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

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Jul 08, 2016 at 11:53:28AM -0400, Jeff Layton wrote:
> On Fri, 2016-07-08 at 11:14 -0400, Oleg Drokin wrote:
> > On Jul 8, 2016, at 7:02 AM, Jeff Layton wrote:
> > 
> > > On Thu, 2016-07-07 at 21:47 -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(-)
> > > > 
> > > 
> > > 
> > > nit: subject says EPERM, but I think you mean EACCES. The mnemonic I've
> > > always used is that EPERM is "permanent". IOW, changing permissions
> > > won't ever allow the user to do something. For instance, unprivileged
> > > users can never chown a file, so they should get back EPERM there. When
> > > a directory isn't writeable on a create they should get EACCES since
> > > they could do the create if the directory were writeable.
> > 
> > Hm, I see, thanks.
> > Confusing that you get "Permission denied" from perror ;)
> > 
> 
> Yes indeed. It's a subtle and confusing distinction.
> 
> > > > 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);
> > > >  	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;
> > > 
> > > 
> > > Ouch. This means two nfsd_permission calls per create operation. If
> > > it's necessary for correctness then so be it, but is it actually
> > > documented anywhere (POSIX perhaps?) that we must prefer EEXIST over
> > > EACCES in this situation?
> > 
> > Opengroup manpage: http://pubs.opengroup.org/onlinepubs/009695399/functions/mkdir.html
> > newer version is here:
> > http://pubs.opengroup.org/onlinepubs/9699919799/
> > 
> > They tell us that we absolutely must fail with EEXIST if the name is a symlink
> > (so we need to lookup it anyway), and also that EEXIST is the failure code
> > if the path exists.
> > 
> 
> I'm not sure that that verbiage supersedes the fact that you don't have
> write permissions on the directory. Does it?
> 
> ISTM that it's perfectly valid to shortcut looking up the dentry if the
> user doesn't have write permissions on the directory, even when the
> target is a symlink.
> 
> IOW, I'm not sure I see a bug here.

If this is causing real programs to behave incorrectly, then that may
matter more than the letter of the spec.  But I'm a little curious why
we'd be hearing about that just now--did the client or server's behavior
change recently?

> > Are double permission checks really as bad for nfs? it looked like it would
> > call mostly into VFS so even if first call would be expensive, second call should
> > be really cheap?
> > 
> 
> It depends on the underlying fs. In most cases, you're right, but you
> can export things that overload the ->permission op, and those can be
> as expensive as they like (within reason of course).

Weird if the expense of a second permission call is significant compared
to following the mkdir and sync.  But, what do I know.

--b.