linux-kernel - [PATCH 14/37] binder: avoid race conditions when enqueuing txn

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Message-Id: <20170629190211.16927-15-tkjos@google.com>
Date:   Thu, 29 Jun 2017 12:01:48 -0700
From:   Todd Kjos <tkjos@...roid.com>
To:     gregkh@...uxfoundation.org, arve@...roid.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        maco@...gle.com, tkjos@...gle.com
Subject: [PATCH 14/37] binder: avoid race conditions when enqueuing txn

Currently, the transaction complete work item is queued
after the transaction. This means that it is possible
for the transaction to be handled and a reply to be
enqueued in the current thread before the transaction
complete is enqueued, which violates the protocol
with userspace who may not expect the transaction
complete. Fixed by always enqueing the transaction
complete first.

Also, once the transaction is enqueued, it is unsafe
to access since it might be freed. Currently,
t->flags is accessed to determine whether a sync
wake is needed. Changed to access tr->flags
instead.

Signed-off-by: Todd Kjos <tkjos@...gle.com>
---
 drivers/android/binder.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index f17d1dfa5b02..71faf548482d 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -1799,6 +1799,9 @@ static void binder_transaction(struct binder_proc *proc,
 			goto err_bad_object_type;
 		}
 	}
+	tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
+	list_add_tail(&tcomplete->entry, &thread->todo);
+
 	if (reply) {
 		BUG_ON(t->buffer->async_transaction != 0);
 		binder_pop_transaction(target_thread, in_reply_to);
@@ -1818,10 +1821,8 @@ static void binder_transaction(struct binder_proc *proc,
 	}
 	t->work.type = BINDER_WORK_TRANSACTION;
 	list_add_tail(&t->work.entry, target_list);
-	tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
-	list_add_tail(&tcomplete->entry, &thread->todo);
 	if (target_wait) {
-		if (reply || !(t->flags & TF_ONE_WAY))
+		if (reply || !(tr->flags & TF_ONE_WAY))
 			wake_up_interruptible_sync(target_wait);
 		else
 			wake_up_interruptible(target_wait);
-- 
2.13.2.725.g09c95d1e9-goog