Re: [PATCH kernel v4 01/11] vfio_pci: Test for extended capabilities if config space > 256 bytes

[Alex Williamson]

On Fri, 29 Apr 2016 18:55:14 +1000
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:

> PCI-Express spec says that reading 4 bytes at offset 100h should return
> zero if there is no extended capability so VFIO reads this dword to
> know if there are extended capabilities.
> 
> However it is not always possible to access the extended space so
> generic PCI code in pci_cfg_space_size_ext() checks if
> pci_read_config_dword() can read beyond 100h and if the check fails,
> it sets the config space size to 100h.
> 
> VFIO does its own extended capabilities check by reading at offset 100h
> which may produce 0xffffffff which VFIO treats as the extended config
> space presense and calls vfio_ecap_init() which fails to parse
> capabilities (which is expected) but right before the exit, it writes
> zero at offset 100h which is beyond the buffer allocated for
> vdev->vconfig (which is 256 bytes) which leads to random memory
> corruption.
> 
> This makes VFIO only check for the extended capabilities if
> the discovered config size is more than 256 bytes.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> ---
> Changes:
> v2:
> * instead of checking for 0xffffffff, this only does the check if
> device's config size is big enough
> ---

I'll take this patch separately, please drop from this series.  Thanks,

Alex

>  drivers/vfio/pci/vfio_pci_config.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index 142c533..d0c4358 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -1124,9 +1124,12 @@ static int vfio_cap_len(struct vfio_pci_device *vdev, u8 cap, u8 pos)
>  			return pcibios_err_to_errno(ret);
>  
>  		if (PCI_X_CMD_VERSION(word)) {
> -			/* Test for extended capabilities */
> -			pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> -			vdev->extended_caps = (dword != 0);
> +			if (pdev->cfg_size > PCI_CFG_SPACE_SIZE) {
> +				/* Test for extended capabilities */
> +				pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE,
> +						&dword);
> +				vdev->extended_caps = (dword != 0);
> +			}
>  			return PCI_CAP_PCIX_SIZEOF_V2;
>  		} else
>  			return PCI_CAP_PCIX_SIZEOF_V0;
> @@ -1138,9 +1141,11 @@ static int vfio_cap_len(struct vfio_pci_device *vdev, u8 cap, u8 pos)
>  
>  		return byte;
>  	case PCI_CAP_ID_EXP:
> -		/* Test for extended capabilities */
> -		pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> -		vdev->extended_caps = (dword != 0);
> +		if (pdev->cfg_size > PCI_CFG_SPACE_SIZE) {
> +			/* Test for extended capabilities */
> +			pci_read_config_dword(pdev, PCI_CFG_SPACE_SIZE, &dword);
> +			vdev->extended_caps = dword != 0;
> +		}
>  
>  		/* length based on version */
>  		if ((pcie_caps_reg(pdev) & PCI_EXP_FLAGS_VERS) == 1)