In the Linux kernel, the following vulnerability has been resolved: nfsd: hold rcu across...

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

nfsd: hold rcu across localio cmpxchg retry

nfsd_file objects are freed via call_rcu (filecache.c:296), and nfsd_file_slab is created without SLAB_TYPESAFE_BY_RCU (KMEM_CACHE(nfsd_file, 0) at filecache.c:789), so the slab page backing a freed nfsd_file becomes freely reclaimable once the RCU grace period elapses.

The again: retry block in nfsd_open_local_fh() loads a pointer with cmpxchg and then calls nfsd_file_get(new) (which is refcount_inc_not_zero) without holding rcu_read_lock. The sole caller nfs_open_local_fh() drops rcu_read_lock before invoking this helper, so no outer reader-side critical section covers the load.

CPU 0 (nfsd_open_local_fh)        CPU 1 (nfsd_file_put_local)
-----                             -----
new = cmpxchg(pnf, NULL, ...)
                                  nf = xchg(pnf, NULL)
                                  nfsd_file_put(nf)
                                    last ref -> call_rcu()
                                  /* grace period elapses;
                                     slab page recycled */
nfsd_file_get(new)
  refcount_inc_not_zero(&new->nf_ref)
  /* operates on recycled memory */

A non-zero word at the nf_ref offset of the recycled object makes the refcount bump appear to succeed, and the caller then dereferences new->nf_net and new->nf_file out of freed memory.

Fix by taking rcu_read_lock() immediately before the cmpxchg and releasing it on all three exits of the if (new) block: the goto-again retry, the lost-race cleanup path, and the install-succeeded path. nfsd_file_put() and nfsd_net_put() stay outside the RCU section so they remain free to block.