[RFC][PATCH 2/2] iommu: Avoid crash if iommu_group is null

[John Stultz]

In trying to handle a possible driver probe ordering issue
brought up by Robin Murphy, I ran across a separate null pointer
crash in the iommu core in iommu_group_remove_device():
[    2.732803] dwc3-qcom a6f8800.usb: failed to get usb-ddr path: -517
[    2.739281] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000c0
...
[    2.775619] [00000000000000c0] user address but active_mm is swapper
[    2.782039] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[    2.787670] Modules linked in:
[    2.790769] CPU: 6 PID: 1 Comm: swapper/0 Tainted: G        W         5.10.0-rc1-mainline-00025-g272a618fc36-dirty #3973
[    2.801719] Hardware name: Thundercomm Dragonboard 845c (DT)
[    2.807431] pstate: 00c00005 (nzcv daif +PAN +UAO -TCO BTYPE=--)
[    2.813508] pc : iommu_group_remove_device+0x30/0x1b0
[    2.818611] lr : iommu_release_device+0x4c/0x78
[    2.823189] sp : ffffffc01005b950
...
[    2.907082] Call trace:
[    2.909566]  iommu_group_remove_device+0x30/0x1b0
[    2.914323]  iommu_release_device+0x4c/0x78
[    2.918559]  iommu_bus_notifier+0xe8/0x108
[    2.922708]  blocking_notifier_call_chain+0x78/0xb8
[    2.927641]  device_del+0x2ac/0x3d0
[    2.931177]  platform_device_del.part.9+0x20/0x98
[    2.935933]  platform_device_unregister+0x2c/0x40
[    2.940694]  of_platform_device_destroy+0xd8/0xe0
[    2.945450]  device_for_each_child_reverse+0x58/0xb0
[    2.950471]  of_platform_depopulate+0x4c/0x78
[    2.954886]  dwc3_qcom_probe+0x93c/0xcb8
[    2.958858]  platform_drv_probe+0x58/0xa8
[    2.962917]  really_probe+0xec/0x398
[    2.966531]  driver_probe_device+0x5c/0xb8
[    2.970677]  device_driver_attach+0x74/0x98
[    2.974911]  __driver_attach+0x60/0xe8
[    2.978700]  bus_for_each_dev+0x84/0xd8
[    2.982581]  driver_attach+0x30/0x40
[    2.986194]  bus_add_driver+0x160/0x208
[    2.990076]  driver_register+0x64/0x110
[    2.993957]  __platform_driver_register+0x58/0x68
[    2.998716]  dwc3_qcom_driver_init+0x20/0x28
[    3.003041]  do_one_initcall+0x6c/0x2d0
[    3.006925]  kernel_init_freeable+0x214/0x268
[    3.011339]  kernel_init+0x18/0x118
[    3.014876]  ret_from_fork+0x10/0x18
[    3.018495] Code: d0006a21 f9417295 91130021 910162b6 (b940c2a2)

In the case above, the arm-smmu driver fails to probe with
EPROBE_DEFER, and I'm guessing I'm guessing that causes
iommu_group_add_device() to fail and sets the
dev->iommu_group = NULL, then somehow we hit
iommu_group_remove_device() and trip over the null value?
I'm not really sure...

Anyway, adding the null check seems to avoid the issue and the
system boots fine after the arm-smmu driver later reprobed.

Feedback or better ideas for a solution would be appreciated!

Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Andy Gross <agross@xxxxxxxxxx>
Cc: Maulik Shah <mkshah@xxxxxxxxxxxxxx>
Cc: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
Cc: Saravana Kannan <saravanak@xxxxxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Lina Iyer <ilina@xxxxxxxxxxxxxx>
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-arm-msm <linux-arm-msm@xxxxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
 drivers/iommu/iommu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 8c470f451a323..44639b88e77db 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -877,6 +877,10 @@ void iommu_group_remove_device(struct device *dev)
 	struct iommu_group *group = dev->iommu_group;
 	struct group_device *tmp_device, *device = NULL;
 
+	/* Avoid crash if iommu_group value is null */
+	if (!group)
+		return;
+
 	dev_info(dev, "Removing from iommu group %d\n", group->id);
 
 	/* Pre-notify listeners that a device is being removed. */
-- 
2.17.1