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epoll: Keep a reference on files added to the check list

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commit a9ed4a6560 upstream.

When adding a new fd to an epoll, and that this new fd is an
epoll fd itself, we recursively scan the fds attached to it
to detect cycles, and add non-epool files to a "check list"
that gets subsequently parsed.

However, this check list isn't completely safe when deletions
can happen concurrently. To sidestep the issue, make sure that
a struct file placed on the check list sees its f_count increased,
ensuring that a concurrent deletion won't result in the file
disapearing from under our feet.

Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Marc Zyngier
2020-08-19 17:12:17 +01:00
committed by Chris
parent 969ea19702
commit 673364c284
1 changed files with 7 additions and 2 deletions
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9
fs/eventpoll.c
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@@ -1749,9 +1749,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
* not already there, and calling reverse_path_check()
* during ep_insert().
*/
if (list_empty(&epi->ffd.file->f_tfile_llink))
if (list_empty(&epi->ffd.file->f_tfile_llink)) {
get_file(epi->ffd.file);
list_add(&epi->ffd.file->f_tfile_llink,
&tfile_check_list);
}
}
}
mutex_unlock(&ep->mtx);
@@ -1795,6 +1797,7 @@ static void clear_tfile_check_list(void)
file = list_first_entry(&tfile_check_list, struct file,
f_tfile_llink);
list_del_init(&file->f_tfile_llink);
fput(file);
}
INIT_LIST_HEAD(&tfile_check_list);
}
@@ -1949,9 +1952,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
clear_tfile_check_list();
goto error_tgt_fput;
}
} else
} else {
get_file(tf.file);
list_add(&tf.file->f_tfile_llink,
&tfile_check_list);
}
mutex_lock_nested(&ep->mtx, 0);
if (is_file_epoll(tf.file)) {
tep = tf.file->private_data;