[PATCH v2] select: Fix indefinitely sleeping task in poll_schedule_timeout()

[Jan Kara <jack@suse.cz>]

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 fdget() in the setup of poll_wqueues
fails now so we never find 'fd' is ready for reading and sleep in
poll_schedule_timeout() indefinitely.

Treat 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>
---
 fs/select.c | 63 ++++++++++++++++++++++++++++-------------------------
 1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 945896d0ac9e..5edffee1162c 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -458,9 +458,11 @@ 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,
@@ -527,6 +529,7 @@ 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,
@@ -534,34 +537,34 @@ 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++;
-						wait->_qproc = NULL;
-					}
-					if ((mask & POLLOUT_SET) && (out & bit)) {
-						res_out |= bit;
-						retval++;
-						wait->_qproc = NULL;
-					}
-					if ((mask & POLLEX_SET) && (ex & bit)) {
-						res_ex |= bit;
-						retval++;
-						wait->_qproc = NULL;
-					}
-					/* got something, stop busy polling */
-					if (retval) {
-						can_busy_loop = false;
-						busy_flag = 0;
-
-					/*
-					 * only remember a returned
-					 * POLL_BUSY_LOOP if we asked for it
-					 */
-					} else if (busy_flag & mask)
-						can_busy_loop = true;
-
 				}
+				if ((mask & POLLIN_SET) && (in & bit)) {
+					res_in |= bit;
+					retval++;
+					wait->_qproc = NULL;
+				}
+				if ((mask & POLLOUT_SET) && (out & bit)) {
+					res_out |= bit;
+					retval++;
+					wait->_qproc = NULL;
+				}
+				if ((mask & POLLEX_SET) && (ex & bit)) {
+					res_ex |= bit;
+					retval++;
+					wait->_qproc = NULL;
+				}
+				/* got something, stop busy polling */
+				if (retval) {
+					can_busy_loop = false;
+					busy_flag = 0;
+
+				/*
+				 * only remember a returned
+				 * POLL_BUSY_LOOP if we asked for it
+				 */
+				} else if (busy_flag & mask)
+					can_busy_loop = true;
+
 			}
 			if (res_in)
 				*rinp = res_in;
-- 
2.31.1