RDMA/mlx5: Use different doorbell memory for different processes

In a fork scenario, the parent and child can have same virtual address and
also share the uverbs fd.  That causes to the list_for_each_entry search
return same doorbell physical page for all processes, even though that
page has been COW' or copied.

This patch takes the mm_struct into consideration during search, to make
sure that VA's belonging to different processes are not intermixed.

Resolves the malfunction of uverbs after fork in some specific cases.

Fixes: e126ba9 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Link: https://lore.kernel.org/r/feacc23fe0bc6e1088c6824d5583798745e72405.1622726212.git.leonro@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/doorbell.c

@@ -41,6 +41,7 @@ struct mlx5_ib_user_db_page {
 	struct ib_umem	       *umem;
 	unsigned long		user_virt;
 	int			refcnt;
+	struct mm_struct	*mm;
 };
 
 int mlx5_ib_db_map_user(struct mlx5_ib_ucontext *context,
@@ -53,7 +54,8 @@ int mlx5_ib_db_map_user(struct mlx5_ib_ucontext *context,
 	mutex_lock(&context->db_page_mutex);
 
 	list_for_each_entry(page, &context->db_page_list, list)
-		if (page->user_virt == (virt & PAGE_MASK))
+		if ((current->mm == page->mm) &&
+		    (page->user_virt == (virt & PAGE_MASK)))
 			goto found;
 
 	page = kmalloc(sizeof(*page), GFP_KERNEL);
@@ -71,6 +73,8 @@ int mlx5_ib_db_map_user(struct mlx5_ib_ucontext *context,
 		kfree(page);
 		goto out;
 	}
+	mmgrab(current->mm);
+	page->mm = current->mm;
 
 	list_add(&page->list, &context->db_page_list);
 
@@ -91,6 +95,7 @@ void mlx5_ib_db_unmap_user(struct mlx5_ib_ucontext *context, struct mlx5_db *db)
 
 	if (!--db->u.user_page->refcnt) {
 		list_del(&db->u.user_page->list);
+		mmdrop(db->u.user_page->mm);
 		ib_umem_release(db->u.user_page->umem);
 		kfree(db->u.user_page);
 	}