Linux kernel nfsd: unserialized grace_ended flag lets two contexts double-free every client reclaim record

Impact

nfsd4_end_grace() guarded its drain path with a plain boolean read followed by a write, with nothing serializing the two. The laundromat worker and a RECLAIM_COMPLETE compound can both observe the flag clear, both set it, and both enter nfs4_release_reclaim(), which walks reclaim_str_hashtbl unlocked and list_del + kfree's every entry - list corruption and a double free of every nfs4_client_reclaim, with a concurrent lookup reading freed memory. The window is server restart plus reclaim, so it is reachable by NFS clients reconnecting after the server comes back. On a GPU cluster this matters where nodes export or re-export NFS for datasets and checkpoints: the failure mode is a kernel crash on the file server, which stalls every job holding that mount. The fix converts the flags to an unsigned long word and uses test_and_set_bit().

Who can reach it

Remote NFS clients performing reclaim against an in-kernel nfsd server during the grace period after a restart. No authentication beyond what the export already requires; it is a race, so it is not reliably triggerable on demand.

What to do

Update to a stable kernel with the linked commits and reboot the NFS servers - a reboot per file server, which means an outage window for every client holding those mounts. No runtime mitigation; the race only opens around nfsd grace-period end, so minimizing nfsd restarts reduces exposure without removing it.

References

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