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

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Message-ID: <20230816133650.GC3425284@fedora>
Date:   Wed, 16 Aug 2023 09:36:50 -0400
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "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 Tue, Aug 15, 2023 at 03:23:50PM +0000, David Laight wrote:
> 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.

Yes, Kevin had a similar comment about this text. What I meant was that
it's safest to have a single memory layout across all architectures
(with explicit padding) so that there are no surprises. I'm going to
remove the statement about information leaks because it's confusing.

> 
> 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.

Yes, that would break compatibility but I don't see any changes in this
patch series that modifies the offsets of later fields. Have I missed
something?

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

Can you elaborate?

Thanks,
Stefan

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