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From:   Ryusuke Konishi <konishi.ryusuke@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-nilfs@vger.kernel.org,
        syzbot <syzbot+b08ebcc22f8f3e6be43a@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: [PATCH] nilfs2: fix potential UAF of struct nilfs_sc_info in nilfs_segctor_thread()
Date:   Tue, 28 Mar 2023 02:53:18 +0900
Message-Id: <20230327175318.8060-1-konishi.ryusuke@gmail.com>
In-Reply-To: <00000000000000660d05f7dfa877@google.com>
References: <00000000000000660d05f7dfa877@google.com>
MIME-Version: 1.0
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The finalization of nilfs_segctor_thread() can race with
nilfs_segctor_kill_thread() which terminates that thread, potentially
causing a use-after-free BUG as KASAN detected.

At the end of nilfs_segctor_thread(), it assigns NULL to "sc_task" member
of "struct nilfs_sc_info" to indicate the thread has finished, and then
notifies nilfs_segctor_kill_thread() of this using waitqueue
"sc_wait_task" on the struct nilfs_sc_info.

However, here, immediately after the NULL assignment to "sc_task",
it is possible that nilfs_segctor_kill_thread() will detect it and return
to continue the deallocation, freeing the nilfs_sc_info structure before
the thread does the notification.

This fixes the issue by protecting the NULL assignment to "sc_task" and
its notification, with spinlock "sc_state_lock" of the struct
nilfs_sc_info.  Since nilfs_segctor_kill_thread() does a final check to
see if "sc_task" is NULL with "sc_state_lock" locked, this can eliminate
the race.

Reported-by: syzbot+b08ebcc22f8f3e6be43a@syzkaller.appspotmail.com
Link: https://lkml.kernel.org/r/00000000000000660d05f7dfa877@google.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
 fs/nilfs2/segment.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 19446a8243d7..6ad41390fa74 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2609,11 +2609,10 @@ static int nilfs_segctor_thread(void *arg)
 	goto loop;
 
  end_thread:
-	spin_unlock(&sci->sc_state_lock);
-
 	/* end sync. */
 	sci->sc_task = NULL;
 	wake_up(&sci->sc_wait_task); /* for nilfs_segctor_kill_thread() */
+	spin_unlock(&sci->sc_state_lock);
 	return 0;
 }
 
-- 
2.34.1