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

[Greg Kroah-Hartman]

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

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

From: Kirill Tkhai <ktk...@virtuozzo.com>

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+4e975615ca01f2277...@syzkaller.appspotmail.com
Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszer...@redhat.com>
Fixes: 46c34a348b0a ("fuse: no fc->lock for pqueue parts")
Cc: <sta...@vger.kernel.org> # v4.2
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 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;