RE: [PATCH] vfio/pci: Fix SR-IOV VF handling with MMIO blocking

[Wang, Haiyue]

> -----Original Message-----
> From: kvm-owner@xxxxxxxxxxxxxxx <kvm-owner@xxxxxxxxxxxxxxx> On Behalf Of Alex Williamson
> Sent: Friday, June 26, 2020 00:57
> To: alex.williamson@xxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; maxime.coquelin@xxxxxxxxxx
> Subject: [PATCH] vfio/pci: Fix SR-IOV VF handling with MMIO blocking
> 
> SR-IOV VFs do not implement the memory enable bit of the command
> register, therefore this bit is not set in config space after
> pci_enable_device().  This leads to an unintended difference
> between PF and VF in hand-off state to the user.  We can correct
> this by setting the initial value of the memory enable bit in our
> virtualized config space.  There's really no need however to
> ever fault a user on a VF though as this would only indicate an
> error in the user's management of the enable bit, versus a PF
> where the same access could trigger hardware faults.
> 
> Fixes: abafbc551fdd ("vfio-pci: Invalidate mmaps and block MMIO access on disabled memory")
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
>  drivers/vfio/pci/vfio_pci_config.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index 8746c943247a..d98843feddce 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -398,9 +398,15 @@ static inline void p_setd(struct perm_bits *p, int off, u32 virt, u32 write)
>  /* Caller should hold memory_lock semaphore */
>  bool __vfio_pci_memory_enabled(struct vfio_pci_device *vdev)
>  {
> +	struct pci_dev *pdev = vdev->pdev;
>  	u16 cmd = le16_to_cpu(*(__le16 *)&vdev->vconfig[PCI_COMMAND]);
> 
> -	return cmd & PCI_COMMAND_MEMORY;
> +	/*
> +	 * SR-IOV VF memory enable is handled by the MSE bit in the
> +	 * PF SR-IOV capability, there's therefore no need to trigger
> +	 * faults based on the virtual value.
> +	 */
> +	return pdev->is_virtfn || (cmd & PCI_COMMAND_MEMORY);

Hi Alex,

After set up the initial copy of config space for memory enable bit for VF, is it worth
to trigger SIGBUS into the bad user space process which intentionally try to disable the
memory access command (even it is VF) then access the memory to trigger CVE-2020-12888 ?

BR,
Haiyue

>  }
> 
>  /*
> @@ -1728,6 +1734,15 @@ int vfio_config_init(struct vfio_pci_device *vdev)
>  				 vconfig[PCI_INTERRUPT_PIN]);
> 
>  		vconfig[PCI_INTERRUPT_PIN] = 0; /* Gratuitous for good VFs */
> +
> +		/*
> +		 * VFs do no implement the memory enable bit of the COMMAND
> +		 * register therefore we'll not have it set in our initial
> +		 * copy of config space after pci_enable_device().  For
> +		 * consistency with PFs, set the virtual enable bit here.
> +		 */
> +		*(__le16 *)&vconfig[PCI_COMMAND] |=
> +					cpu_to_le16(PCI_COMMAND_MEMORY);
>  	}
> 
>  	if (!IS_ENABLED(CONFIG_VFIO_PCI_INTX) || vdev->nointx)