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?
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
