[PATCH -next] iommu/of: Check for valid fwspec after of_pci_iommu_init

[Sudeep Holla <sudeep.holla@arm.com>]

The successful return from of_pci_iommu_init doesn't ensure valid
fwspec if it's IOMMU is disabled.

Accessing dev->iommu_fwspec->ops without checking dev->iommu_fwspec
could result in NULL pointer dereference.

Unable to handle kernel NULL pointer dereference at virtual address 00000000
task: ffff800976880000 task.stack: ffff800976888000
PC is at of_iommu_configure+0x130/0x138
LR is at of_iommu_configure+0x118/0x138
 of_iommu_configure+0x130/0x138
 of_dma_configure+0xa8/0x190
 dma_configure+0x40/0xe8
 driver_probe_device+0x190/0x2d0
 __device_attach_driver+0x9c/0xf8
 bus_for_each_drv+0x58/0x98
 __device_attach+0xc4/0x138
 device_attach+0x10/0x18
 pci_bus_add_device+0x4c/0xa8
 pci_bus_add_devices+0x44/0x90
 pci_bus_add_devices+0x74/0x90
 pci_host_common_probe+0x14c/0x3a8
 gen_pci_probe+0x2c/0x38
 platform_drv_probe+0x58/0xc0
 driver_probe_device+0x214/0x2d0
 __driver_attach+0xac/0xb0
 bus_for_each_dev+0x60/0xa0
 driver_attach+0x20/0x28
 bus_add_driver+0x110/0x230
 driver_register+0x60/0xf8
 __platform_driver_register+0x44/0x50
 gen_pci_driver_init+0x18/0x20
 do_one_initcall+0x38/0x120
 kernel_init_freeable+0x184/0x224
 kernel_init+0x10/0x100

This patch adds the check for dev->iommu_fwspec and fixes the above.

Fixes: d87beb749281 ("iommu/of: Handle PCI aliases properly")
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
---
 drivers/iommu/of_iommu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Hi Robin,

I see this crash on Juno with linux-next. Enabling PCIe SMMU fixes the
issue, but we may need to handle the disabled SMMU case to continue to
work with old DTBs.

Regards,
Sudeep

diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
index 34160e7a8dd7..11e08ff2db57 100644
--- a/drivers/iommu/of_iommu.c
+++ b/drivers/iommu/of_iommu.c
@@ -200,7 +200,8 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
 					     of_pci_iommu_init, &info);
 		if (err) /* err > 0 means the walk stopped, but non-fatally */
 			ops = ERR_PTR(min(err, 0));
-		else /* success implies both fwspec and ops are now valid */
+		/* success may not imply fwspec is valid */
+		else if (dev->iommu_fwspec)
 			ops = dev->iommu_fwspec->ops;
 	} else {
 		struct of_phandle_args iommu_spec;
--
2.7.4