In the Linux kernel, the following vulnerability has been resolved:
nfsd: guard nfsd_serv deref in nfsd_file_net_dispose
nfsd_file_net_dispose() is the consumer side of l->freeme: the nfsd
service thread loop calls it to drain entries that the filecache
garbage collector and shrinker append via
nfsd_file_dispose_list_delayed(). During per-net teardown,
nn->nfsd_serv is cleared before the filecache laundrette is shut
down, so the service thread can still run a dispose pass that finds
more than eight entries on l->freeme and dereferences a NULL
svc_serv:
nfsd service thread loop
nfsd_file_net_dispose(nn)
if (!list_empty(&l->freeme)) {
…
svc_wake_up(nn->nfsd_serv); /* nn->nfsd_serv == NULL */
}
The sibling helper nfsd_file_dispose_list_delayed() already documents
this ordering and caches nn->nfsd_serv into a local before testing it
for NULL. nfsd_file_net_dispose() was introduced with the same raw
svc_wake_up(nn->nfsd_serv) call and never picked up the guard.
Fix by loading nn->nfsd_serv into a local svc_serv pointer and only
calling svc_wake_up() when it is non-NULL, matching the pattern in
nfsd_file_dispose_list_delayed().
