cfq-iosched: do not leak ioc_data across iosched switches

When switching scheduler from cfq, cfq_exit_queue() does not clear
ioc->ioc_data, leaving a dangling pointer that can deceive the following
lookups when the iosched is switched back to cfq.  The pattern that can
trigger that is the following:

    - elevator switch from cfq to something else;
    - module unloading, with elv_unregister() that calls cfq_free_io_context()
      on ioc freeing the cic (via the .trim op);
    - module gets reloaded and the elevator switches back to cfq;
    - reallocation of a cic at the same address as before (with a valid key).

To fix it just assign NULL to ioc_data in __cfq_exit_single_io_context(),
that is called from the regular exit path and from the elevator switching
code.  The only path that frees a cic and is not covered is the error handling
one, but cic's freed in this way are never cached in ioc_data.

Signed-off-by: Fabio Checconi <fabio@gandalf.sssup.it>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f26da2bfcc15..f4e1006c253d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1214,6 +1214,8 @@ static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
                                          struct cfq_io_context *cic)
 {
+        struct io_context *ioc = cic->ioc;
+
         list_del_init(&cic->queue_list);
 
         /*
@@ -1223,6 +1225,9 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
         cic->dead_key = (unsigned long) cic->key;
         cic->key = NULL;
 
+        if (ioc->ioc_data == cic)
+                rcu_assign_pointer(ioc->ioc_data, NULL);
+
         if (cic->cfqq[ASYNC]) {
                 cfq_exit_cfqq(cfqd, cic->cfqq[ASYNC]);
                 cic->cfqq[ASYNC] = NULL;
@@ -1255,7 +1260,6 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
  */
 static void cfq_exit_io_context(struct io_context *ioc)
 {
-        rcu_assign_pointer(ioc->ioc_data, NULL);
         call_for_each_cic(ioc, cfq_exit_single_io_context);
 }
 
@@ -1478,8 +1482,7 @@ cfq_drop_dead_cic(struct cfq_data *cfqd, struct io_context *ioc,
 
         spin_lock_irqsave(&ioc->lock, flags);
 
-        if (ioc->ioc_data == cic)
-                rcu_assign_pointer(ioc->ioc_data, NULL);
+        BUG_ON(ioc->ioc_data == cic);
 
         radix_tree_delete(&ioc->radix_root, (unsigned long) cfqd);
         hlist_del_rcu(&cic->cic_list);