iommu: Make __iommu_group_set_domain() handle error unwind

Let's try to have a consistent and clear strategy for error handling
during domain attach failures.

There are two broad categories, the first is callers doing destruction and
trying to set the domain back to a previously good domain. These cases
cannot handle failure during destruction flows and must succeed, or at
least avoid a UAF on the current group->domain which is likely about to be
freed.

Many of the drivers are well behaved here and will not hit the WARN_ON's
or a UAF, but some are doing hypercalls/etc that can fail unpredictably
and don't meet the expectations.

The second case is attaching a domain for the first time in a failable
context, failure should restore the attachment back to group->domain using
the above unfailable operation.

Have __iommu_group_set_domain_internal() execute a common algorithm that
tries to achieve this, and in the worst case, would leave a device
"detached" or assigned to a global blocking domain. This relies on some
existing common driver behaviors where attach failure will also do detatch
and true IOMMU_DOMAIN_BLOCK implementations that are not allowed to ever
fail.

Name the first case with __iommu_group_set_domain_nofail() to make it
clear.

Pull all the error handling and WARN_ON generation into
__iommu_group_set_domain_internal().

Avoid the obfuscating use of __iommu_group_for_each_dev() and be more
careful about what should happen during failures by only touching devices
we've already touched.
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v5-1b99ae392328+44574-iommu_err_unwind_jgg@nvidia.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -101,8 +101,26 @@ static int __iommu_attach_device(struct iommu_domain *domain,
 				 struct device *dev);
 static int __iommu_attach_group(struct iommu_domain *domain,
 				struct iommu_group *group);
+
+enum {
+	IOMMU_SET_DOMAIN_MUST_SUCCEED = 1 << 0,
+};
+
+static int __iommu_group_set_domain_internal(struct iommu_group *group,
+					     struct iommu_domain *new_domain,
+					     unsigned int flags);
 static int __iommu_group_set_domain(struct iommu_group *group,
-				    struct iommu_domain *new_domain);
+				    struct iommu_domain *new_domain)
+{
+	return __iommu_group_set_domain_internal(group, new_domain, 0);
+}
+static void __iommu_group_set_domain_nofail(struct iommu_group *group,
+					    struct iommu_domain *new_domain)
+{
+	WARN_ON(__iommu_group_set_domain_internal(
+		group, new_domain, IOMMU_SET_DOMAIN_MUST_SUCCEED));
+}
+
 static int iommu_create_device_direct_mappings(struct iommu_group *group,
 					       struct device *dev);
 static struct iommu_group *iommu_group_get_for_dev(struct device *dev);
@@ -2022,15 +2040,13 @@ EXPORT_SYMBOL_GPL(iommu_domain_free);
 static void __iommu_group_set_core_domain(struct iommu_group *group)
 {
 	struct iommu_domain *new_domain;
-	int ret;
 
 	if (group->owner)
 		new_domain = group->blocking_domain;
 	else
 		new_domain = group->default_domain;
 
-	ret = __iommu_group_set_domain(group, new_domain);
-	WARN(ret, "iommu driver failed to attach the default/blocking domain");
+	__iommu_group_set_domain_nofail(group, new_domain);
 }
 
 static int __iommu_attach_device(struct iommu_domain *domain,
@@ -2215,21 +2231,55 @@ int iommu_attach_group(struct iommu_domain *domain, struct iommu_group *group)
 }
 EXPORT_SYMBOL_GPL(iommu_attach_group);
 
-static int iommu_group_do_set_platform_dma(struct device *dev, void *data)
+static int __iommu_device_set_domain(struct iommu_group *group,
+				     struct device *dev,
+				     struct iommu_domain *new_domain,
+				     unsigned int flags)
 {
-	const struct iommu_ops *ops = dev_iommu_ops(dev);
-
-	if (!WARN_ON(!ops->set_platform_dma_ops))
-		ops->set_platform_dma_ops(dev);
+	int ret;
 
+	ret = __iommu_attach_device(new_domain, dev);
+	if (ret) {
+		/*
+		 * If we have a blocking domain then try to attach that in hopes
+		 * of avoiding a UAF. Modern drivers should implement blocking
+		 * domains as global statics that cannot fail.
+		 */
+		if ((flags & IOMMU_SET_DOMAIN_MUST_SUCCEED) &&
+		    group->blocking_domain &&
+		    group->blocking_domain != new_domain)
+			__iommu_attach_device(group->blocking_domain, dev);
+		return ret;
+	}
 	return 0;
 }
 
-static int __iommu_group_set_domain(struct iommu_group *group,
-				    struct iommu_domain *new_domain)
-{
+/*
+ * If 0 is returned the group's domain is new_domain. If an error is returned
+ * then the group's domain will be set back to the existing domain unless
+ * IOMMU_SET_DOMAIN_MUST_SUCCEED, otherwise an error is returned and the group's
+ * domains is left inconsistent. This is a driver bug to fail attach with a
+ * previously good domain. We try to avoid a kernel UAF because of this.
+ *
+ * IOMMU groups are really the natural working unit of the IOMMU, but the IOMMU
+ * API works on domains and devices.  Bridge that gap by iterating over the
+ * devices in a group.  Ideally we'd have a single device which represents the
+ * requestor ID of the group, but we also allow IOMMU drivers to create policy
+ * defined minimum sets, where the physical hardware may be able to distiguish
+ * members, but we wish to group them at a higher level (ex. untrusted
+ * multi-function PCI devices).  Thus we attach each device.
+ */
+static int __iommu_group_set_domain_internal(struct iommu_group *group,
+					     struct iommu_domain *new_domain,
+					     unsigned int flags)
+{
+	struct group_device *last_gdev;
+	struct group_device *gdev;
+	int result;
 	int ret;
 
+	lockdep_assert_held(&group->mutex);
+
 	if (group->domain == new_domain)
 		return 0;
 
@@ -2239,8 +2289,12 @@ static int __iommu_group_set_domain(struct iommu_group *group,
 	 * platform specific behavior.
 	 */
 	if (!new_domain) {
-		__iommu_group_for_each_dev(group, NULL,
-					   iommu_group_do_set_platform_dma);
+		for_each_group_device(group, gdev) {
+			const struct iommu_ops *ops = dev_iommu_ops(gdev->dev);
+
+			if (!WARN_ON(!ops->set_platform_dma_ops))
+				ops->set_platform_dma_ops(gdev->dev);
+		}
 		group->domain = NULL;
 		return 0;
 	}
@@ -2250,16 +2304,52 @@ static int __iommu_group_set_domain(struct iommu_group *group,
 	 * domain. This switch does not have to be atomic and DMA can be
 	 * discarded during the transition. DMA must only be able to access
 	 * either new_domain or group->domain, never something else.
-	 *
-	 * Note that this is called in error unwind paths, attaching to a
-	 * domain that has already been attached cannot fail.
 	 */
-	ret = __iommu_group_for_each_dev(group, new_domain,
-					 iommu_group_do_attach_device);
-	if (ret)
-		return ret;
+	result = 0;
+	for_each_group_device(group, gdev) {
+		ret = __iommu_device_set_domain(group, gdev->dev, new_domain,
+						flags);
+		if (ret) {
+			result = ret;
+			/*
+			 * Keep trying the other devices in the group. If a
+			 * driver fails attach to an otherwise good domain, and
+			 * does not support blocking domains, it should at least
+			 * drop its reference on the current domain so we don't
+			 * UAF.
+			 */
+			if (flags & IOMMU_SET_DOMAIN_MUST_SUCCEED)
+				continue;
+			goto err_revert;
+		}
+	}
 	group->domain = new_domain;
-	return 0;
+	return result;
+
+err_revert:
+	/*
+	 * This is called in error unwind paths. A well behaved driver should
+	 * always allow us to attach to a domain that was already attached.
+	 */
+	last_gdev = gdev;
+	for_each_group_device(group, gdev) {
+		const struct iommu_ops *ops = dev_iommu_ops(gdev->dev);
+
+		/*
+		 * If set_platform_dma_ops is not present a NULL domain can
+		 * happen only for first probe, in which case we leave
+		 * group->domain as NULL and let release clean everything up.
+		 */
+		if (group->domain)
+			WARN_ON(__iommu_device_set_domain(
+				group, gdev->dev, group->domain,
+				IOMMU_SET_DOMAIN_MUST_SUCCEED));
+		else if (ops->set_platform_dma_ops)
+			ops->set_platform_dma_ops(gdev->dev);
+		if (gdev == last_gdev)
+			break;
+	}
+	return ret;
 }
 
 void iommu_detach_group(struct iommu_domain *domain, struct iommu_group *group)
@@ -3176,16 +3266,13 @@ EXPORT_SYMBOL_GPL(iommu_device_claim_dma_owner);
 
 static void __iommu_release_dma_ownership(struct iommu_group *group)
 {
-	int ret;
-
 	if (WARN_ON(!group->owner_cnt || !group->owner ||
 		    !xa_empty(&group->pasid_array)))
 		return;
 
 	group->owner_cnt = 0;
 	group->owner = NULL;
-	ret = __iommu_group_set_domain(group, group->default_domain);
-	WARN(ret, "iommu driver failed to attach the default domain");
+	__iommu_group_set_domain_nofail(group, group->default_domain);
 }
 
 /**