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select: Fix indefinitely sleeping task in poll_schedule_timeout()

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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>
This commit is contained in:
Jan Kara 2022-01-10 19:19:23 +01:00 committed by Linus Torvalds
parent fe8152b38d
commit 68514dacf2
1 changed files with 33 additions and 30 deletions
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63
fs/select.c
View File

@ -459,9 +459,11 @@ 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,
@ -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,
@ -535,34 +538,34 @@ 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;