Re: [PATCH v1 1/2] NFS: fix RCU safety in nfs_compare_super_address

[Benjamin Coddington]

On 8 Apr 2026, at 12:14, Sean Chang wrote:

> The cl_xprt pointer in struct rpc_clnt is marked as __rcu. Accessing
> it directly in nfs_compare_super_address() without RCU protection is
> unsafe and triggers Sparse warnings about dereferencing noderef
> expressions.
>
> Fix this by wrapping the access with rcu_read_lock() and using
> rcu_dereference() to safely retrieve the transport pointer. This
> ensures the xprt remains valid during the comparison of network
> namespaces and addresses, preventing potential use-after-free during
> concurrent transport updates.
>
> Signed-off-by: Sean Chang <seanwascoding@xxxxxxxxx>

Fixes: 7e3fcf61abde ("nfs: don't share mounts between network namespaces")

> ---
>  fs/nfs/super.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> index 7a318581f85b..071337f9ea37 100644
> --- a/fs/nfs/super.c
> +++ b/fs/nfs/super.c
> @@ -1166,43 +1166,55 @@ static int nfs_set_super(struct super_block *s, struct fs_context *fc)
>  static int nfs_compare_super_address(struct nfs_server *server1,
>  				     struct nfs_server *server2)
>  {
> +	struct rpc_xprt *xprt1, *xprt2;
>  	struct sockaddr *sap1, *sap2;
> -	struct rpc_xprt *xprt1 = server1->client->cl_xprt;
> -	struct rpc_xprt *xprt2 = server2->client->cl_xprt;
> +	int ret = 0;
> +
> +	rcu_read_lock();
> +
> +	xprt1 = rcu_dereference(server1->client->cl_xprt);
> +	xprt2 = rcu_dereference(server2->client->cl_xprt);
> +
> +	if (!xprt1 || !xprt2)
> +		goto out;

^^ I'm not sure that's a great test, the rpc_xprt objects are refcounted.
These might not be null but you could still race with a freeing path?

However, I think you might just be safe inside the RCU section here because
rpc_switch_client_transport() uses synchronize_rcu() before xprt_put(old).
I didn't audit all the freeing paths.

>  	if (!net_eq(xprt1->xprt_net, xprt2->xprt_net))
> -		return 0;
> +		goto out;

Probably safe to drop the RCU protection scope after this point.  No need to
hold it over all the other checks..

Ben

>
>  	sap1 = (struct sockaddr *)&server1->nfs_client->cl_addr;
>  	sap2 = (struct sockaddr *)&server2->nfs_client->cl_addr;
>
>  	if (sap1->sa_family != sap2->sa_family)
> -		return 0;
> +		goto out;
>
>  	switch (sap1->sa_family) {
>  	case AF_INET: {
>  		struct sockaddr_in *sin1 = (struct sockaddr_in *)sap1;
>  		struct sockaddr_in *sin2 = (struct sockaddr_in *)sap2;
>  		if (sin1->sin_addr.s_addr != sin2->sin_addr.s_addr)
> -			return 0;
> +			goto out;
>  		if (sin1->sin_port != sin2->sin_port)
> -			return 0;
> +			goto out;
>  		break;
>  	}
>  	case AF_INET6: {
>  		struct sockaddr_in6 *sin1 = (struct sockaddr_in6 *)sap1;
>  		struct sockaddr_in6 *sin2 = (struct sockaddr_in6 *)sap2;
>  		if (!ipv6_addr_equal(&sin1->sin6_addr, &sin2->sin6_addr))
> -			return 0;
> +			goto out;
>  		if (sin1->sin6_port != sin2->sin6_port)
> -			return 0;
> +			goto out;
>  		break;
>  	}
>  	default:
> -		return 0;
> +		goto out;
>  	}
>
> -	return 1;
> +	ret = 1;
> +
> +out:
> +	rcu_read_unlock();
> +	return ret;
>  }
>
>  static int nfs_compare_userns(const struct nfs_server *old,
> -- 
> 2.34.1