Re: [PATCH v1] sunrpc: pin gss module across auth_domain RCU free

[Jeff Layton <jlayton@kernel.org>]

On Sun, 2026-09-13 at 12:07 -0400, Chuck Lever wrote:
> From: Chris Mason <clm@meta.com>
> 
> A gss auth_domain stores a raw pointer to svcauthops_gss in
> dom->flavour, and its release queues svcauth_gss_domain_release_rcu()
> via call_rcu(). Both live in auth_rpcgss.ko text, and nothing pins the
> module while a domain referencing it is reachable. rmmod auth_rpcgss
> can therefore unmap that text while an NFSD export still holds a
> reference to the domain.
> 
> The export cache is one such holder. When the export is finally
> released, the domain reference drops from module text:
> 
>     svc_export_put()
>       auth_domain_put(exp->ex_client)
>         auth_domain_release()
>           dom->flavour->domain_release(dom)   /* into module text */
>             svcauth_gss_domain_release()
>               call_rcu(&dom->rcu_head,
>                        svcauth_gss_domain_release_rcu)
> 
> Once auth_rpcgss is unloaded, the domain_release call lands in
> unmapped memory. The rcu_barrier() in exit_rpcsec_gss() does not
> help: it waits only for callbacks already queued, and this one is
> not queued until the export goes away.
> 
> NFSD's NFSv4 objects import symbols from auth_rpcgss.ko, so only a
> server built without CONFIG_NFSD_V4 can unload auth_rpcgss while
> exports still name a gss domain.
> 
> Take a module reference in svcauth_gss_register_pseudoflavor() before
> publishing the domain and drop it at the end of
> svcauth_gss_domain_release_rcu(). The rcu_barrier() then holds the
> module image until this callback has returned.
> 
> Fixes: 608a0ab2f54a ("SUNRPC: Add lockless lookup of the server's auth domain")
> Assisted-by: kres:claude-opus-4-7
> Signed-off-by: Chris Mason <clm@meta.com>
> [ cel: rewrote description ]
> Signed-off-by: Chuck Lever <cel@kernel.org>
> ---
>  net/sunrpc/auth_gss/svcauth_gss.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/sunrpc/auth_gss/svcauth_gss.c b/net/sunrpc/auth_gss/svcauth_gss.c
> index b319c4423d78..43839e805531 100644
> --- a/net/sunrpc/auth_gss/svcauth_gss.c
> +++ b/net/sunrpc/auth_gss/svcauth_gss.c
> @@ -824,12 +824,18 @@ svcauth_gss_register_pseudoflavor(u32 pseudoflavor, char * name)
>  	new->h.flavour = &svcauthops_gss;
>  	new->pseudoflavor = pseudoflavor;
>  
> +	if (!try_module_get(svcauthops_gss.owner)) {
> +		stat = -ENODEV;
> +		goto out_free_name;
> +	}
> +
>  	test = auth_domain_lookup(name, &new->h);
>  	if (test != &new->h) {
>  		pr_warn("svc: duplicate registration of gss pseudo flavour %s.\n",
>  			name);
>  		stat = -EADDRINUSE;
>  		auth_domain_put(test);
> +		module_put(svcauthops_gss.owner);
>  		goto out_free_name;
>  	}
>  	return test;
> @@ -2004,9 +2010,11 @@ svcauth_gss_domain_release_rcu(struct rcu_head *head)
>  {
>  	struct auth_domain *dom = container_of(head, struct auth_domain, rcu_head);
>  	struct gss_domain *gd = container_of(dom, struct gss_domain, h);
> +	struct module *owner = dom->flavour->owner;
>  
>  	kfree(dom->name);
>  	kfree(gd);
> +	module_put(owner);
>  }
>  
>  static void

Reviewed-by: Jeff Layton <jlayton@kernel.org>