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SubjectRe: [PATCH] ppp: remove the PPPIOCDETACH ioctl
On Tue, May 22, 2018 at 08:59:52PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> 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.
>
> Reported-by: syzbot+16363c99d4134717c05b@syzkaller.appspotmail.com
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> Documentation/networking/ppp_generic.txt | 6 -----
> drivers/net/ppp/ppp_generic.c | 29 ------------------------
> fs/compat_ioctl.c | 1 -
> include/uapi/linux/ppp-ioctl.h | 1 -
> 4 files changed, 37 deletions(-)
>
> 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
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index dc7c7ec43202..dce8812fe802 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -603,35 +603,6 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> goto out;
> }
>
> - if (cmd == PPPIOCDETACH) {
> - /*
> - * We have to be careful here... if the file descriptor
> - * has been dup'd, we could have another process in the
> - * middle of a poll using the same file *, so we had
> - * better not free the interface data structures -
> - * instead we fail the ioctl. Even in this case, we
> - * shut down the interface if we are the owner of it.
> - * Actually, we should get rid of PPPIOCDETACH, userland
> - * (i.e. pppd) could achieve the same effect by closing
> - * this fd and reopening /dev/ppp.
> - */
> - err = -EINVAL;
> - if (pf->kind == INTERFACE) {
> - ppp = PF_TO_PPP(pf);
> - rtnl_lock();
> - if (file == ppp->owner)
> - unregister_netdevice(ppp->dev);
> - rtnl_unlock();
> - }
> - if (atomic_long_read(&file->f_count) < 2) {
> - ppp_release(NULL, file);
> - err = 0;
> - } else
> - pr_warn("PPPIOCDETACH file->f_count=%ld\n",
> - atomic_long_read(&file->f_count));
> - goto out;
> - }
> -

I'd rather add
+ if (cmd == PPPIOCDETACH) {
+ err = -EINVAL;
+ goto out;
+ }

Making PPPIOCDETACH unknown to ppp_generic means that the ioctl would
be handled by the underlying channel when pf->kind == CHANNEL (see the
chan->ops->ioctl() call further down). That shouldn't be a problem per
se, but even though PPPIOCDETACH is unsupported, I feel that it should
remain a ppp_generic thing. I don't really want its value to be reused
for other purposes in the future or have different behaviour depending
on the underlying channel.

Also PPPIOCDETACH can already fail with -EINVAL. Therefore, if ever
there really were programs out there using this call, they'd already
have to handle this case. Unconditionally returning -EINVAL would
further minimise possibilities for breakage.

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