[PATCH] ppp: remove the PPPIOCDETACH ioctl

[PATCH] ppp: remove the PPPIOCDETACH ioctl

[Eric Biggers]

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;
-	}
-
 	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 */
-- 
2.17.0

Re: [PATCH] ppp: remove the PPPIOCDETACH ioctl

[Guillaume Nault]

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.

Re: [PATCH] ppp: remove the PPPIOCDETACH ioctl

[David Miller]

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.

Re: [PATCH] ppp: remove the PPPIOCDETACH ioctl

[Eric Biggers]

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

[PATCH v2] ppp: remove the PPPIOCDETACH ioctl

[Eric Biggers]

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.  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")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---

v2: leave a stub in place, rather than removing the ioctl completely.

 Documentation/networking/ppp_generic.txt |  6 ------
 drivers/net/ppp/ppp_generic.c            | 27 +++++-------------------
 include/uapi/linux/ppp-ioctl.h           |  2 +-
 3 files changed, 6 insertions(+), 29 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..02ad03a2fab7 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -605,30 +605,13 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	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.
+		 * PPPIOCDETACH is no longer supported as it was heavily broken,
+		 * and is only known to have been used by pppd older than
+		 * ppp-2.4.2 (released November 2003).
 		 */
+		pr_warn_once("%s (%d) used obsolete PPPIOCDETACH ioctl\n",
+			     current->comm, current->pid);
 		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;
 	}
 
diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h
index b19a9c249b15..784c2e3e572e 100644
--- a/include/uapi/linux/ppp-ioctl.h
+++ b/include/uapi/linux/ppp-ioctl.h
@@ -106,7 +106,7 @@ 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 PPPIOCDETACH	_IOW('t', 60, int)	/* obsolete, do not use */
 #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 */
-- 
2.17.0.441.gb46fe60e1d-goog

Re: [PATCH v2] ppp: remove the PPPIOCDETACH ioctl

[Paul Mackerras]

On Wed, May 23, 2018 at 02:37:38PM -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.  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")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Paul Mackerras <paulus@ozlabs.org>

Re: [PATCH v2] ppp: remove the PPPIOCDETACH ioctl

[Guillaume Nault]

On Wed, May 23, 2018 at 02:37:38PM -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.  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")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> 
> v2: leave a stub in place, rather than removing the ioctl completely.
> 
Thanks a lot for your help on this matter.

BTW, netdev has its own rules wrt. stable backports. You didn't need to
CC: stable@. David handles -stable submissions himself.
Using a 'PATCH net' subject prefix would have made it clear that this
patch was fixing some released code and should be considered for -stable
backport.

Reviewed-by: Guillaume Nault <g.nault@alphalink.fr>
Tested-by: Guillaume Nault <g.nault@alphalink.fr>

Re: [PATCH v2] ppp: remove the PPPIOCDETACH ioctl

[David Miller]

From: Eric Biggers <ebiggers3@gmail.com>
Date: Wed, 23 May 2018 14:37:38 -0700

> 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.  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 <ebiggers@google.com>

Applied and queued up for -stable.

Re: [PATCH v2] ppp: remove the PPPIOCDETACH ioctl

[Walter Harms]

> Eric Biggers <ebiggers3@gmail.com> hat am 23. Mai 2018 um 23:37 geschrieben:
> 
> 
> 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.  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")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> 
> v2: leave a stub in place, rather than removing the ioctl completely.
> 
>  Documentation/networking/ppp_generic.txt |  6 ------
>  drivers/net/ppp/ppp_generic.c            | 27 +++++-------------------
>  include/uapi/linux/ppp-ioctl.h           |  2 +-
>  3 files changed, 6 insertions(+), 29 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..02ad03a2fab7 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -605,30 +605,13 @@ static long ppp_ioctl(struct file *file, unsigned int
> cmd, unsigned long arg)
>  
>  	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.
> +		 * PPPIOCDETACH is no longer supported as it was heavily broken,
> +		 * and is only known to have been used by pppd older than
> +		 * ppp-2.4.2 (released November 2003).
>  		 */
> +		pr_warn_once("%s (%d) used obsolete PPPIOCDETACH ioctl\n",
> +			     current->comm, current->pid);
>  		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;
>  	}
>  
> diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h
> index b19a9c249b15..784c2e3e572e 100644
> --- a/include/uapi/linux/ppp-ioctl.h
> +++ b/include/uapi/linux/ppp-ioctl.h
> @@ -106,7 +106,7 @@ 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 PPPIOCDETACH	_IOW('t', 60, int)	/* obsolete, do not use */

It's a bit late but ...
i would suggest a stronger wording here. like /* removed 2003 */ 
for me "obsolete" sounds like "maybe it will be removed in a distant future"

just my 2 cents,

re,
 wh

>  #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 */
> -- 
> 2.17.0.441.gb46fe60e1d-goog
> 
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