This is a note to let you know that I've just added the patch titled select: Fix indefinitely sleeping task in poll_schedule_timeout() to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: select-fix-indefinitely-sleeping-task-in-poll_schedule_timeout.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 68514dacf2715d11b91ca50d88de047c086fea9c Mon Sep 17 00:00:00 2001 From: Jan Kara <jack@xxxxxxx> Date: Mon, 10 Jan 2022 19:19:23 +0100 Subject: select: Fix indefinitely sleeping task in poll_schedule_timeout() From: Jan Kara <jack@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> CC: stable@xxxxxxxxxxxxxxx Signed-off-by: Jan Kara <jack@xxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/select.c | 63 +++++++++++++++++++++++++++++++----------------------------- 1 file changed, 33 insertions(+), 30 deletions(-) --- a/fs/select.c +++ b/fs/select.c @@ -458,9 +458,11 @@ 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, @@ -527,6 +529,7 @@ 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, @@ -534,34 +537,34 @@ 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; Patches currently in stable-queue which might be from jack@xxxxxxx are queue-5.15/select-fix-indefinitely-sleeping-task-in-poll_schedule_timeout.patch