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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>
Subject: [PATCH 4.19 3/3] select: Fix indefinitely sleeping task in poll_schedule_timeout()
Date:   Thu, 27 Jan 2022 19:09:03 +0100
Message-Id: <20220127180256.943514512@linuxfoundation.org>
In-Reply-To: <20220127180256.837257619@linuxfoundation.org>
References: <20220127180256.837257619@linuxfoundation.org>
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From: Jan Kara <jack@suse.cz>

commit 68514dacf2715d11b91ca50d88de047c086fea9c upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/select.c |   63 +++++++++++++++++++++++++++++++-----------------------------
 1 file changed, 33 insertions(+), 30 deletions(-)

--- a/fs/select.c
+++ b/fs/select.c
@@ -431,9 +431,11 @@ get_max:
 	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,
@@ -500,6 +502,7 @@ static int do_select(int n, fd_set_bits
 					break;
 				if (!(bit & all_bits))
 					continue;
+				mask = EPOLLNVAL;
 				f = fdget(i);
 				if (f.file) {
 					wait_key_set(wait, in, out, bit,
@@ -507,34 +510,34 @@ static int do_select(int n, fd_set_bits
 					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;