CVE-2026-89540
massUninitialized-mutex race in Linux kernel sunrpc use-gss-proxy proc entry
The Linux kernel's sunrpc code publishes /proc/net/rpc/use-gss-proxy via proc_create_data() before it initializes the sn->gssp_lock mutex, so a local user who writes to the freshly created proc file in that window locks a zero-initialized struct mutex. The window is narrow (only between proc_create_data() returning and init_gssp_clnt() running) but widens on auth_rpcgss module load, when the entry is created for every live network namespace whose tasks are already running. A winning writer takes the lock via the CMPXCHG fast path on a zeroed lock, allowing a second concurrent writer to enter set_gssp_clnt(), shut down an RPC client that is still in use (use-after-free-style corruption or crash), and leak the losing writer's client; debug kernels instead emit a 'lock used without init' splat. Any system that loads sunrpc/auth_rpcgss — typically NFS servers and clients using Kerberos (RPCSEC_GSS) or gss-proxy — and hosts untrusted local accounts is exposed, with CVSS 3.1 of 7.8 (local vector, C:H/I:H/A:H). No public PoC exists and the issue is not in CISA's KEV, so no exploitation is known.
What to do: Install a kernel containing the fix that initializes gssp_lock in sunrpc_init_net() — track your distribution's advisory for CVE-2026-89540 rather than a specific version number. Until patched, check with lsmod whether sunrpc/auth_rpcgss is loaded on NFS/Kerberos hosts, restrict untrusted local accounts there, and consider blocking auto-load of auth_rpcgss if RPCSEC_GSS is not needed. On CONFIG_DEBUG_MUTEXES kernels, any 'lock used without init' splat referencing gssp_lock is evidence the race was hit and warrants investigation.
| Linux kernel (net/sunrpc, auth_rpcgss) | — |
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
In the Linux kernel, the following vulnerability has been resolved: sunrpc: init gssp_lock before publishing proc entry create_use_gss_proxy_proc_entry() publishes /proc/net/rpc/use-gss-proxy via proc_create_data() before init_gssp_clnt() runs mutex_init() on sn->gssp_lock. Once the dentry is linked under proc_subdir_lock it is immediately reachable from userspace, so a write that lands in the window drives set_gssp_clnt() into mutex_lock() on a zero-initialized struct mutex. create_use_gss_proxy_proc_entry(net) proc_create_data("use-gss-proxy", ...) /* dentry live */ init_gssp_clnt(sn) mutex_init(&sn->gssp_lock) /* too late */ write_gssp() set_gssp_clnt(net) mutex_lock(&sn->gssp_lock) /* uninitialized */ gssp_rpc_create(...) sn->gssp_clnt = clnt mutex_unlock(&sn->gssp_lock) The window spans only the two statements between proc_create_data() returning and init_gssp_clnt(), so a writer reaches it only if the registering thread is preempted there while another task is already opening the freshly published file. register_pernet_subsys() runs in preemptible context under pernet_ops_rwsem, so that preemption is possible, and the window widens on auth_rpcgss module load, when the proc entry is created for every live net namespace whose tasks are already running. A writer that wins the race locks a zero-filled struct mutex. On CONFIG_DEBUG_MUTEXES the missing magic value trips a "lock used without init" splat; on a production kernel the fast path acquires the lock via CMPXCHG(owner, 0, current). In the latter case a second writer that arrives before init_gssp_clnt() re-zeroes owner can enter set_gssp_clnt() concurrently, shut down the first writer's clnt while it is still in use, and leak the loser's clnt. Fix by initializing sn->gssp_lock in sunrpc_init_net() so its lifetime matches the sunrpc_net it lives in. sn->gssp_clnt is already NULL from the kzalloc that backs net_generic storage, so the lazy helper is no longer needed; drop init_gssp_clnt(), its prototype, and the call from create_use_gss_proxy_proc_entry(). sunrpc.ko is a build-time dependency of auth_rpcgss.ko, so sunrpc_init_net() has always run on every netns before any auth_gss pernet init can publish the proc entry.
- Vector
- CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
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