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Date:   Mon, 10 May 2021 09:10:14 +0800
From:   Yuan Yao <yuan.yao@linux.intel.com>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     tkffaul@outlook.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vfio/pci: Sanity check IGD OpRegion Size
Message-ID: <20210510011014.q6xfcmqopbqgepbq@yy-desk-7060>
References: <162041357421.21800.16214130780777455390.stgit@omen>
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On Fri, May 07, 2021 at 12:53:17PM -0600, Alex Williamson wrote:
> The size field of the IGD OpRegion table is supposed to indicate table
> size in KB, but we've seen at least one report of a BIOS that appears
> to incorrectly report size in bytes.  The default size is 8 (*1024 =
> 8KB), but an incorrect implementation may report 8192 (*1024 = 8MB)
> and can cause a variety of mapping errors.
> 
> It's believed that 8MB would be an implausible, if not absurd, actual
> size, so we can probably be pretty safe in assuming this is a BIOS bug
> where the intended size is likely 8KB.
> 
> Reported-by: Travis Faulhaber <tkffaul@outlook.com>
> Tested-by: Travis Faulhaber <tkffaul@outlook.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/vfio/pci/vfio_pci_igd.c |   11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_igd.c b/drivers/vfio/pci/vfio_pci_igd.c
> index 228df565e9bc..c89a4797cd18 100644
> --- a/drivers/vfio/pci/vfio_pci_igd.c
> +++ b/drivers/vfio/pci/vfio_pci_igd.c
> @@ -86,7 +86,16 @@ static int vfio_pci_igd_opregion_init(struct vfio_pci_device *vdev)
>  		return -EINVAL;
>  	}
>  
> -	size *= 1024; /* In KB */
> +	/*
> +	 * The OpRegion size field is specified as size in KB, but there have been
> +	 * user reports where this field appears to report size in bytes.  If we
> +	 * read 8192, assume this is the case.
> +	 */
> +	if (size == OPREGION_SIZE)

Is "size >= OPREGION_SIZE" or "size >= smaller but still implausible value
(like 4096)" better for covering more bad BIOS implementation cases ?

> +		pci_warn(vdev->pdev,
> +			 "BIOS Bug, IGD OpRegion reports invalid size, assuming default 8KB\n");
> +	else
> +		size *= 1024; /* In KB */
>  
>  	/*
>  	 * Support opregion v2.1+
> 
>