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

[Matt Evans]

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@xxxxxxxx>
---

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@xxxxxxxx/

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 */
 
-- 
2.47.3