[2/4] vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info

  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
32-bit userspace on a 64-bit kernel breakage.

This patch increases the struct size on x32 but this is safe because of
the struct's argsz field. The kernel may grow the struct as long as it
still supports smaller argsz values from userspace (e.g. applications
compiled against older kernel headers).

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/uapi/linux/vfio.h        | 3 ++-
 drivers/gpu/drm/i915/gvt/kvmgt.c | 4 +++-
 samples/vfio-mdev/mbochs.c       | 6 ++++--
 samples/vfio-mdev/mdpy.c         | 4 +++-
 4 files changed, 12 insertions(+), 5 deletions(-)
  

> From: Stefan Hajnoczi <stefanha@redhat.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/

> @@ -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)?
  

On Thu, Aug 10, 2023 at 03:22:56AM +0000, Tian, Kevin wrote:
> > From: Stefan Hajnoczi <stefanha@redhat.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
  

On Thu, Aug 10, 2023 at 03:22:56AM +0000, Tian, Kevin wrote:
> > From: Stefan Hajnoczi <stefanha@redhat.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/

Sounds good.

> 
> > @@ -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)?

I'll fix this in the next revision (it should be dmabuf.argsz).

Stefan
  

From: Stefan Hajnoczi
> Sent: 09 August 2023 22:03
> 
> 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
> 32-bit userspace on a 64-bit kernel breakage.

Isn't the hole likely to cause an information leak?
Forcing it to be there doesn't make any difference.
I'd add an explicit pad as well.

It is a shame there isn't an __attribute__(()) to error padded structures.

> 
> This patch increases the struct size on x32 but this is safe because of
> the struct's argsz field. The kernel may grow the struct as long as it
> still supports smaller argsz values from userspace (e.g. applications
> compiled against older kernel headers).

Doesn't changing the offset of later fields break compatibility?
The size field (probably) only lets you extend the structure.

Oh, for sanity do min(variable, constant).

	David

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