On Tue, Jul 30, 2019 at 10:02:37AM +1000, NeilBrown wrote:
> On Mon, Jul 29 2019, J. Bruce Fields wrote:
>
> > On Fri, Jul 26, 2019 at 06:33:01PM -0400, Dave Wysochanski wrote:
> >> The sunrpc cache interface is susceptible to being fooled by a rogue
> >> process just reading a 'channel' file. If this happens the kernel
> >> may think a valid daemon exists to service the cache when it does not.
> >> For example, the following may fool the kernel:
> >> cat /proc/net/rpc/auth.unix.gid/channel
> >>
> >> Change the tracking of readers to writers when considering whether a
> >> listener exists as all valid daemon processes either open a channel
> >> file O_RDWR or O_WRONLY. While this does not prevent a rogue process
> >> from "stealing" a message from the kernel, it does at least improve
> >> the kernels perception of whether a valid process servicing the cache
> >> exists.
> >>
> >> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> >> ---
> >> include/linux/sunrpc/cache.h | 6 +++---
> >> net/sunrpc/cache.c | 12 ++++++++----
> >> 2 files changed, 11 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
> >> index c7f38e8..f7d086b 100644
> >> --- a/include/linux/sunrpc/cache.h
> >> +++ b/include/linux/sunrpc/cache.h
> >> @@ -107,9 +107,9 @@ struct cache_detail {
> >> /* fields for communication over channel */
> >> struct list_head queue;
> >>
> >> - atomic_t readers; /* how many time is /chennel open */
> >> - time_t last_close; /* if no readers, when did last close */
> >> - time_t last_warn; /* when we last warned about no readers */
> >> + atomic_t writers; /* how many time is /channel open */
> >> + time_t last_close; /* if no writers, when did last close */
> >> + time_t last_warn; /* when we last warned about no writers */
> >>
> >> union {
> >> struct proc_dir_entry *procfs;
> >> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> >> index 6f1528f..a6a6190 100644
> >> --- a/net/sunrpc/cache.c
> >> +++ b/net/sunrpc/cache.c
> >> @@ -373,7 +373,7 @@ void sunrpc_init_cache_detail(struct cache_detail *cd)
> >> spin_lock(&cache_list_lock);
> >> cd->nextcheck = 0;
> >> cd->entries = 0;
> >> - atomic_set(&cd->readers, 0);
> >> + atomic_set(&cd->writers, 0);
> >> cd->last_close = 0;
> >> cd->last_warn = -1;
> >> list_add(&cd->others, &cache_list);
> >> @@ -1029,11 +1029,13 @@ static int cache_open(struct inode *inode, struct file *filp,
> >> }
> >> rp->offset = 0;
> >> rp->q.reader = 1;
> >> - atomic_inc(&cd->readers);
> >> +
> >> spin_lock(&queue_lock);
> >> list_add(&rp->q.list, &cd->queue);
> >> spin_unlock(&queue_lock);
> >> }
> >> + if (filp->f_mode & FMODE_WRITE)
> >> + atomic_inc(&cd->writers);
> >
> > This patch would be even simpler if we just modified the condition of
> > the preceding if clause:
> >
> > - if (filp->f_mode & FMODE_READ) {
> > + if (filp->f_mode & FMODE_WRITE) {
> >
> > and then we could drop the following chunk completely.
> >
> > Is there any reason not to do that?
>
> I can see how this would be tempting, but I think the reason not to do
> that is it is ... wrong.
>
> The bulk of the code is for setting up context to support reading, so it
> really should be conditional on FMODE_READ.
> We always want to set that up, because if a process opens for read, and
> not write, we want to respond properly to read requests. This is useful
> for debugging.
How is it useful for debugging?
--b.
> I think this patch from Dave is good. A process opening for read might
> just be inquisitive. A program opening for write is making more of a
> commitment to being involved in managing the cache.
>
> Reviewed-by: NeilBrown <neilb@xxxxxxxx>
>
> Thanks,
> NeilBrown
>
>
> >
> > Or if the resulting behavior isn't right for write-only openers, we
> > could make the condition ((filp->f_mode & FMODE_READ) && (filp->f_mode &
> > FMODE_WRITE)).
> >
> > --b.
> >
> >> filp->private_data = rp;
> >> return 0;
> >> }
> >> @@ -1062,8 +1064,10 @@ static int cache_release(struct inode *inode, struct file *filp,
> >> filp->private_data = NULL;
> >> kfree(rp);
> >>
> >> + }
> >> + if (filp->f_mode & FMODE_WRITE) {
> >> + atomic_dec(&cd->writers);
> >> cd->last_close = seconds_since_boot();
> >> - atomic_dec(&cd->readers);
> >> }
> >> module_put(cd->owner);
> >> return 0;
> >> @@ -1171,7 +1175,7 @@ static void warn_no_listener(struct cache_detail *detail)
> >>
> >> static bool cache_listeners_exist(struct cache_detail *detail)
> >> {
> >> - if (atomic_read(&detail->readers))
> >> + if (atomic_read(&detail->writers))
> >> return true;
> >> if (detail->last_close == 0)
> >> /* This cache was never opened */
> >> --
> >> 1.8.3.1
