Re: [PATCH] sunrpc: hold RPC client while gss_auth has a link to it

[Trond Myklebust <trondmy@kernel.org>]

On Thu, 2025-08-28 at 00:48 +0100, Calum Mackay wrote:
> On 28/08/2025 12:33 am, Trond Myklebust wrote:
> > On Wed, 2025-08-27 at 23:38 +0100, Calum Mackay wrote:
> > > We have seen crashes where the RPC client has been freed, while a
> > > gss_auth
> > > still has a pointer to it. Subsequently, GSS attempts to send a
> > > NULL
> > > call,
> > > resulting in a use-after-free in xprt_iter_get_next.
> > > 
> > > Fix that by having GSS take a reference on the RPC client, in
> > > gss_create_new, and release it in gss_free.
> > > 
> > > The crashes we've seen have been on a v5.4-based kernel. They are
> > > not
> > > reproducible on demand, happening once every few months under
> > > heavy
> > > load.
> > > 
> > > The crashing thread is an rpc_async_release worker:
> > > 
> > > xprt_iter_ops (net/sunrpc/xprtmultipath.c:184:43)
> > > xprt_iter_get_next (net/sunrpc/xprtmultipath.c:527:35)
> > > rpc_task_get_next_xprt (net/sunrpc/clnt.c:1062:9)
> > > rpc_task_set_transport (net/sunrpc/clnt.c:1073:19)
> > > rpc_task_set_transport (net/sunrpc/clnt.c:1066:6)
> > > rpc_task_set_client (net/sunrpc/clnt.c:1081:3)
> > > rpc_run_task (net/sunrpc/clnt.c:1133:2)
> > > rpc_call_null_helper (net/sunrpc/clnt.c:2766:9)
> > > rpc_call_null (net/sunrpc/clnt.c:2771:9)
> > > gss_send_destroy_context (net/sunrpc/auth_gss/auth_gss.c:1274:10)
> > > gss_destroy_cred (net/sunrpc/auth_gss/auth_gss.c:1341:3)
> > > put_rpccred (net/sunrpc/auth.c:758:2)
> > > put_rpccred (net/sunrpc/auth.c:733:1)
> > > __put_nfs_open_context (fs/nfs/inode.c:1010:2)
> > > put_nfs_open_context (fs/nfs/inode.c:1017:2)
> > > nfs_pgio_data_destroy (fs/nfs/pagelist.c:562:3)
> > > nfs_pgio_header_free (fs/nfs/pagelist.c:573:2)
> > > nfs_read_completion (fs/nfs/read.c:200:2)
> > > nfs_pgio_release (fs/nfs/pagelist.c:699:2)
> > > rpc_release_calldata (net/sunrpc/sched.c:890:3)
> > > rpc_free_task (net/sunrpc/sched.c:1171:2)
> > > rpc_async_release (net/sunrpc/sched.c:1183:2)
> > > process_one_work (kernel/workqueue.c:2295:2)
> > > worker_thread (kernel/workqueue.c:2448:4)
> > > kthread (kernel/kthread.c:296:9)
> > > ret_from_fork+0x2b/0x36 (arch/x86/entry/entry_64.S:358)
> > > 
> > > Immediately before __put_nfs_open_context calls put_rpccred,
> > > above,
> > > it calls nfs_sb_deactive, which frees the RPC client:
> > > 
> > > rpc_free_client+189 [sunrpc]
> > > rpc_release_client+98 [sunrpc]
> > > rpc_shutdown_client+98 [sunrpc]
> > > nfs_free_client+123 [nfs]
> > > nfs_put_client+217 [nfs]
> > > nfs_free_server+81 [nfs]
> > > nfs_kill_super+49 [nfs]
> > > deactivate_locked_super+58
> > > deactivate_super+89
> > > nfs_sb_deactive+36 [nfs]
> > > 
> > > After the RPC client is freed here, __put_nfs_open_context calls
> > > put_rpccred, which wants to destroy the cred, since its refcnt is
> > > now
> > > zero. Since the cred is not marked as UPTODATE,
> > > gss_send_destroy_context
> > > needs to send a NULL call to the server, to get it to release all
> > > state
> > > for this context.  For this NULL call, we need an RPC client with
> > > an
> > > associated xprt; whilst looking for one, we trip over the freed
> > > xpi,
> > > that we freed earlier from nfs_sb_deactive.
> > > 
> > > Ensuring that the RPC client refcnt is incremented whilst
> > > gss_auth
> > > has a
> > > pointer to it would ensure that the client can't be freed until
> > > gss_auth
> > > has been freed.
> > > 
> > > Whilst the above occurred on a v5.4 kernel, I don't see anything
> > > obvious
> > > that would stop this happening to current code.
> > > 
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Calum Mackay <calum.mackay@oracle.com>
> > > ---
> > >   net/sunrpc/auth_gss/auth_gss.c | 16 +++++++++++++++-
> > >   1 file changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/net/sunrpc/auth_gss/auth_gss.c
> > > b/net/sunrpc/auth_gss/auth_gss.c
> > > index 5c095cb8cb20..8c2cc92c6cd6 100644
> > > --- a/net/sunrpc/auth_gss/auth_gss.c
> > > +++ b/net/sunrpc/auth_gss/auth_gss.c
> > > @@ -1026,6 +1026,13 @@ gss_create_new(const struct
> > > rpc_auth_create_args *args, struct rpc_clnt *clnt)
> > >   
> > >   	if (!try_module_get(THIS_MODULE))
> > >   		return ERR_PTR(err);
> > > +
> > > +	/*
> > > +	 * Make sure the RPC client can't be freed while
> > > gss_auth
> > > has
> > > +	 * a link to it
> > > +	 */
> > > +	refcount_inc(&clnt->cl_count);
> > > +
> > 
> > NACK. We can't have the client hold a reference to the auth_gss
> > struct
> > and then have the same auth_gss hold a reference back to the
> > client.
> > That's a guaranteed leak of both objects.
> 
> Thanks Trond.
> 
> Would an acceptable alternative be for gss_send_destroy_context to 
> simply not attempt the NULL RPC call if gss_auth->client has already 
> been freed?
> 

The expectation is normally that if something depends on the gss_auth,
then it should be holding a reference to the gss_auth->client (and not
necessarily to the gss_auth itself). Normally, this happens through the
rpc_clone_client() mechanism.

If there are corner cases where we're currently failing to grab a
reference to the gss_auth->client, then we want to understand how that
is falling through the cracks, so that we can fix it up.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com