Re: [PATCH] nfsd: Close a race between access checking/setting in nfs4_get_vfs_file

[Oleg Drokin <green@linuxhacker.ru>]

On Jun 9, 2016, at 5:01 PM, Oleg Drokin wrote:

> Currently there's an unprotected access mode check in nfs4_upgrade_open
> that then calls nfs4_get_vfs_file which in turn assumes whatever
> access mode was present in the state is still valid which is racy.
> Two nfs4_get_vfs_file van enter the same path as result and get two
> references to nfs4_file, but later drop would only happens once because
> access mode is only denoted by bits, so no refcounting.
> 
> The locking around access mode testing is introduced to avoid this race.
> 
> Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
> ---
> 
> This patch performs equally well to the st_rwsem -> mutex conversion,
> but is a bit ligher-weight I imagine.
> For one it seems to allow truncates in parallel if we ever want it.
> 
> fs/nfsd/nfs4state.c | 28 +++++++++++++++++++++++++---
> 1 file changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index f5f82e1..d4b9eba 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -3958,6 +3958,11 @@ static __be32 nfs4_get_vfs_file(struct svc_rqst *rqstp, struct nfs4_file *fp,
> 
> 	spin_lock(&fp->fi_lock);
> 
> +	if (test_access(open->op_share_access, stp)) {
> +		spin_unlock(&fp->fi_lock);
> +		return nfserr_eagain;
> +	}
> +
> 	/*
> 	 * Are we trying to set a deny mode that would conflict with
> 	 * current access?
> @@ -4017,11 +4022,21 @@ nfs4_upgrade_open(struct svc_rqst *rqstp, struct nfs4_file *fp, struct svc_fh *c
> 	__be32 status;
> 	unsigned char old_deny_bmap = stp->st_deny_bmap;
> 
> -	if (!test_access(open->op_share_access, stp))
> -		return nfs4_get_vfs_file(rqstp, fp, cur_fh, stp, open);
> +again:
> +	spin_lock(&fp->fi_lock);
> +	if (!test_access(open->op_share_access, stp)) {
> +		spin_unlock(&fp->fi_lock);
> +		status = nfs4_get_vfs_file(rqstp, fp, cur_fh, stp, open);
> +		/*
> +		 * Somebody won the race for access while we did not hold
> +		 * the lock here
> +		 */
> +		if (status == nfserr_eagain)
> +			goto again;
> +		return status;
> +	}
> 
> 	/* test and set deny mode */
> -	spin_lock(&fp->fi_lock);
> 	status = nfs4_file_check_deny(fp, open->op_share_deny);
> 	if (status == nfs_ok) {
> 		set_deny(open->op_share_deny, stp);
> @@ -4361,6 +4376,13 @@ nfsd4_process_open2(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nf
> 		status = nfs4_get_vfs_file(rqstp, fp, current_fh, stp, open);
> 		if (status) {
> 			up_read(&stp->st_rwsem);
> +			/*
> +			 * EAGAIN is returned when there's a racing access,
> +			 * this should never happen as we are the only user
> +			 * of this new state, and since it's not yet hashed,
> +			 * nobody can find it
> +			 */
> +			WARN_ON(status == nfserr_eagain);

Ok, some more testing shows that this CAN happen.
So this patch is inferior to the mutex one after all.

> 			release_open_stateid(stp);
> 			goto out;
> 		}
> -- 
> 2.7.4