linux-kernel - Re: [PATCH 2/4] vfio: use __aligned_u64 in struct vfio_device_gfx_plane

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Date:   Thu, 10 Aug 2023 10:24:20 -0400
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH 2/4] vfio: use __aligned_u64 in struct
 vfio_device_gfx_plane_info

On Thu, Aug 10, 2023 at 03:22:56AM +0000, Tian, Kevin wrote:
> > From: Stefan Hajnoczi <stefanha@...hat.com>
> > Sent: Thursday, August 10, 2023 5:03 AM
> > 
> > The memory layout of struct vfio_device_gfx_plane_info is
> > architecture-dependent due to a u64 field and a struct size that is not
> > a multiple of 8 bytes:
> > - On x86_64 the struct size is padded to a multiple of 8 bytes.
> > - On x32 the struct size is only a multiple of 4 bytes, not 8.
> > - Other architectures may vary.
> > 
> > Use __aligned_u64 to make memory layout consistent. This reduces the
> > chance of holes that result in an information leak and the chance that
> 
> I didn't quite get this. The leak example [1] from your earlier fix is really
> not caused by the use of __u64. Instead it's a counter example that on
> x32 there is no hole with 4byte alignment for __u64.
> 
> I'd remove the hole part and just keep the compat reason.
> 
> [1] https://lore.kernel.org/lkml/20230801103114.757d7992.alex.williamson@redhat.com/T/

Okay.

> 
> > @@ -1392,6 +1392,8 @@ static long intel_vgpu_ioctl(struct vfio_device
> > *vfio_dev, unsigned int cmd,
> >  		if (dmabuf.argsz < minsz)
> >  			return -EINVAL;
> > 
> > +		minsz = min(minsz, sizeof(dmabuf));
> > +
> 
> Is there a case where minsz could be greater than sizeof(dmabuf)?

Thanks for spotting this, it's a bug in the patch. It should be
min(dmabuf.argsz, sizeof(dmabuf)).

Stefan

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