[PATCH 4.9 18/83] fuse: Fix use-after-free in fuse_dev_do_read()

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>

commit bc78abbd55dd28e2287ec6d6502b842321a17c87 upstream.

We may pick freed req in this way:

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

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

Reported-by: syzbot+4e975615ca01f2277bdd@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Fixes: 46c34a348b0a ("fuse: no fc->lock for pqueue parts")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.2
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/fuse/dev.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1303,12 +1303,14 @@ static ssize_t fuse_dev_do_read(struct f
 		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;