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

[Calum Mackay <calum.mackay@oracle.com>]

On 28/08/2025 1:04 am, Trond Myklebust wrote:
> 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.

thanks Trond, I'll keep digging.

cheers,
c.