select: Fix indefinitely sleeping task in poll_schedule_timeout()

A task can end up indefinitely sleeping in do_select() ->
poll_schedule_timeout() when the following race happens:

  TASK1 (thread1)             TASK2                   TASK1 (thread2)
  do_select()
    setup poll_wqueues table
    with 'fd'
                              write data to 'fd'
                                pollwake()
                                  table->triggered = 1
                                                      closes 'fd' thread1 is
                                                        waiting for
    poll_schedule_timeout()
      - sees table->triggered
      table->triggered = 0
      return -EINTR
    loop back in do_select()

But at this point when TASK1 loops back, the fdget() in the setup of
poll_wqueues fails.  So now so we never find 'fd' is ready for reading
and sleep in poll_schedule_timeout() indefinitely.

Treat an fd that got closed as a fd on which some event happened.  This
makes sure cannot block indefinitely in do_select().

Another option would be to return -EBADF in this case but that has a
potential of subtly breaking applications that excercise this behavior
and it happens to work for them.  So returning fd as active seems like a
safer choice.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/select.c

@@ -459,9 +459,11 @@ static int max_select_fd(unsigned long n, fd_set_bits *fds)
 	return max;
 }
 
-#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR)
-#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR)
-#define POLLEX_SET (EPOLLPRI)
+#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR |\
+			EPOLLNVAL)
+#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR |\
+			 EPOLLNVAL)
+#define POLLEX_SET (EPOLLPRI | EPOLLNVAL)
 
 static inline void wait_key_set(poll_table *wait, unsigned long in,
 				unsigned long out, unsigned long bit,
@@ -528,6 +530,7 @@ static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
 					break;
 				if (!(bit & all_bits))
 					continue;
+				mask = EPOLLNVAL;
 				f = fdget(i);
 				if (f.file) {
 					wait_key_set(wait, in, out, bit,
@@ -535,6 +538,7 @@ static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
 					mask = vfs_poll(f.file, wait);
 
 					fdput(f);
+				}
 				if ((mask & POLLIN_SET) && (in & bit)) {
 					res_in |= bit;
 					retval++;
@@ -563,7 +567,6 @@ static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
 					can_busy_loop = true;
 
 			}
-			}
 			if (res_in)
 				*rinp = res_in;
 			if (res_out)