linux-kernel - Re: [PATCH] vfio_pci: Enable memory accesses before calling pci_map

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Date:   Wed, 13 Feb 2019 11:28:21 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, alex.williamson@...hat.com
Subject: Re: [PATCH] vfio_pci: Enable memory accesses before calling
 pci_map_rom

On Wed, Feb 13, 2019 at 11:14:06AM +0100, Eric Auger wrote:
> pci_map_rom/pci_get_rom_size() performs memory access in the ROM.
> In case the Memory Space accesses were disabled, readw() is likely to
> crash the host with a synchronous external abort (aarch64).

Ouch. Is there an CVE for this?

Also I think this can cause x86 machines to blow up.

See https://xenbits.xen.org/xsa/advisory-120.html
> 
> In case memory accesses were disabled, re-enable them before the call
> and disable them back again just after.
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> ---
>  drivers/vfio/pci/vfio_pci.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index ff60bd1ea587..96b8bbd909d7 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -706,8 +706,10 @@ static long vfio_pci_ioctl(void *device_data,
>  			break;
>  		case VFIO_PCI_ROM_REGION_INDEX:
>  		{
> +			bool mem_access_disabled;
>  			void __iomem *io;
>  			size_t size;
> +			u16 cmd;
>  
>  			info.offset = VFIO_PCI_INDEX_TO_OFFSET(info.index);
>  			info.flags = 0;
> @@ -723,6 +725,13 @@ static long vfio_pci_ioctl(void *device_data,
>  					break;
>  			}
>  
> +			pci_read_config_word(pdev, PCI_COMMAND, &cmd);
> +			mem_access_disabled = !(cmd & PCI_COMMAND_MEMORY);
> +			if (mem_access_disabled) {
> +				cmd |= PCI_COMMAND_MEMORY;
> +				pci_write_config_word(pdev, PCI_COMMAND, cmd);
> +			}
> +
>  			/* Is it really there? */
>  			io = pci_map_rom(pdev, &size);
>  			if (!io || !size) {
> @@ -731,6 +740,11 @@ static long vfio_pci_ioctl(void *device_data,
>  			}
>  			pci_unmap_rom(pdev, io);
>  
> +			if (mem_access_disabled) {
> +				cmd &= ~PCI_COMMAND_MEMORY;
> +				pci_write_config_word(pdev, PCI_COMMAND, cmd);
> +			}
> +
>  			info.flags = VFIO_REGION_INFO_FLAG_READ;
>  			break;
>  		}
> -- 
> 2.20.1
>