Re: oopsable race in xen-gntdev (unsafe vma access)

From: Konrad Rzeszutek Wilk
Date: Fri Dec 21 2012 - 15:18:32 EST

On Sat, Dec 15, 2012 at 06:12:11PM +0000, Al Viro wrote:
> 1) find_vma() is *not* safe without ->mmap_sem and its result may
> very well be freed just as it's returned to caller. IOW,
> gntdev_ioctl_get_offset_for_vaddr() is racy and may end up with
> dereferencing freed memory.
> 2) gntdev_vma_close() is putting NULL into map->vma with only
> ->mmap_sem held by caller. Things like
> if (!map->vma)
> continue;
> if (map->vma->vm_start >= end)
> continue;
> if (map->vma->vm_end <= start)
> done with just priv->lock held are racy.
> I'm not familiar with the code, but it looks like we need to
> protect gntdev_vma_close() guts with the same spinlock and probably
> hold ->mmap_sem shared around the "find_vma()+get to map->{index,count}"
> in the ioctl. Or replace the logics in ioctl with search through the
> list of grant_map under the same spinlock...
> Comments?
Hey Al,

Thank you for your analysis.

CC-ing Daniel, David and Stefano. I recall we had some priv->lock movement
in the past and there is also interaction with another piece of code -
the balloon code so we better be circumspect of not blowing up.

Al, it is around holidays and folks are mostly gone - so this will take
a bit of time to get sorted out.
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