| Message ID | 20180523035952.25768-1-ebiggers3@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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.
From: Guillaume Nault <g.nault@alphalink.fr> Date: Wed, 23 May 2018 15:57:08 +0200 > 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. I agree.
On Wed, May 23, 2018 at 11:56:36AM -0400, David Miller wrote: > From: Guillaume Nault <g.nault@alphalink.fr> > Date: Wed, 23 May 2018 15:57:08 +0200 > > > 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. > > I agree. Okay, I'll do that and leave the ioctl number reserved. I will add a pr_warn_once() too. Thanks, - Eric
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; - } - if (pf->kind == CHANNEL) { struct channel *pch; struct ppp_channel *chan; diff --git a/fs/compat_ioctl.c b/fs/compat_ioctl.c index ef80085ed564..8285b570d635 100644 --- a/fs/compat_ioctl.c +++ b/fs/compat_ioctl.c @@ -917,7 +917,6 @@ COMPATIBLE_IOCTL(PPPIOCSDEBUG) /* PPPIOCGIDLE is translated */ COMPATIBLE_IOCTL(PPPIOCNEWUNIT) COMPATIBLE_IOCTL(PPPIOCATTACH) -COMPATIBLE_IOCTL(PPPIOCDETACH) COMPATIBLE_IOCTL(PPPIOCSMRRU) COMPATIBLE_IOCTL(PPPIOCCONNECT) COMPATIBLE_IOCTL(PPPIOCDISCONN) diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h index b19a9c249b15..d46caf217ea4 100644 --- a/include/uapi/linux/ppp-ioctl.h +++ b/include/uapi/linux/ppp-ioctl.h @@ -106,7 +106,6 @@ struct pppol2tp_ioc_stats { #define PPPIOCGIDLE _IOR('t', 63, struct ppp_idle) /* get idle time */ #define PPPIOCNEWUNIT _IOWR('t', 62, int) /* create new ppp unit */ #define PPPIOCATTACH _IOW('t', 61, int) /* attach to ppp unit */ -#define PPPIOCDETACH _IOW('t', 60, int) /* detach from ppp unit/chan */ #define PPPIOCSMRRU _IOW('t', 59, int) /* set multilink MRU */ #define PPPIOCCONNECT _IOW('t', 58, int) /* connect channel to unit */ #define PPPIOCDISCONN _IO('t', 57) /* disconnect channel */