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

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Message-ID: <20121221201824.GA31554@phenom.dumpdata.com>
Date:	Fri, 21 Dec 2012 15:18:24 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Al Viro <viro@...IV.linux.org.uk>, dgdegra@...ho.nsa.gov,
	xen-devel@...ts.xensource.com, david.vrabel@...rix.com,
	stefano.stabellini@...citrix.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: oopsable race in xen-gntdev (unsafe vma access)

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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