iommufd: Fix iopt_access_list_id overwrite bug

Syzkaller reported the following WARN_ON:
  WARNING: CPU: 1 PID: 4738 at drivers/iommu/iommufd/io_pagetable.c:1360

  Call Trace:
   iommufd_access_change_ioas+0x2fe/0x4e0
   iommufd_access_destroy_object+0x50/0xb0
   iommufd_object_remove+0x2a3/0x490
   iommufd_object_destroy_user
   iommufd_access_destroy+0x71/0xb0
   iommufd_test_staccess_release+0x89/0xd0
   __fput+0x272/0xb50
   __fput_sync+0x4b/0x60
   __do_sys_close
   __se_sys_close
   __x64_sys_close+0x8b/0x110
   do_syscall_x64

The mismatch between the access pointer in the list and the passed-in
pointer is resulting from an overwrite of access->iopt_access_list_id, in
iopt_add_access(). Called from iommufd_access_change_ioas() when
xa_alloc() succeeds but iopt_calculate_iova_alignment() fails.

Add a new_id in iopt_add_access() and only update iopt_access_list_id when
returning successfully.

Cc: stable@vger.kernel.org
Fixes: 9227da7 ("iommufd: Add iommufd_access_change_ioas(_id) helpers")
Link: https://lore.kernel.org/r/2dda7acb25b8562ec5f1310de828ef5da9ef509c.1708636627.git.nicolinc@nvidia.com
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/io_pagetable.c

@@ -1330,20 +1330,23 @@ int iopt_disable_large_pages(struct io_pagetable *iopt)
 
 int iopt_add_access(struct io_pagetable *iopt, struct iommufd_access *access)
 {
+	u32 new_id;
 	int rc;
 
 	down_write(&iopt->domains_rwsem);
 	down_write(&iopt->iova_rwsem);
-	rc = xa_alloc(&iopt->access_list, &access->iopt_access_list_id, access,
-		      xa_limit_16b, GFP_KERNEL_ACCOUNT);
+	rc = xa_alloc(&iopt->access_list, &new_id, access, xa_limit_16b,
+		      GFP_KERNEL_ACCOUNT);
+
 	if (rc)
 		goto out_unlock;
 
 	rc = iopt_calculate_iova_alignment(iopt);
 	if (rc) {
-		xa_erase(&iopt->access_list, access->iopt_access_list_id);
+		xa_erase(&iopt->access_list, new_id);
 		goto out_unlock;
 	}
+	access->iopt_access_list_id = new_id;
 
 out_unlock:
 	up_write(&iopt->iova_rwsem);