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

J. Bruce Fields
2016-07-08T20:54:33Z

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@linuxhacker.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