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Subject[PATCH 1/8] fs/aio: Add explicit RCU grace period when freeing kioctx
Date
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
---
fs/aio.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index a062d75..eb2e0cf 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);

/*
--
2.9.5
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