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

J. Bruce Fields
2016-07-21T20:34:19Z

On Fri, Jul 08, 2016 at 05:53:19PM -0400, Oleg Drokin wrote:
>
> On Jul 8, 2016, at 4:54 PM, J. Bruce Fields wrote:
>
> > 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.
>
> In reality this check is not really needed, I suspect.
> When we call vfs_create/mknod/mkdir later on, it has it's own permission check
> anyway so if there was a race and somebody changed dir access in the middle,
> there's going to be another check anyway and it would be caught.
> Unless there's some weird server-side permission wiggling as well that makes it
> ineffective, but I imagine that one cannot really change in a racy way?

Yeah, I think I'll just change those NFSD_MAY_EXEC's to NFSD_MAY_NOP's.
We still need the fh_verify there since it's also what does the
filehandle->dentry translation, but we don't need permission checking
here yet.

Applying with that one change. (And I'll followup with some additional
minor cleanup of the create code.)

--b.

>
> > I wonder why the code's structured that way--it's confusing.
>
> Probably years of accumulated "damage" ;)
>
> > --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