Re: [PATCH 1/2] PCI/MSI: Cache the MSIX table size

[Leon Romanovsky]

On Thu, Jan 19, 2023 at 07:06:32PM +0200, Alexander Shishkin wrote:
> A malicious device can change its MSIX table size between the table
> ioremap() and subsequent accesses, resulting in a kernel page fault in
> pci_write_msg_msix().
> 
> To avoid this, cache the table size observed at the moment of table
> ioremap() and use the cached value. This, however, does not help drivers
> that peek at the PCIE_MSIX_FLAGS register directly.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Reviewed-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  drivers/pci/msi/api.c | 7 ++++++-
>  drivers/pci/msi/msi.c | 2 +-
>  include/linux/pci.h   | 1 +
>  3 files changed, 8 insertions(+), 2 deletions(-)

I'm not security expert here, but not sure that this protects from anything.
1. Kernel relies on working and not-malicious HW. There are gazillion ways
to cause crashes other than changing MSI-X.
2. Device can report large table size, kernel will cache it and
malicious device will reduce it back. It is not handled and will cause
to kernel crash too.

Thanks

> 
> diff --git a/drivers/pci/msi/api.c b/drivers/pci/msi/api.c
> index b8009aa11f3c..617ea1256487 100644
> --- a/drivers/pci/msi/api.c
> +++ b/drivers/pci/msi/api.c
> @@ -75,8 +75,13 @@ int pci_msix_vec_count(struct pci_dev *dev)
>  	if (!dev->msix_cap)
>  		return -EINVAL;
>  
> +	if (dev->flags_qsize)
> +		return dev->flags_qsize;
> +
>  	pci_read_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS, &control);
> -	return msix_table_size(control);
> +	dev->flags_qsize = msix_table_size(control);
> +
> +	return dev->flags_qsize;
>  }
>  EXPORT_SYMBOL(pci_msix_vec_count);
>  
> diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
> index 1f716624ca56..d50cd45119f1 100644
> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -715,7 +715,7 @@ static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
>  
>  	pci_read_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS, &control);
>  	/* Request & Map MSI-X table region */
> -	tsize = msix_table_size(control);
> +	tsize = pci_msix_vec_count(dev);
>  	dev->msix_base = msix_map_region(dev, tsize);
>  	if (!dev->msix_base) {
>  		ret = -ENOMEM;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index adffd65e84b4..2e1a72a2139d 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -352,6 +352,7 @@ struct pci_dev {
>  	u8		rom_base_reg;	/* Config register controlling ROM */
>  	u8		pin;		/* Interrupt pin this device uses */
>  	u16		pcie_flags_reg;	/* Cached PCIe Capabilities Register */
> +	u16		flags_qsize;	/* Cached MSIX table size */
>  	unsigned long	*dma_alias_mask;/* Mask of enabled devfn aliases */
>  
>  	struct pci_driver *driver;	/* Driver bound to this device */
> -- 
> 2.39.0
>