linux-kernel - Re: KASAN: use-after-free Read in _

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Message-ID: <20200901031945.GE1236603@ZenIV.linux.org.uk>
Date:   Tue, 1 Sep 2020 04:19:45 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+c282923e5da93549fa27@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in __fput (3)

On Tue, Sep 01, 2020 at 10:35:47AM +0800, Hillf Danton wrote:

> Any light on how a9ed4a6560b8 ends up with __fput called twice is
> highly appreciated.

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e0decff22ae2..8107e06d7f6f 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1995,9 +1995,9 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
 			 * during ep_insert().
 			 */
 			if (list_empty(&epi->ffd.file->f_tfile_llink)) {
-				get_file(epi->ffd.file);
-				list_add(&epi->ffd.file->f_tfile_llink,
-					 &tfile_check_list);
+				if (get_file_rcu(epi->ffd.file))
+					list_add(&epi->ffd.file->f_tfile_llink,
+						 &tfile_check_list);
 			}
 		}
 	}