[PATCH] cxl/regs: Reject register blocks in an unassigned BAR

[Junjie Cao <junjie.cao@intel.com>]

cxl_decode_regblock() only checks that the Register Locator offset fits
pci_resource_len().  A BAR the PCI core could not place is reset to zero
start, end and flags while config space keeps the firmware value, so a
zero offset passes and the block is mapped at physical address 0.  On
x86 ioremap() of the reserved low megabyte succeeds, the component
register header does not match, and a switch port fails with "HDM
decoder capability not found", pointing at the HDM decoders rather than
at the BAR.

Reject a BAR with no length or still unassigned, next to the existing size
check.  Skip empty Register Locator entries first: they decode as BAR0
offset 0 and would trip the check, and nothing looks them up.

Seen with edk2-stable202602 (f6489621b8ae, reverted in stable202605),
which places 64-bit non-prefetchable BARs behind a bridge in the
prefetchable window.  Linux refuses to claim them and the switch
upstream and downstream port BAR0s find no room in the 32-bit window.
QEMU q35 with a CXL switch, Fedora 43 edk2-ovmf-20260213, before:

pci 0000:0d:00.0: BAR 0 [mem size 0x00010000 64bit]: can't assign; no space
cxl_port port2: HDM decoder capability not found

after:

pcieport 0000:0d:00.0: BAR0: not assigned (type: 1)
cxl_port port2: No component registers mapped
cxl_port port2: Failed to map HDM decoder capability

Link: https://github.com/tianocore/edk2/issues/13104
Signed-off-by: Junjie Cao <junjie.cao@intel.com>
---
 drivers/cxl/core/regs.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/cxl/core/regs.c b/drivers/cxl/core/regs.c
index 20c2d9fbcfe7..41416fa2ce4a 100644
--- a/drivers/cxl/core/regs.c
+++ b/drivers/cxl/core/regs.c
@@ -277,6 +277,20 @@ static bool cxl_decode_regblock(struct pci_dev *pdev, u32 reg_lo, u32 reg_hi,
 	u64 offset = ((u64)reg_hi << 32) |
 		     (reg_lo & PCI_DVSEC_CXL_REG_LOCATOR_BLOCK_OFF_LOW);
 
+	if (reg_type == CXL_REGLOC_RBI_EMPTY)
+		return false;
+
+	/*
+	 * A BAR the PCI core could not place is reset to zero; decoding it
+	 * would map the block at physical address 0.
+	 */
+	if (!pci_resource_len(pdev, bar) ||
+	    (pci_resource_flags(pdev, bar) & IORESOURCE_UNSET)) {
+		dev_warn(&pdev->dev, "BAR%d: not assigned (type: %d)\n", bar,
+			 reg_type);
+		return false;
+	}
+
 	if (offset > pci_resource_len(pdev, bar)) {
 		dev_warn(&pdev->dev,
 			 "BAR%d: %pr: too small (offset: %pa, type: %d)\n", bar,
-- 
2.43.0