Re: [PATCH] nfsd: use dynamic allocation for oversized NFSv4.0 replay cache

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2026-02-24 at 14:33 -0500, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Commit 1e8e9913672a ("nfsd: fix heap overflow in NFSv4.0 LOCK
> replay cache") capped the replay cache copy at NFSD4_REPLAY_ISIZE
> to prevent a heap overflow, but set rp_buflen to zero when the
> encoded response exceeded the inline buffer. A retransmitted LOCK
> reaching the replay path then produced only a status code with no
> operation body, resulting in a malformed XDR response.
> 
> When the encoded response exceeds the 112-byte inline rp_ibuf, a
> buffer is kmalloc'd to hold it. If the allocation fails, rp_buflen
> remains zero, preserving the behavior from the capped-copy fix.
> The buffer is freed when the stateowner is released or when a
> subsequent operation's response fits in the inline buffer.
> 
> Fixes: 1e8e9913672a ("nfsd: fix heap overflow in NFSv4.0 LOCK replay cache")
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs4state.c | 16 ++++++++++++++++
>  fs/nfsd/nfs4xdr.c   | 23 ++++++++++++++++-------
>  fs/nfsd/state.h     | 12 +++++++-----
>  3 files changed, 39 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index ba49f49bb93b..b4d0e82b2690 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1496,8 +1496,24 @@ release_all_access(struct nfs4_ol_stateid *stp)
>  	}
>  }
>  
> +/**
> + * nfs4_replay_free_cache - release dynamically allocated replay buffer
> + * @rp: replay cache to reset
> + *
> + * If @rp->rp_buf points to a kmalloc'd buffer, free it and reset
> + * rp_buf to the inline rp_ibuf. Always zeroes rp_buflen.
> + */
> +void nfs4_replay_free_cache(struct nfs4_replay *rp)
> +{
> +	if (rp->rp_buf != rp->rp_ibuf)
> +		kfree(rp->rp_buf);
> +	rp->rp_buf = rp->rp_ibuf;
> +	rp->rp_buflen = 0;
> +}
> +
>  static inline void nfs4_free_stateowner(struct nfs4_stateowner *sop)
>  {
> +	nfs4_replay_free_cache(&sop->so_replay);
>  	kfree(sop->so_owner.data);
>  	sop->so_ops->so_free(sop);
>  }
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index 690f7a3122ec..2a0946c630e1 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -6282,14 +6282,23 @@ 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;
> -		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;
> +		if (len > NFSD4_REPLAY_ISIZE) {
> +			char *buf = kmalloc(len, GFP_KERNEL);
> +
> +			nfs4_replay_free_cache(&so->so_replay);
> +			if (buf) {
> +				so->so_replay.rp_buf = buf;
> +			} else {
> +				/* rp_buflen already zeroed; skip caching */
> +				goto status;
> +			}
> +		} else if (so->so_replay.rp_buf != so->so_replay.rp_ibuf) {
> +			nfs4_replay_free_cache(&so->so_replay);
>  		}
> +		so->so_replay.rp_buflen = len;
> +		read_bytes_from_xdr_buf(xdr->buf,
> +					op_status_offset + XDR_UNIT,
> +					so->so_replay.rp_buf, len);
>  	}
>  status:
>  	op->status = nfsd4_map_status(op->status,
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 3159c7b67f50..9b05462da4cc 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -554,10 +554,10 @@ struct nfs4_client_reclaim {
>   *   ~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.
> + * lock owner, which can be up to 1024 bytes (NFS4_OPAQUE_LIMIT). When a
> + * response exceeds REPLAY_ISIZE, a buffer is dynamically allocated. If
> + * that allocation fails, only rp_status is saved. Enlarging this constant
> + * increases the size of every nfs4_stateowner.
>   */
>  
>  #define NFSD4_REPLAY_ISIZE       112 
> @@ -569,12 +569,14 @@ struct nfs4_client_reclaim {
>  struct nfs4_replay {
>  	__be32			rp_status;
>  	unsigned int		rp_buflen;
> -	char			*rp_buf;
> +	char			*rp_buf; /* rp_ibuf or kmalloc'd */
>  	struct knfsd_fh		rp_openfh;
>  	int			rp_locked;
>  	char			rp_ibuf[NFSD4_REPLAY_ISIZE];
>  };
>  
> +extern void nfs4_replay_free_cache(struct nfs4_replay *rp);
> +
>  struct nfs4_stateowner;
>  
>  struct nfs4_stateowner_operations {


Certainly a reasonable approach if we care about full correctness when
dealing with a large lockowner on NFSv4.0. Do we?
-- 
Jeff Layton <jlayton@kernel.org>