On 1/31/2017 9:21 PM, Jan Kara wrote:
On Tue 31-01-17 20:08:57, Sahitya Tummala wrote:
Below is the synchronization issue between unmount and kjournald2
contexts, which results into use after free issue in kjournald2().
Fix this issue by using journal->j_state_lock to synchronize the
wait_event() done in journal_kill_thread() and the wake_up() done
in kjournald2().
TASK 1:
umount cmd:
|--jbd2_journal_destroy() {
|--journal_kill_thread() {
write_lock(&journal->j_state_lock);
journal->j_flags |= JBD2_UNMOUNT;
...
write_unlock(&journal->j_state_lock);
wake_up(&journal->j_wait_commit); TASK 2 wakes up here:
kjournald2() {
...
checks JBD2_UNMOUNT flag and calls goto end-loop;
...
end_loop:
write_unlock(&journal->j_state_lock);
journal->j_task = NULL; --> If this thread gets
pre-empted here, then TASK 1 wait_event will
exit even before this thread is completely
done.
wait_event(journal->j_wait_done_commit, journal->j_task == NULL);
...
write_lock(&journal->j_state_lock);
write_unlock(&journal->j_state_lock);
}
|--kfree(journal);
}
}
wake_up(&journal->j_wait_done_commit); --> this step
now results into use after free issue.
}
Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
Yeah, what you write looks possible (although rather unlikely). Thanks for
catching this. One small nit below:
Yes, it was observed only once and is very hard to reproduce.
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index a097048..f5cd3c0 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -278,9 +278,11 @@ static int kjournald2(void *arg)
end_loop:
write_unlock(&journal->j_state_lock);
del_timer_sync(&journal->j_commit_timer);
+ write_lock(&journal->j_state_lock);
There's no good reason to do del_timer_sync() outside of j_state_lock. This
is not performance critical code and commit_timeout is trivial and cannot
block on anything. So just keep j_state_lock locked upto the place where
you unlock it now...
Sure, I will update the patch.
Honza
journal->j_task = NULL;
wake_up(&journal->j_wait_done_commit);
jbd_debug(1, "Journal thread exiting.\n");
+ write_unlock(&journal->j_state_lock);
return 0;
}
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
