linux-kernel - Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM

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Date:	Fri, 8 Jul 2016 16:54:13 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Oleg Drokin <green@...uxhacker.ru>
Cc:	Jeff Layton <jlayton@...chiereds.net>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of
 EPERM

On Thu, Jul 07, 2016 at 09:47:46PM -0400, Oleg Drokin wrote:
> It looks like we are bit overzealous about failing mkdir/create/mknod
> with permission denied if the parent dir is not writeable.
> Need to make sure the name does not exist first, because we need to
> return EEXIST in that case.
> 
> Signed-off-by: Oleg Drokin <green@...uxhacker.ru>
> ---
> A very similar problem exists with symlinks, but the patch is more
> involved, so assuming this one is ok, I'll send a symlink one separately.
>  fs/nfsd/nfs4proc.c |  6 +++++-
>  fs/nfsd/vfs.c      | 11 ++++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index de1ff1d..0067520 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -605,8 +605,12 @@ nfsd4_create(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>  
>  	fh_init(&resfh, NFS4_FHSIZE);
>  
> +	/*
> +	 * We just check thta parent is accessible here, nfsd_* do their
> +	 * own access permission checks
> +	 */
>  	status = fh_verify(rqstp, &cstate->current_fh, S_IFDIR,
> -			   NFSD_MAY_CREATE);
> +			   NFSD_MAY_EXEC);
>  	if (status)
>  		return status;
>  
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 6fbd81e..6a45ec6 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1161,7 +1161,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	if (isdotent(fname, flen))
>  		goto out;
>  
> -	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_CREATE);
> +	/*
> +	 * Even though it is a create, first we see if we are even allowed
> +	 * to peek inside the parent
> +	 */
> +	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_EXEC);

Looks like in the v3 case we haven't actually locked the directory yet
at this point so this check is a little race-prone.

I wonder why the code's structured that way--it's confusing.

--b.

>  	if (err)
>  		goto out;
>  
> @@ -1211,6 +1215,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  		goto out; 
>  	}
>  
> +	/* Now let's see if we actually have permissions to create */
> +	err = nfsd_permission(rqstp, fhp->fh_export, dentry, NFSD_MAY_CREATE);
> +	if (err)
> +		goto out;
> +
>  	if (!(iap->ia_valid & ATTR_MODE))
>  		iap->ia_mode = 0;
>  	iap->ia_mode = (iap->ia_mode & S_IALLUGO) | type;
> -- 
> 2.7.4