[PATCH 3/4] fs, afs: convert afs_server.usage from atomic_t to refcount_t

[Reshetova, Elena]

> -----Original Message-----
> From: David Howells [mailto:dhowells at redhat.com]
> Sent: Wednesday, February 22, 2017 6:32 PM
> To: Reshetova, Elena <elena.reshetova at intel.com>
> Cc: dhowells at redhat.com; linux-kernel at vger.kernel.org; linux-
> afs at lists.infradead.org; peterz at infradead.org; gregkh at linuxfoundation.org;
> Hans Liljestrand <ishkamiel at gmail.com>; Kees Cook
> <keescook at chromium.org>; David Windsor <dwindsor at gmail.com>
> Subject: Re: [PATCH 3/4] fs, afs: convert afs_server.usage from atomic_t to
> refcount_t
> 
> Elena Reshetova <elena.reshetova at intel.com> wrote:
> 
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> 
> Although I don't see an assertion for this (the window is too small), it is
> possible for a dead server record to get resurrected.  Take a look at
> afs_put_server() and note there's a check around the move to the graveyard.

This one is harder to transform correctly using +1 scheme. As far as I understand, I would need to take 
an additional lock in afs_put_server() to accommodate this section:

@@ -230,12 +230,17 @@ void afs_put_server(struct afs_server *server)
 
     _debug("PUT SERVER %d", refcount_read(&server->usage));
 
-    ASSERTCMP(refcount_read(&server->usage), >, 0);
+    ASSERTCMP(refcount_read(&server->usage), >, 1);
 
-    if (likely(!refcount_dec_and_test(&server->usage))) {
+    /* need a write lock for  &server the following section */
+    refcount_dec(&server->usage);
+
+    if (likely(refcount_read(&server->usage) > 1)) {
+        spin_unlock(&old_server->fs_lock);
         _leave("");
         return;
     }
+    /* end of lock section */

And smth tells me that you don't want to have any locks here... Right?

Same story is with afs_vlocation_lookup().

Best Regards,
Elena.





> 
> So, please hold this patch also.
> 
> David