On Wed, 4 Jun 2008 18:41:20 -0400
"J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> 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)?
>
Hmm. You may be right, though I'd think the race is pretty unlikely in
normal usage. I guess the comment you're referring to is this one:
if (nfsd_serv)
/* Cannot change versions without updating
* nfsd_serv->sv_xdrsize, and reallocing
* rq_argp and rq_resp
*/
return -EBUSY;
...so the race would have to be:
nfsd is down
write versions is called and gets past
nfsd_serv NULL ptr check
nfsd accepts a call
write versions disables the NFS version that
was in the call
A pretty unlikely race, I think, but might be possible. Holding the
nfsd_mutex over the life of write_versions is probably the right thing
to do here. I'll plan a respin to add that (and I'll check that it
doesn't cause any problems).
> That'd be a preexisting problem, nothing to do with your work--I was
> just grepping for uses of nfsd_serv....
>
This is actually Neil's work...I only added the signed-off-by since I
added and cleaned up some comments. ;-)
--
Jeff Layton <jlayton@xxxxxxxxxx>
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