FAILED: patch "[PATCH] nfsd: fix heap overflow in NFSv4.0 LOCK replay cache" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 5133b61aaf437e5f25b1b396b14242a6bb0508e2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026032010-shredding-stargazer-b481@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5133b61aaf437e5f25b1b396b14242a6bb0508e2 Mon Sep 17 00:00:00 2001
From: Jeff Layton <[email protected]>
Date: Tue, 24 Feb 2026 11:33:35 -0500
Subject: [PATCH] nfsd: fix heap overflow in NFSv4.0 LOCK replay cache

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.

index 5172dbd0cb05..fa16b34fae50 100644
@@ -6281,9 +6281,14 @@ nfsd4_encode_operation(struct nfsd4_compoundres *resp, struct nfsd4_op *op)
 		int len = xdr->buf->len - (op_status_offset + XDR_UNIT);
 
 		so->so_replay.rp_status = op->status;
-		so->so_replay.rp_buflen = len;
-		read_bytes_from_xdr_buf(xdr->buf, op_status_offset + XDR_UNIT,
+		if (len <= NFSD4_REPLAY_ISIZE) {
+			so->so_replay.rp_buflen = len;
+			read_bytes_from_xdr_buf(xdr->buf,
+						op_status_offset + XDR_UNIT,
 						so->so_replay.rp_buf, len);
+		} else {
+			so->so_replay.rp_buflen = 0;
+		}
 	}
 status:
 	op->status = nfsd4_map_status(op->status,

index 6fcbf1e427d4..c0ca115c3b74 100644
@@ -541,11 +541,18 @@ 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 

From: Jeff Layton <[email protected]>

[ Upstream commit 5133b61aaf437e5f25b1b396b14242a6bb0508e2 ]

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.

---
 fs/nfsd/nfs4xdr.c |  9 +++++++--
 fs/nfsd/state.h   | 17 ++++++++++++-----
 2 files changed, 19 insertions(+), 7 deletions(-)

index 46a7fd731ba0a..e02c5650e2ac4 100644
@@ -5439,9 +5439,14 @@ 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:
 	*p = op->status;

index 477828dbfc665..53298bdcfb3d0 100644
@@ -430,11 +430,18 @@ 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 

2.51.0

Hi,

This patch does not appear to be queued in 5.15.y yet, although it is already present in 5.10.y and 6.1.y.

Could this please also be queued for 5.15.y?
It seems it may have been missed inadvertently.

Thanks,
Alok

Odd, I did miss it, now added, thanks!

greg k-h