On Tue, May 22, 2018 at 08:59:52PM -0700, Eric Biggers wrote: > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > --- > 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.
