadlock in "drm/exynos: fix wrong pointer access at vm
> > > close"
> > >
> > > 	You have drm_dev->struct_mutex grabbed before ->mmap_sem in
> > > exynos_drm_gem_mmap_ioctl() and after - in exynos_drm_gem_fault()
> > > (since ->fault() is always called with ->mmap_sem held).  Looks like
> > > a garden-variety AB-BA deadlock...
> > >
> >
> > Right, if mmap system call is requested by another process between -
> >f_op
> > pointer changing and restoring, the deadlock can be incurred.
> >
> > For this, I think we can resolve this issue like below,
> >
> > At exynos_drm_gem_mmap_ioctl()
> > 	down_write(&mm->mmap_sem);
> > 	mutex_lock(&dev->struct_mutex);
> > 	...
> 
> Umm...  If you do it that way, why bother with changing ->private_data
> at all?  You can as well stash obj in dev->dev_private->something after
> you've grabbed the mutex and have ->mmap() pick it there...

I changed ->private_data to a gem object so that mmap_buffer function can
get the gem object. But yes, we can use dev->dev_private->something instead,
and that seems like better way. For this, I missed that we can get
drm_device object from file pointer.

> 
> Said that, I really don't like that approach - both playing with ->f_op
> and the games with private vmas;

Agree. But is there other better way to support direct mapping; not needing
page fault handler to map, and indirect mapping interfaces; needing the page
fault handler?

> exynos_gem_get_vma(), AFAICS, calls
> find_vma() (without bothering to hold ->mmap_sem, BTW - there's nothing
> to prevent the result of find_vma() being freed just as it's returned

I can't see to hold ->mmap_sem when it calls find_vma() anywhere else.

> to caller) and clones it manually, regardless of whether that vma allows
> to clone itself or not.  Quite a few drivers rely on that not happening...
> 

I think that has no any problem because exynos_gem_get_vma() takes a
reference to vma, and also v4l2 side is using same way. I and v4l2 guys
might be missing something what you are concerning. So Could you give us
more comments?

And, I found a bug that we never call exynos_gem_put_vma() when userptr is
freed. That should be fixed.

> IOW, you are already digging deep inside VM guts and this only makes
> it deeper ;-/
> 
> And no, the deadlock doesn't depend on race between ioctl() and mmap()
> from another process; all it takes is

Yes, there was my wrong comments. That is  a mmap
callback(exynos_drm_gem_mmap_buffer) we don't want can be called if we
called mmap system call when ->f_op points to exynos_drm_gem_fops.

> 1) thread A does clone(), creating thread B that shares address space with
> it.
> 2) thread A does that ioctl, creating a mapping
> 3) thread A does that ioctl again, creating another mapping, while thread
> B dereferences an address in the first mapping and triggers a page fault.
> 
> The only race is on step 3 in the above; the question about mmap() vs.
> ioctl() was about mmap(2) _during_ that ioctl() hitting
> exynos_drm_gem_mmap_buffer() instead of exynos_drm_gem_mmap() it would've
> called before or after ioctl().  Here the interesting case is when
> callers of mmap() and ioctl() do *not* share the address space, since
> in that case mmap(2) won't notice ->mmap_sem held by you - it's on the
> different mm_struct, so mmap(2) will get to calling the ->f_op->mmap()
> without waiting for you to restore ->f_op...

Yes, it's a problem. When two more threads are running, and when thread A
called mmap system call, exynos_drm_gem_mmap_buffer() will be called if
thread B called  ioctl() first and f_op was not restored yet. I have no idea
to resolve this issue yet. So do you have any good idea? At any rate, we
should fix this issue.
 
> 
> For another bug in the same area, try building that driver modular and
> watch what happens to use count after a round of this switching ->f_op and
> restoring it back to original; fops_get() in there is wrong and AFAICS
> pointless.

Yes, it would also be another bug. Will check it out. It seems that it's not
good way to change and restore ->f_op, and this way makes mmap part to be
complicated. I will try to find a way to resolve this issue or to find
another better way to support direct and indirect mapping interfaces.

I'd appreciate all the advices I can get.

Thanks,
Inki Dae