io_uring: fix potential req refcount underflow

For multishot mode, there may be cases like:

iowq                                 original context
io_poll_add
  _arm_poll()
  mask = vfs_poll() is not 0
  if mask
(2)  io_poll_complete()
  compl_unlock
   (interruption happens
    tw queued to original
    context)
                                     io_poll_task_func()
                                     compl_lock
                                 (3) done = io_poll_complete() is true
                                     compl_unlock
                                     put req ref
(1) if (poll->flags & EPOLLONESHOT)
      put req ref

EPOLLONESHOT flag in (1) may be from (2) or (3), so there are multiple
combinations that can cause ref underfow.
Let's address it by:
- check the return value in (2) as done
- change (1) to if (done)
    in this way, we only do ref put in (1) if 'oneshot flag' is from
    (2)
- do poll.done check in io_poll_task_func(), so that we won't put ref
  for the second time.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210922101238.7177-4-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ac0c06d..7707cdb 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5367,6 +5367,10 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
        } else {
                bool done;
 
+               if (req->poll.done) {
+                       spin_unlock(&ctx->completion_lock);
+                       return;
+               }
                done = __io_poll_complete(req, req->result);
                if (done) {
                        io_poll_remove_double(req);
@@ -5830,6 +5834,7 @@ static int io_poll_add(struct io_kiocb *req, unsigned int issue_flags)
        struct io_ring_ctx *ctx = req->ctx;
        struct io_poll_table ipt;
        __poll_t mask;
+       bool done;
 
        ipt.pt._qproc = io_poll_queue_proc;
 
@@ -5838,13 +5843,13 @@ static int io_poll_add(struct io_kiocb *req, unsigned int issue_flags)
 
        if (mask) { /* no async, we'd stolen it */
                ipt.error = 0;
-               io_poll_complete(req, mask);
+               done = io_poll_complete(req, mask);
        }
        spin_unlock(&ctx->completion_lock);
 
        if (mask) {
                io_cqring_ev_posted(ctx);
-               if (poll->events & EPOLLONESHOT)
+               if (done)
                        io_put_req(req);
        }
        return ipt.error;