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Re: [patch-4.16] arm/smmuv1,v2: Protect smmu master list with a lock



Hi Michal,

On 26/10/2021 13:29, Michal Orzel wrote:
If a device is added to SMMUv1/v2 from DT and PCI
at the same time, there is a concurrent access
to a smmu master list. This could lead to a
scenario where one is looking into a list that
is being modified at the same time. Add a lock
to prevent this issue.

Did you intend to say "Hold" rather than "Add"?


Reuse the existing spinlock arm_smmu_devices_lock
as it is already protecting find_smmu_master.

ipmmu-smmu and smmuv3 are not impacted by this
issue as there is no access or modification of
a global resource during add_device.

Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
---
This patch aims for 4.16 release.
Benefits:
Remove a bug that could lead to a corruption of the
smmu master list, which would be very hard to debug.

From my understanding, this corruption would only happen with CONFIG_HAS_PCI. At the moment, this is a experimental feature as it is not fully complete.

Risks:
Overall the risk is low as we are touching init code
rather than a runtime one. In case of any issue, the
problem would be catched during system boot or guest
start.

With the PCI feature enabled, this can be called at runtime as this plumbed through a PHYSDEVOP. That said, it doesn't matter here as for supported code (platform/DT device), this will only happen during boot.

The patch looks straighforward, so I would not mind to have it in Xen 4.16. Ian, what do you think?

---
  xen/drivers/passthrough/arm/smmu.c | 25 +++++++++++++++++++------
  1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/xen/drivers/passthrough/arm/smmu.c 
b/xen/drivers/passthrough/arm/smmu.c
index c9dfc4caa0..be62a66a28 100644
--- a/xen/drivers/passthrough/arm/smmu.c
+++ b/xen/drivers/passthrough/arm/smmu.c
@@ -820,21 +820,25 @@ static int arm_smmu_dt_add_device_legacy(struct 
arm_smmu_device *smmu,
                                         struct device *dev,
                                         struct iommu_fwspec *fwspec)
  {
-       int i;
+       int i, ret;
        struct arm_smmu_master *master;
        struct device_node *dev_node = dev_get_dev_node(dev);
+ spin_lock(&arm_smmu_devices_lock);
        master = find_smmu_master(smmu, dev_node);
        if (master) {
                dev_err(dev,
                        "rejecting multiple registrations for master device 
%s\n",
                        dev_node->name);
-               return -EBUSY;
+               ret = -EBUSY;
+               goto out_unlock;
        }
master = devm_kzalloc(dev, sizeof(*master), GFP_KERNEL);
-       if (!master)
-               return -ENOMEM;
+       if (!master) {
+               ret = -ENOMEM;
+               goto out_unlock;
+       }
        master->of_node = dev_node;
/* Xen: Let Xen know that the device is protected by an SMMU */
@@ -846,11 +850,17 @@ static int arm_smmu_dt_add_device_legacy(struct 
arm_smmu_device *smmu,
                        dev_err(dev,
                                "stream ID for master device %s greater than maximum 
allowed (%d)\n",
                                dev_node->name, smmu->num_mapping_groups);
-                       return -ERANGE;
+                       ret = -ERANGE;
+                       goto out_unlock;
                }
                master->cfg.smendx[i] = INVALID_SMENDX;
        }
-       return insert_smmu_master(smmu, master);
+
+       ret = insert_smmu_master(smmu, master);
+
+out_unlock:
+       spin_unlock(&arm_smmu_devices_lock);

Please add a newline here.

+       return ret;
  }
static int register_smmu_master(struct arm_smmu_device *smmu,
@@ -2056,7 +2066,10 @@ static int arm_smmu_add_device(struct device *dev)
        } else {
                struct arm_smmu_master *master;
+ spin_lock(&arm_smmu_devices_lock);
                master = find_smmu_master(smmu, dev->of_node);
+               spin_unlock(&arm_smmu_devices_lock);

At the moment, unlocking here is fine because we don't remove the device. However, there are a series to supporting removing a device (see [1]). So I think it would be preferable to unlock after the last use of 'cfg'.

Looking at the code, I also noticed that the error path is not correct for at least the PCI device and we would leak memory. We also assume that Stream ID == Requester ID. Are both of them in your radar for PCI passthrough?

Cheers,

[1] https://lore.kernel.org/xen-devel/1630562763-390068-9-git-send-email-fnu.vikram@xxxxxxxxxx/

--
Julien Grall



 


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