On Wed, Jun 04, 2008 at 05:58:15PM -0400, bfields wrote:
> On Wed, Jun 04, 2008 at 05:27:52PM -0400, Jeff Layton wrote:
> > On Wed, 4 Jun 2008 17:02:35 -0400
> > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> >
> > > On Wed, Jun 04, 2008 at 11:03:13AM -0400, Jeff Layton wrote:
> > > > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> > > > index 5ac00c4..d601a77 100644
> > > > --- a/fs/nfsd/nfsctl.c
> > > > +++ b/fs/nfsd/nfsctl.c
> > > ...
> > > > @@ -566,14 +574,13 @@ static ssize_t write_versions(struct file *file, char *buf, size_t size)
> > > > return len;
> > > > }
> > > >
> > > > -static ssize_t write_ports(struct file *file, char *buf, size_t size)
> > > > +static ssize_t __write_ports(struct file *file, char *buf, size_t size)
> > > > {
> > > > if (size == 0) {
> > > > int len = 0;
> > > > - lock_kernel();
> > > > +
> > > > if (nfsd_serv)
> > > > len = svc_xprt_names(nfsd_serv, buf, 0);
> > > > - unlock_kernel();
> > >
> > > svc_xprt_names() has to be prepared to accept NULL as a first parameter
> > > (since we've got nothing here any longer to guarantee that nfsd_serv
> > > won't change after we've checked it). And, indeed, it does check for
> > > that (with its local copy, which won't change. So that's OK. But then
> > > could we just ditch this redundant check here? It's confusing.
> > >
> > > Oops, but: what happens if something like this races with svc_destroy,
> > > so svc_xprt_names() is passed a pointer to freed memory?
> > >
> >
> > We do have a guarantee that nfsd_serv won't change after it's checked
> > here. The new nfsd_mutex protects it. write_ports has been renamed to
> > __write_ports, and write_ports has been turned into a wrapper that runs
> > the entire original function under the nfsd_mutex. We also have nfsd
> > hold the nfsd_mutex when svc_exit_thread is called, so svc_destroy
> > should also be called while holding it. That should serialize access
> > to the nfsd_serv.
>
> Of course, you're right; thanks for setting me straight!
One more random point of confusion: is write_versions racy? It assigns
to nfsd_versions, which is used in svc_process() to decide whether a
version is supported or not, without doing the adjustment of rq_argp and
rq_resp which a comment in write_versions() says is necessary. And
there's no locking around the nfsd_serv check there. So in theory could
a write_versions() at the wrong time result in an nfsd that accepted nfs
versions that it shouldn't (and hence could overflow some buffer)?
That'd be a preexisting problem, nothing to do with your work--I was
just grepping for uses of nfsd_serv....
--b.
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