[PATCH v2] drm/amdgpu: fix PASID task_info lookup race

[Fan Wu <fanwu01@zju.edu.cn>]

amdgpu_vm_get_task_info_pasid() currently looks up the VM from the
PASID xarray, drops the xarray lock, and only then grabs the task_info
reference through amdgpu_vm_get_task_info_vm().

Take the task_info reference directly while holding the PASID xarray
lock instead. This keeps the lookup and reference acquisition in the
same critical section.

The task_info is dropped only after the VM is removed from the PASID
mapping, so a regular kref_get() is sufficient here.

Also remove the now unnecessary amdgpu_vm_get_vm_from_pasid() helper.

Fixes: b8f67b9ddf4f ("drm/amdgpu: change vm->task_info handling")
Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
---
v2:
- grab the task_info reference directly under the PASID xarray lock
- drop the extra local-variable comment
- use kref_get() instead of kref_get_unless_zero()
- simplify the changelog

 drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 35 +++++++++++++++-----------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
index f2beb980e3c3..706ca8dd65d2 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
@@ -2468,19 +2468,6 @@ static void amdgpu_vm_destroy_task_info(struct kref *kref)
 	kfree(ti);
 }
 
-static inline struct amdgpu_vm *
-amdgpu_vm_get_vm_from_pasid(struct amdgpu_device *adev, u32 pasid)
-{
-	struct amdgpu_vm *vm;
-	unsigned long flags;
-
-	xa_lock_irqsave(&adev->vm_manager.pasids, flags);
-	vm = xa_load(&adev->vm_manager.pasids, pasid);
-	xa_unlock_irqrestore(&adev->vm_manager.pasids, flags);
-
-	return vm;
-}
-
 /**
  * amdgpu_vm_put_task_info - reference down the vm task_info ptr
  *
@@ -2527,8 +2514,26 @@ amdgpu_vm_get_task_info_vm(struct amdgpu_vm *vm)
 struct amdgpu_task_info *
 amdgpu_vm_get_task_info_pasid(struct amdgpu_device *adev, u32 pasid)
 {
-	return amdgpu_vm_get_task_info_vm(
-			amdgpu_vm_get_vm_from_pasid(adev, pasid));
+	struct amdgpu_vm *vm;
+	unsigned long flags;
+	struct amdgpu_task_info *ti = NULL;
+
+	/*
+	 * Acquire the task_info reference while holding the pasids xarray
+	 * lock to prevent a race with amdgpu_vm_fini() which removes the
+	 * PASID mapping before freeing the VM (embedded in struct amdgpu_fpriv).
+	 * Without this, the VM could be freed between xa_load() return and
+	 * the task_info dereference.
+	 */
+	xa_lock_irqsave(&adev->vm_manager.pasids, flags);
+	vm = xa_load(&adev->vm_manager.pasids, pasid);
+	if (vm && vm->task_info) {
+		ti = vm->task_info;
+		kref_get(&ti->refcount);
+	}
+	xa_unlock_irqrestore(&adev->vm_manager.pasids, flags);
+
+	return ti;
 }
 
 static int amdgpu_vm_create_task_info(struct amdgpu_vm *vm)
-- 
2.34.1