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

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Message-ID: <BN9PR11MB5276BC132EAE86D470B886528C13A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 10 Aug 2023 03:22:56 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     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

> 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/

> @@ -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)?