Overview
About vulnerability
In the Linux kernel, the following vulnerability has been resolved:
NFSD: Prevent client use-after-free during admin state revocation
A stateid holds only a bare pointer to its nfs4_client; a stateid reference does not pin it. The client survives only because __destroy_client() drains its stateids before free_client() runs.
nfsd4_revoke_states() drops nn->client_lock across revoke_one_stid(), which dereferences the client to revoke a stateid and read clp->cl_minorversion. A teardown racing the dropped lock can free the client first.
Pinning cl_rpc_users under client_lock blocks the DESTROY_CLIENTID and EXCHANGE_ID teardown, which refuses while cl_rpc_users is non-zero. force_expire_client() ignores it: once its wait for cl_rpc_users to reach zero has passed, a later pin goes unnoticed.
Under client_lock, skip a client whose cl_time is already zero – force_expire_client() clears it there before waiting – otherwise pin cl_rpc_users before dropping the lock. The walk then either sees the expiry and skips, or pins in time for that wait to cover the revoke.
Details
- Affected product:
- Debian 10 ELS , Debian 11 ELS
- Affected packages:
- linux @ 4.19.0 (+1 more)