nfsd: fix heap overflow in NFSv4.0 LOCK replay cache

jira VULN-180161
cve CVE-2026-31402
commit-author Jeff Layton <jlayton@kernel.org>
commit 5133b61
upstream-diff Used `post_err_offset' instead of `op_status_offset +
  XDR_UNIT' in the `read_bytes_from_xdr_buf()' call, as the LTS 8.6
  version is missing ef3675b ("NFSD:
  Encode COMPOUND operation status on page boundaries")

The NFSv4.0 replay cache uses a fixed 112-byte inline buffer
(rp_ibuf[NFSD4_REPLAY_ISIZE]) to store encoded operation responses.
This size was calculated based on OPEN responses and does not account
for LOCK denied responses, which include the conflicting lock owner as
a variable-length field up to 1024 bytes (NFS4_OPAQUE_LIMIT).

When a LOCK operation is denied due to a conflict with an existing lock
that has a large owner, nfsd4_encode_operation() copies the full encoded
response into the undersized replay buffer via read_bytes_from_xdr_buf()
with no bounds check. This results in a slab-out-of-bounds write of up
to 944 bytes past the end of the buffer, corrupting adjacent heap memory.

This can be triggered remotely by an unauthenticated attacker with two
cooperating NFSv4.0 clients: one sets a lock with a large owner string,
then the other requests a conflicting lock to provoke the denial.

We could fix this by increasing NFSD4_REPLAY_ISIZE to allow for a full
opaque, but that would increase the size of every stateowner, when most
lockowners are not that large.

Instead, fix this by checking the encoded response length against
NFSD4_REPLAY_ISIZE before copying into the replay buffer. If the
response is too large, set rp_buflen to 0 to skip caching the replay
payload. The status is still cached, and the client already received the
correct response on the original request.

Fixes: 1da177e ("Linux-2.6.12-rc2")
	Cc: stable@kernel.org
	Reported-by: Nicholas Carlini <npc@anthropic.com>
	Tested-by: Nicholas Carlini <npc@anthropic.com>
	Signed-off-by: Jeff Layton <jlayton@kernel.org>
	Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
(cherry picked from commit 5133b61)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

fs/nfsd/nfs4xdr.c

@@ -4942,9 +4942,14 @@ nfsd4_encode_operation(struct nfsd4_compoundres *resp, struct nfsd4_op *op)
 		int len = xdr->buf->len - post_err_offset;
 
 		so->so_replay.rp_status = op->status;
-		so->so_replay.rp_buflen = len;
-		read_bytes_from_xdr_buf(xdr->buf, post_err_offset,
+		if (len <= NFSD4_REPLAY_ISIZE) {
+			so->so_replay.rp_buflen = len;
+			read_bytes_from_xdr_buf(xdr->buf,
+						post_err_offset,
 						so->so_replay.rp_buf, len);
+		} else {
+			so->so_replay.rp_buflen = 0;
+		}
 	}
 status:
 	/* Note that op->status is already in network byte order: */

fs/nfsd/state.h

@@ -379,11 +379,18 @@ struct nfs4_client_reclaim {
 	struct xdr_netobj	cr_princhash;
 };
 
-/* A reasonable value for REPLAY_ISIZE was estimated as follows:  
- * The OPEN response, typically the largest, requires 
- *   4(status) + 8(stateid) + 20(changeinfo) + 4(rflags) +  8(verifier) + 
- *   4(deleg. type) + 8(deleg. stateid) + 4(deleg. recall flag) + 
- *   20(deleg. space limit) + ~32(deleg. ace) = 112 bytes 
+/*
+ * REPLAY_ISIZE is sized for an OPEN response with delegation:
+ *   4(status) + 8(stateid) + 20(changeinfo) + 4(rflags) +
+ *   8(verifier) + 4(deleg. type) + 8(deleg. stateid) +
+ *   4(deleg. recall flag) + 20(deleg. space limit) +
+ *   ~32(deleg. ace) = 112 bytes
+ *
+ * Some responses can exceed this. A LOCK denial includes the conflicting
+ * lock owner, which can be up to 1024 bytes (NFS4_OPAQUE_LIMIT). Responses
+ * larger than REPLAY_ISIZE are not cached in rp_ibuf; only rp_status is
+ * saved. Enlarging this constant increases the size of every
+ * nfs4_stateowner.
  */
 
 #define NFSD4_REPLAY_ISIZE       112