Re: [PATCH rdma-rc] RDMA/odp: Ensure the mm is still alive before creating an implicit child

[Jason Gunthorpe <jgg@xxxxxxxx>]

On Thu, Feb 27, 2020 at 01:41:18PM +0200, Leon Romanovsky wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> 
> Registration of a mmu_notifier requires the caller to hold a mmget() on
> the mm as registration is not permitted to race with exit_mmap(). There is
> a BUG_ON inside the mmu_notifier to guard against this.
> 
> Normally creating a umem is done against current which implicitly holds
> the mmget(), however an implicit ODP child is created from a pagefault
> work queue and is not guaranteed to have a mmget().
> 
> Call mmget() around this registration and abort faulting if the MM has
> gone to exit_mmap().
> 
> Before the patch below the notifier was registered when the implicit ODP
> parent was created, so there was no chance to register a notifier outside
> of current.
> 
> Fixes: c571feca2dc9 ("RDMA/odp: use mmu_notifier_get/put for 'struct ib_ucontext_per_mm'")
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>
> ---
>  drivers/infiniband/core/umem_odp.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
 
> diff --git a/drivers/infiniband/core/umem_odp.c b/drivers/infiniband/core/umem_odp.c
> index b8c657b28380..168f4f260c23 100644
> --- a/drivers/infiniband/core/umem_odp.c
> +++ b/drivers/infiniband/core/umem_odp.c
> @@ -181,14 +181,27 @@ ib_umem_odp_alloc_child(struct ib_umem_odp *root, unsigned long addr,
>  	odp_data->page_shift = PAGE_SHIFT;
>  	odp_data->notifier.ops = ops;
>  
> +	/*
> +	 * A mmget must be held when registering a notifier, the owming_mm only
> +	 * has a mm_grab at this point.
> +	 */
> +	if (!mmget_not_zero(umem->owning_mm)) {
> +		ret = -EFAULT;
> +		goto out_free;
> +	}
> +
>  	odp_data->tgid = get_pid(root->tgid);
>  	ret = ib_init_umem_odp(odp_data, ops);
> -	if (ret) {
> -		put_pid(odp_data->tgid);

This put_pid got lost, I put it back before applying to for-rc:

diff --git a/drivers/infiniband/core/umem_odp.c b/drivers/infiniband/core/umem_odp.c
index b8c657b2838048..cd656ad4953bfc 100644
--- a/drivers/infiniband/core/umem_odp.c
+++ b/drivers/infiniband/core/umem_odp.c
@@ -181,14 +181,28 @@ ib_umem_odp_alloc_child(struct ib_umem_odp *root, unsigned long addr,
 	odp_data->page_shift = PAGE_SHIFT;
 	odp_data->notifier.ops = ops;
 
+	/*
+	 * A mmget must be held when registering a notifier, the owming_mm only
+	 * has a mm_grab at this point.
+	 */
+	if (!mmget_not_zero(umem->owning_mm)) {
+		ret = -EFAULT;
+		goto out_free;
+	}
+
 	odp_data->tgid = get_pid(root->tgid);
 	ret = ib_init_umem_odp(odp_data, ops);
-	if (ret) {
-		put_pid(odp_data->tgid);
-		kfree(odp_data);
-		return ERR_PTR(ret);
-	}
+	if (ret)
+		goto out_tgid;
+	mmput(umem->owning_mm);
 	return odp_data;
+
+out_tgid:
+	put_pid(odp_data->tgid);
+	mmput(umem->owning_mm);
+out_free:
+	kfree(odp_data);
+	return ERR_PTR(ret);
 }
 EXPORT_SYMBOL(ib_umem_odp_alloc_child);