[PATCH 6.1.y] nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke

[Sasha Levin <sashal@kernel.org>]

From: Jeff Layton <jlayton@kernel.org>

[ Upstream commit ca94ba36172046be6a694a7986f6931e47ed4d51 ]

nfsd4_alloc_layout_stateid reads fp->fi_deleg_file without holding
fi_lock when the parent stateid is a delegation. A concurrent delegation
revoke via the laundromat can clear fi_deleg_file under fi_lock, causing
nfsd_file_get() to return NULL and triggering the BUG_ON.

This race is client-reachable: two NFS clients can trigger it by having
one hold a delegation while another opens the same file to force a
recall. When the first client doesn't respond to the recall, the
laundromat revokes it. A concurrent LAYOUTGET from any client using the
delegation stateid hits the race window.

Fix this by taking fi_lock around the fi_deleg_file read in the
SC_TYPE_DELEG path, matching the locking discipline of the
find_any_file() arm, and replacing the BUG_ON with a graceful error
return that cleans up the partially-initialized layout stateid.

Fixes: c5c707f96fc9 ("nfsd: implement pNFS layout recalls")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-1-f27e8eb4d974@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/nfsd/nfs4layouts.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/fs/nfsd/nfs4layouts.c b/fs/nfsd/nfs4layouts.c
index 84bb200e24ade..03c8e71d1974a 100644
--- a/fs/nfsd/nfs4layouts.c
+++ b/fs/nfsd/nfs4layouts.c
@@ -168,9 +168,12 @@ nfsd4_free_layout_stateid(struct nfs4_stid *stid)
 	list_del_init(&ls->ls_perfile);
 	spin_unlock(&fp->fi_lock);
 
-	if (!nfsd4_layout_ops[ls->ls_layout_type]->disable_recalls)
-		vfs_setlease(ls->ls_file->nf_file, F_UNLCK, NULL, (void **)&ls);
-	nfsd_file_put(ls->ls_file);
+	if (ls->ls_file) {
+		if (!nfsd4_layout_ops[ls->ls_layout_type]->disable_recalls)
+			vfs_setlease(ls->ls_file->nf_file, F_UNLCK, NULL,
+				     (void **)&ls);
+		nfsd_file_put(ls->ls_file);
+	}
 
 	if (ls->ls_recalled)
 		atomic_dec(&ls->ls_stid.sc_file->fi_lo_recalls);
@@ -235,11 +238,17 @@ nfsd4_alloc_layout_stateid(struct nfsd4_compound_state *cstate,
 	nfsd4_init_cb(&ls->ls_recall, clp, &nfsd4_cb_layout_ops,
 			NFSPROC4_CLNT_CB_LAYOUT);
 
-	if (parent->sc_type == NFS4_DELEG_STID)
+	if (parent->sc_type == NFS4_DELEG_STID) {
+		spin_lock(&fp->fi_lock);
 		ls->ls_file = nfsd_file_get(fp->fi_deleg_file);
-	else
+		spin_unlock(&fp->fi_lock);
+	} else {
 		ls->ls_file = find_any_file(fp);
-	BUG_ON(!ls->ls_file);
+	}
+	if (!ls->ls_file) {
+		nfs4_put_stid(stp);
+		return NULL;
+	}
 
 	if (nfsd4_layout_setlease(ls)) {
 		nfs4_put_stid(stp);
-- 
2.53.0