[PATCH 2/8] nfsd: hold rcu across localio cmpxchg retry

[Jeff Layton <jlayton@kernel.org>]

From: Chris Mason <clm@meta.com>

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.

Fixes: e6f7e1487ab5 ("nfs_localio: simplify interface to nfsd for getting nfsd_file")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
---
 fs/nfsd/localio.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/localio.c b/fs/nfsd/localio.c
index be710d809a3b..c3eb0557b3e1 100644
--- a/fs/nfsd/localio.c
+++ b/fs/nfsd/localio.c
@@ -97,11 +97,15 @@ nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
 		}
 		nfsd_file_get(localio);
 	again:
+		rcu_read_lock();
 		new = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(localio)));
 		if (new) {
 			/* Some other thread installed an nfsd_file */
-			if (nfsd_file_get(new) == NULL)
+			if (nfsd_file_get(new) == NULL) {
+				rcu_read_unlock();
 				goto again;
+			}
+			rcu_read_unlock();
 			/*
 			 * Drop the ref we were going to install (both file and
 			 * net) and the one we were going to return (only file).
@@ -110,6 +114,8 @@ nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
 			nfsd_net_put(net);
 			nfsd_file_put(localio);
 			localio = new;
+		} else {
+			rcu_read_unlock();
 		}
 	} else
 		nfsd_net_put(net);

-- 
2.54.0