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

[Chuck Lever <cel@kernel.org>]

On 2/24/26 2:51 PM, Jeff Layton wrote:
> On Tue, 2026-02-24 at 14:42 -0500, Chuck Lever wrote:
>> On 2/24/26 2:39 PM, Jeff Layton wrote:
>>> 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?
>>
>> The idea would be to either:
>>
>> o Backport your fix and not this update, or
>> o Squash these two together, and backport both
>>
>> Admittedly this is a narrow corner case for a minor version that is
>> destined for the scrap heap.
>>
> 
> Right. I ask because I looked at this approach when I was fixing this,
> and decided it wasn't worthwhile. I certainly won't stand in your way
> if you decide you want to handle long lockowner blobs, but I doubt any
> legitimate user will ever care.

I don't disagree at all. My concern is handling replay compliantly.
Maybe there's another approach.


-- 
Chuck Lever