SUNRPC: Track writers of the 'channel' file to improve cache_listeners_exist

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@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
index c7f38e8971747cd0c7633abf981f7fb0f3860a31..f7d086b77a21a152d3f0352cb169ae1b17c48a80 100644 (file)
--- 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 6f1528f271eed6ea0b7343c1c81e67702cb31dd5..a6a6190ad37a98f3ccfb0d92e5edab063f0e85de 100644 (file)
--- 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);
        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 */