Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

nfsd: convert nfsd_net boolean flags to unsigned long flags word

nfsd_net contains several boolean fields that are accessed from concurrent contexts without serialization. In particular, nfsd4_end_grace() guards its drain path with a plain bool:

if (nn->grace_ended) return; nn->grace_ended = true;

The read and the write are independent, and nothing in struct nfsd_net serializes them. At least two contexts can reach this code with no lock held:

laundromat path laundry_wq kworker nfs4_laundromat() nfsd4_end_grace()

RECLAIM_COMPLETE path nfsd compound kthread nfsd4_reclaim_complete() inc_reclaim_complete() nfsd4_end_grace()

Both callers can observe grace_ended == false on different CPUs, both store true, and both proceed into nfsd4_record_grace_done(), which invokes the active client_tracking_ops->grace_done callback. For tracking ops that drain reclaim_str_hashtbl (legacy_tracking_ops via nfsd4_recdir_purge_old, and the cld v1+ ops via nfsd4_cld_grace_done), grace_done calls nfs4_release_reclaim(), which walks every bucket of reclaim_str_hashtbl with no lock and calls nfs4_remove_reclaim_record() (list_del + kfree) on each entry. Two concurrent walkers corrupt the list and double-free every nfs4_client_reclaim. A concurrent nfsd4_find_reclaim_client() iterating the same bucket reads through freed memory.

A third…