fs/aio: Add explicit RCU grace period when freeing kioctx

While fixing refcounting, e34ecee2ae79 ("aio: Fix a trinity splat")
incorrectly removed explicit RCU grace period before freeing kioctx.
The intention seems to be depending on the internal RCU grace periods
of percpu_ref; however, percpu_ref uses a different flavor of RCU,
sched-RCU.  This can lead to kioctx being freed while RCU read
protected dereferences are still in progress.

Fix it by updating free_ioctx() to go through call_rcu() explicitly.

v2: Comment added to explain double bouncing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jann Horn <jannh@google.com>
Fixes: e34ecee2ae79 ("aio: Fix a trinity splat")
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable@vger.kernel.org # v3.13+

1 files changed, 19 insertions, 4 deletions

diff --git a/fs/aio.c b/fs/aio.c
index a062d75109cb..eb2e0cfbe2d2 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -115,7 +115,8 @@ struct kioctx {
         struct page             **ring_pages;
         long                    nr_pages;
 
-        struct work_struct      free_work;
+        struct rcu_head         free_rcu;
+        struct work_struct      free_work;      /* see free_ioctx() */
 
         /*
          * signals when all in-flight requests are done
@@ -588,6 +589,12 @@ static int kiocb_cancel(struct aio_kiocb *kiocb)
         return cancel(&kiocb->common);
 }
 
+/*
+ * free_ioctx() should be RCU delayed to synchronize against the RCU
+ * protected lookup_ioctx() and also needs process context to call
+ * aio_free_ring(), so the double bouncing through kioctx->free_rcu and
+ * ->free_work.
+ */
 static void free_ioctx(struct work_struct *work)
 {
         struct kioctx *ctx = container_of(work, struct kioctx, free_work);
@@ -601,6 +608,14 @@ static void free_ioctx(struct work_struct *work)
         kmem_cache_free(kioctx_cachep, ctx);
 }
 
+static void free_ioctx_rcufn(struct rcu_head *head)
+{
+        struct kioctx *ctx = container_of(head, struct kioctx, free_rcu);
+
+        INIT_WORK(&ctx->free_work, free_ioctx);
+        schedule_work(&ctx->free_work);
+}
+
 static void free_ioctx_reqs(struct percpu_ref *ref)
 {
         struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
@@ -609,8 +624,8 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
         if (ctx->rq_wait && atomic_dec_and_test(&ctx->rq_wait->count))
                 complete(&ctx->rq_wait->comp);
 
-        INIT_WORK(&ctx->free_work, free_ioctx);
-        schedule_work(&ctx->free_work);
+        /* Synchronize against RCU protected table->table[] dereferences */
+        call_rcu(&ctx->free_rcu, free_ioctx_rcufn);
 }
 
 /*
@@ -838,7 +853,7 @@ static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
         table->table[ctx->id] = NULL;
         spin_unlock(&mm->ioctx_lock);
 
-        /* percpu_ref_kill() will do the necessary call_rcu() */
+        /* free_ioctx_reqs() will do the necessary RCU synchronization */
         wake_up_all(&ctx->wait);
 
         /*