From af8d3c7c001ae7df1ed2b2715f058113efc86187 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Wed, 23 May 2018 14:37:38 -0700 Subject: ppp: remove the PPPIOCDETACH ioctl The PPPIOCDETACH ioctl effectively tries to "close" the given ppp file before f_count has reached 0, which is fundamentally a bad idea. It does check 'f_count < 2', which excludes concurrent operations on the file since they would only be possible with a shared fd table, in which case each fdget() would take a file reference. However, it fails to account for the fact that even with 'f_count == 1' the file can still be linked into epoll instances. As reported by syzbot, this can trivially be used to cause a use-after-free. Yet, the only known user of PPPIOCDETACH is pppd versions older than ppp-2.4.2, which was released almost 15 years ago (November 2003). Also, PPPIOCDETACH apparently stopped working reliably at around the same time, when the f_count check was added to the kernel, e.g. see https://lkml.org/lkml/2002/12/31/83. Also, the current 'f_count < 2' check makes PPPIOCDETACH only work in single-threaded applications; it always fails if called from a multithreaded application. All pppd versions released in the last 15 years just close() the file descriptor instead. Therefore, instead of hacking around this bug by exporting epoll internals to modules, and probably missing other related bugs, just remove the PPPIOCDETACH ioctl and see if anyone actually notices. Leave a stub in place that prints a one-time warning and returns EINVAL. Reported-by: syzbot+16363c99d4134717c05b@syzkaller.appspotmail.com Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Eric Biggers Acked-by: Paul Mackerras Reviewed-by: Guillaume Nault Tested-by: Guillaume Nault Signed-off-by: David S. Miller --- Documentation/networking/ppp_generic.txt | 6 ------ 1 file changed, 6 deletions(-) (limited to 'Documentation') diff --git a/Documentation/networking/ppp_generic.txt b/Documentation/networking/ppp_generic.txt index 091d20273dcb..61daf4b39600 100644 --- a/Documentation/networking/ppp_generic.txt +++ b/Documentation/networking/ppp_generic.txt @@ -300,12 +300,6 @@ unattached instance are: The ioctl calls available on an instance of /dev/ppp attached to a channel are: -* PPPIOCDETACH detaches the instance from the channel. This ioctl is - deprecated since the same effect can be achieved by closing the - instance. In order to prevent possible races this ioctl will fail - with an EINVAL error if more than one file descriptor refers to this - instance (i.e. as a result of dup(), dup2() or fork()). - * PPPIOCCONNECT connects this channel to a PPP interface. The argument should point to an int containing the interface unit number. It will return an EINVAL error if the channel is already -- cgit v1.2.3-59-g8ed1b