Re: [Xen-devel] oopsable race in xen-gntdev [PATCH 0/3]

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jan 02, 2013 at 05:57:10PM -0500, Daniel De Graaf wrote:
> On 12/21/2012 03:18 PM, Konrad Rzeszutek Wilk wrote:
> > 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.
> 
> I agree, this one should be fixed by taking mmap_sem in
> gntdev_ioctl_get_offset_for_vaddr. Iterating the grant_map list here
> will not work under HVM, where map->vma is not filled in.
> 
> >>    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?
> 
> Although I don't think the mmu notifier is ever called without mmap_sem
> on this particular device file (we map only with VM_DONTCOPY and other
> paths like truncate generally aren't triggered), it's probably best not
> to rely on that behavior, so adding the spinlock in gntdev_vma_close
> seems to be the best solution.
> 
> > Hey Al,
> > 
> > Thank you for your analysis.

Are you OK with the patches or have comments about them? Thanks.

> > 
> > 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.
> 
> While I was digging in this code, I found a related bug in
> mn_invl_range_start: if gntdev_ioctl_unmap_grant_ref is called on
> a range before unmapping it, the entry is removed from priv->maps and
> the later call to mn_invl_range_start won't find it to do the unmapping.
> This could be fixed by using find_vma, but I don't think there's a safe
> way to do that from inside the mmu notifier, so instead I created a list
> of these unlinked but still mapped pages.
> 
> The third patch is a fix to an unrelated bug that I found while testing
> the fixes in the other two patches.
> 
> [PATCH 1/3] xen/gntdev: fix unsafe vma access
> [PATCH 2/3] xen/gntdev: correctly unmap unlinked maps in mmu
> [PATCH 3/3] xen/gntdev: remove erronous use of copy_to_user

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