From: Sahitya Tummala <stummala@codeaurora.org>
Date: Thu, 2 Feb 2017 01:49:35 +0000 (-0500)
Subject: jbd2: fix use after free in kjournald2()
X-Git-Tag: v4.1.52~76
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=0759c1c2f140ec56a9fec96dddf9e5beb89f3b48;p=users%2Fdwmw2%2Flinux.git

jbd2: fix use after free in kjournald2()

[ Upstream commit dbfcef6b0f4012c57bc0b6e0e660d5ed12a5eaed ]

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@codeaurora.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 97f29405e5c6b..0bb394b4f04b7 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -275,11 +275,11 @@ loop:
 	goto loop;
 
 end_loop:
-	write_unlock(&journal->j_state_lock);
 	del_timer_sync(&journal->j_commit_timer);
 	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;
 }