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@xxxxxxxxxxxxxxx > > > Signed-off-by: Calum Mackay <calum.mackay@xxxxxxxxxx> > > > --- > > > 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@xxxxxxxxxx, trond.myklebust@xxxxxxxxxxxxxxx
