linux-kernel - Re: [PATCH v2] vfio_pci: Enable memory accesses before calling pci_map

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Date:   Wed, 13 Feb 2019 10:52:00 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2] vfio_pci: Enable memory accesses before calling
 pci_map_rom

On Wed, 13 Feb 2019 12:06:10 +0100
Eric Auger <eric.auger@...hat.com> 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).

As implied in response to Konrad, the likeliness really depends on the
whole platform, not just the CPU architecture.  It's a class of
problems that depends on OS control or error handling, which we simply
don't have on many systems.  But we can fix this instance of it.

> 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>

This has been around since the beginning, but maybe a Fixes tag would
be useful:

Fixes: 89e1f7d4c66d ("vfio: Add PCI device driver")

> 
> ---
> 
> v1 -> v2:
> - also re-enable in case of error
> ---
>  drivers/vfio/pci/vfio_pci.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index ff60bd1ea587..721aa55424a4 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;
> @@ -723,15 +725,28 @@ 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) {
>  				info.size = 0;
> -				break;
> +				goto rom_info_out;
>  			}
>  			pci_unmap_rom(pdev, io);
>  
>  			info.flags = VFIO_REGION_INFO_FLAG_READ;
> +rom_info_out:
> +			if (mem_access_disabled) {
> +				cmd &= ~PCI_COMMAND_MEMORY;
> +				pci_write_config_word(pdev, PCI_COMMAND, cmd);
> +			}
> +
>  			break;
>  		}
>  		case VFIO_PCI_VGA_REGION_INDEX:

I don't think we need to be so timid about the command register and we
can also avoid the goto by modifying the test (testing io and size in
the original is probably overly paranoid), perhaps simply:

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index ff60bd1ea587..659b7c1ea8fb 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -708,6 +708,7 @@ static long vfio_pci_ioctl(void *device_data,
 		{
 			void __iomem *io;
 			size_t size;
+			u16 orig_cmd;
 
 			info.offset = VFIO_PCI_INDEX_TO_OFFSET(info.index);
 			info.flags = 0;
@@ -723,15 +724,23 @@ static long vfio_pci_ioctl(void *device_data,
 					break;
 			}
 
-			/* Is it really there? */
+			/*
+			 * Is it really there?  Enable memory decode for
+			 * implicit access in pci_map_rom().
+			 */
+			pci_read_config_word(pdev, PCI_COMMAND, &orig_cmd);
+			pci_write_config_word(pdev, PCI_COMMAND,
+					      orig_cmd | PCI_COMMAND_MEMORY);
+
 			io = pci_map_rom(pdev, &size);
-			if (!io || !size) {
+			if (io) {
+				info.flags = VFIO_REGION_INFO_FLAG_READ;
+				pci_unmap_rom(pdev, io);
+			} else
 				info.size = 0;
-				break;
-			}
-			pci_unmap_rom(pdev, io);
 
-			info.flags = VFIO_REGION_INFO_FLAG_READ;
+			pci_write_config_word(pdev, PCI_COMMAND, orig_cmd);
+
 			break;
 		}
 		case VFIO_PCI_VGA_REGION_INDEX: