Re: [PATCH RFC] nfsd: don't hand out write delegations on O_WRONLY opens

Jeff Layton
2023-07-31T22:44:50Z

On Tue, 2023-08-01 at 08:11 +1000, NeilBrown wrote:
> On Tue, 01 Aug 2023, Jeff Layton wrote:
> > I noticed that xfstests generic/001 was failing against linux-next nfsd.
> >
> > The client would request a OPEN4_SHARE_ACCESS_WRITE open, and the server
> > would hand out a write delegation. The client would then try to use that
> > write delegation as the source stateid in a COPY or CLONE operation, and
> > the server would respond with NFS4ERR_STALE.
> >
> > The problem is that the struct file associated with the delegation does
> > not necessarily have read permissions. It's handing out a write
> > delegation on what is effectively an O_WRONLY open. RFC 8881 states:
> >
> > "An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its
> > own, all opens."
> >
> > Given that the client didn't request any read permissions, and that nfsd
> > didn't check for any, it seems wrong to give out a write delegation.
> >
> > Don't hand out a delegation if the client didn't request
> > OPEN4_SHARE_ACCESS_BOTH.
> >
> > This fixes xfstest generic/001.
> >
> > Closes: https://bugzilla.linux-nfs.org/show_bug.cgi?id=412
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> > fs/nfsd/nfs4state.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index ef7118ebee00..9f1c90afed72 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -5462,6 +5462,8 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> > return ERR_PTR(-EAGAIN);
> >
> > if (open->op_share_access & NFS4_SHARE_ACCESS_WRITE) {
> > + if (!(open->op_share_access & NFS4_SHARE_ACCESS_READ))
> > + return ERR_PTR(-EBADF);
> <bikeshed>
> The actual error code returned by nfs4_set_delegation() is ignored -
> only the fact of an error is relevant.
> Given that, how did you choose -EBADF. nfsd doesn't use file
> descriptors, and doesn't use EBADF anywhere else.
> Given that you have just tested access, EACCES might be justifiable.
> But I would prefer if nfs4_set_delegation() returns NULL if it could not
> find or create a delegation, without bothering with giving a reason.
> </bikeshed>
>

I chose EBADF because the fcntl code uses it for similar purposes. From
the manpage:

EBADF cmd is F_SETLK or F_SETLKW and the file descriptor open
mode doesn't match with the type of lock requested.

We're requesting a "lock" here in a delegation, so this made some sense
to me. I'm not particular here though. If another error makes more
sense, then that's fine.

> Reviewed-by: NeilBrown <neilb@suse.de>
>

Thanks!

> NeilBrown
>
> > nf = find_writeable_file(fp);
> > dl_type = NFS4_OPEN_DELEGATE_WRITE;
> > } else {
> >
> > ---
> > base-commit: ec89391563792edd11d138a853901bce76d11f44
> > change-id: 20230731-wdeleg-bbdb6b25a3c6
> >
> > Best regards,
> > --
> > Jeff Layton <jlayton@kernel.org>
> >
> >