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Subject: [PATCH] fuse: Fix use-after-free in fuse_dev_do_read()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     miklos@szeredi.hu,
        syzbot+4e975615ca01f2277bdd@syzkaller.appspotmail.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        ktkhai@virtuozzo.com
Date:   Tue, 25 Sep 2018 12:28:55 +0300
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We may pick freed req in this way:

[cpu0]                                              [cpu1]
fuse_dev_do_read()                                  fuse_dev_do_write()
   list_move_tail(&req->list, &fpq->processing);      ...
   spin_unlock(&fpq->lock);                           ...
   ...                                                request_end(fc, req);
   ...                                                  fuse_put_request(fc, req);
   if (test_bit(FR_INTERRUPTED, &req->flags))
         queue_interrupt(fiq, req);

Fix that by keeping req alive till we finish all manipulations.

Reported-by: syzbot+4e975615ca01f2277bdd@syzkaller.appspotmail.com
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 fs/fuse/dev.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 11ea2c4a38ab..675caed3e655 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1311,12 +1311,14 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 		goto out_end;
 	}
 	list_move_tail(&req->list, &fpq->processing);
+	__fuse_get_request(req);
 	spin_unlock(&fpq->lock);
 	set_bit(FR_SENT, &req->flags);
 	/* matches barrier in request_wait_answer() */
 	smp_mb__after_atomic();
 	if (test_bit(FR_INTERRUPTED, &req->flags))
 		queue_interrupt(fiq, req);
+	fuse_put_request(fc, req);
 
 	return reqsize;