[PATCH] iommu: Introduce reset_mutex to avoid circular locking dependency

[David Matlack <dmatlack@google.com>]

Introduce a dedicated reset_mutex inside struct iommu_group to serialize
device reset operations and domain resetting state without acquiring
group->mutex.

Commit f5b16b802174 ("PCI: Suspend iommu function prior to resetting a
device") introduced pci_dev_reset_iommu_prepare() and
pci_dev_reset_iommu_done(), which acquire group->mutex during PCI device
resets. In drivers such as VFIO, device reset handlers (e.g.
pci_try_reset_function()) are executed while holding driver locks such
as vdev->memory_lock.

This introduces a circular locking dependency that can lead to tasks
being permanently stuck in a deadlock:

[vdev->memory_lock]  ──────(1. VFIO Reset)──────>  [iommu_group->mutex]
         ▲
         │                                                    │
 (3. VFIO BAR Page Fault)                                    ... [*]
         │                                                    │
         │                                                    │
         └───────────  [mm->mmap_lock]  <─────────────────────┘

Fix this by using group->reset_mutex instead of group->mutex in
pci_dev_reset_iommu_prepare() and pci_dev_reset_iommu_done(). To ensure
other iommu_group operations stay properly synchronized with resets,
also take group->reset_mutex when checking group->recovery_cnt.

[*] The iommu_group->mutex --> mm->mmap_lock dependency was reported as
a transitive chain: iommu_group->mutex --> cpu_hotplug_lock -->
i_mutex_dir_key --> mm->mmap_lock.

Reported-by: Vipin Sharma <vipinsh@google.com>
Closes: https://lore.kernel.org/kvm/20260821193502.92431-1-vipinsh@google.com/
Fixes: f5b16b802174 ("PCI: Suspend iommu function prior to resetting a device")
Signed-off-by: David Matlack <dmatlack@google.com>
---
Cc: Alex Williamson <alex@shazbot.org>

 drivers/iommu/iommu.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index e8f13dcebbde..29e63697211f 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -56,6 +56,7 @@ struct iommu_group {
 	struct list_head devices;
 	struct xarray pasid_array;
 	struct mutex mutex;
+	struct mutex reset_mutex;
 	void *iommu_data;
 	void (*iommu_data_release)(void *iommu_data);
 	char *name;
@@ -1080,6 +1081,7 @@ struct iommu_group *iommu_group_alloc(void)
 
 	group->kobj.kset = iommu_group_kset;
 	mutex_init(&group->mutex);
+	mutex_init(&group->reset_mutex);
 	INIT_LIST_HEAD(&group->devices);
 	INIT_LIST_HEAD(&group->entry);
 	xa_init(&group->pasid_array);
@@ -2476,6 +2478,7 @@ static int __iommu_group_set_domain_internal(struct iommu_group *group,
 	 * pci_dev_reset_iommu_done() attaches the device to group->domain, if
 	 * IOMMU_SET_DOMAIN_MUST_SUCCEED is not set.
 	 */
+	guard(mutex)(&group->reset_mutex);
 	if (group->recovery_cnt && !(flags & IOMMU_SET_DOMAIN_MUST_SUCCEED))
 		return -EBUSY;
 
@@ -3652,6 +3655,7 @@ int iommu_attach_device_pasid(struct iommu_domain *domain,
 	 * This is a concurrent attach during device recovery. Reject it until
 	 * pci_dev_reset_iommu_done() attaches the device to group->domain.
 	 */
+	guard(mutex)(&group->reset_mutex);
 	if (group->recovery_cnt) {
 		ret = -EBUSY;
 		goto out_unlock;
@@ -3745,6 +3749,7 @@ int iommu_replace_device_pasid(struct iommu_domain *domain,
 	 * This is a concurrent attach during device recovery. Reject it until
 	 * pci_dev_reset_iommu_done() attaches the device to group->domain.
 	 */
+	guard(mutex)(&group->reset_mutex);
 	if (group->recovery_cnt) {
 		ret = -EBUSY;
 		goto out_unlock;
@@ -4042,7 +4047,7 @@ int pci_dev_reset_iommu_prepare(struct pci_dev *pdev)
 	if (!pci_ats_supported(pdev) || !dev_has_iommu(&pdev->dev))
 		return 0;
 
-	guard(mutex)(&group->mutex);
+	guard(mutex)(&group->reset_mutex);
 
 	gdev = __dev_to_gdev(&pdev->dev);
 	if (WARN_ON(!gdev))
@@ -4153,7 +4158,7 @@ void pci_dev_reset_iommu_done(struct pci_dev *pdev)
 	if (!pci_ats_supported(pdev) || !dev_has_iommu(&pdev->dev))
 		return;
 
-	guard(mutex)(&group->mutex);
+	guard(mutex)(&group->reset_mutex);
 
 	gdev = __dev_to_gdev(&pdev->dev);
 	if (WARN_ON(!gdev))

base-commit: 37ffa24c9d07edcd414d34283e02af3f3866cf12
-- 
2.55.0.897.gb25b4bd76c-goog