Re: [PATCH] PCI/P2PDMA: Avoid returning a provider for non_mappable_bars

[Alex Williamson <alex@shazbot.org>]

On Tue, 21 Apr 2026 10:43:51 -0700
Matt Evans <mattev@meta.com> wrote:

> Extend pcim_p2pdma_provider()'s checks to exclude functions that have
> pdev->non_mappable_bars set.
> 
> Consumers such as VFIO were previously able to map these for access by
> the CPU or P2P.  Update the comment on non_mappable_bars to show it
> refers to any access, not just userspace CPU access.
> 
> Fixes: 372d6d1b8ae3c ("PCI/P2PDMA: Refactor to separate core P2P functionality from memory allocation")
> Signed-off-by: Matt Evans <mattev@meta.com>
> ---
> 
> This arises from Alex Williamson's suggestion to test
> non_mappable_bars when getting the provider, with discussion here:
> 
>  https://lore.kernel.org/kvm/20260415181623.1021090-1-mattev@meta.com/
> 
> The goal was to prevent a hole where VFIO could export DMABUFs for
> BARs marked non-mappable, and to fix for all users of the provider
> rather than just VFIO.  Alex observed that non_mappable_bars should be
> taken to mean BARs weren't usable by the CPU _or_ peers and,
> considering that, its comment about userspace access wasn't quite
> right.
> 
> 
>  drivers/pci/p2pdma.c | 3 ++-
>  include/linux/pci.h  | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 7c898542af8d..4a783413f466 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -318,7 +318,8 @@ struct p2pdma_provider *pcim_p2pdma_provider(struct pci_dev *pdev, int bar)
>  {
>  	struct pci_p2pdma *p2p;
>  
> -	if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM))
> +	if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM) ||
> +	    pdev->non_mappable_bars)
>  		return NULL;
>  
>  	p2p = rcu_dereference_protected(pdev->p2pdma, 1);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 2c4454583c11..1e6802017d6b 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -508,7 +508,7 @@ struct pci_dev {
>  	unsigned int	no_command_memory:1;	/* No PCI_COMMAND_MEMORY */
>  	unsigned int	rom_bar_overlap:1;	/* ROM BAR disable broken */
>  	unsigned int	rom_attr_enabled:1;	/* Display of ROM attribute enabled? */
> -	unsigned int	non_mappable_bars:1;	/* BARs can't be mapped to user-space  */
> +	unsigned int	non_mappable_bars:1;	/* BARs can't be mapped by CPU or peers */
>  	pci_dev_flags_t dev_flags;
>  	atomic_t	enable_cnt;	/* pci_enable_device has been called */
>  

Should pcim_p2pdma_init() separately test pdev->non_mappable_bars
before the rcu-deref/kzalloc of the pci_p2pdma object and return
-EOPNOTSUPP?

That then invokes the same error paths we'd see if we simply don't have
p2pdma in the kernel and handles the pci_p2pdma_add_resource() path
automatically as well.

This pcim_p2pdma_provider() test is really then just suppressing the
WARN_ON that we'd otherwise see by not finding the p2p object on the
device.  Thanks,

Alex