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Merge branch 'add-ppp_generic-ioctls-to-bridge-channels'

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Tom Parkin says:

====================
add ppp_generic ioctl(s) to bridge channels
Following on from my previous RFC[1], this series adds two ioctl calls
to the ppp code to implement "channel bridging".

When two ppp channels are bridged, frames presented to ppp_input() on
one channel are passed to the other channel's ->start_xmit function for
transmission.

The primary use-case for this functionality is in an L2TP Access
Concentrator where PPP frames are typically presented in a PPPoE session
(e.g. from a home broadband user) and are forwarded to the ISP network in
a PPPoL2TP session.

The two new ioctls, PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN form a
symmetric pair.

Userspace code testing and illustrating use of the ioctl calls is
available in the go-l2tp[2] and l2tp-ktest[3] repositories.

[1]. Previous RFC series:

https://lore.kernel.org/netdev/20201106181647.16358-1-tparkin@katalix.com/

[2]. go-l2tp: a Go library for building L2TP applications on Linux
systems. Support for the PPPIOCBRIDGECHAN ioctl is on a branch:

https://github.com/katalix/go-l2tp/tree/tp_002_pppoe_2

[3]. l2tp-ktest: a test suite for the Linux Kernel L2TP subsystem.
Support for the PPPIOCBRIDGECHAN ioctl is on a branch:

https://github.com/katalix/l2tp-ktest/tree/tp_ac_pppoe_tests_2

Changelog:

v4:
    * Fix NULL-pointer access in PPPIOCBRIDGECHAN in the case that the
      ID of the channel to be bridged wasn't found.
    * Add comment in ppp_unbridge_channels to better document the
      unbridge process.

v3:
    * Use rcu_dereference_protected for accessing struct channel
      'bridge' field during updates with lock 'upl' held.
    * Avoid race in ppp_unbridge_channels by ensuring that each channel
      in the bridge points to it's peer before decrementing refcounts.

v2:
    * Add missing __rcu annotation to struct channel 'bridge' field in
      order to squash a sparse warning from a C=1 build
    * Integrate review comments from gnault@redhat.com
    * Have ppp_unbridge_channels return -EINVAL if the channel isn't
      part of a bridge: this better aligns with the return code from
      ppp_disconnect_channel.
    * Improve docs update by including information on ioctl arguments
      and error return codes.
====================

Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2020-12-10 13:57:36 -08:00
commit 91163f8214
3 changed files with 167 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
Documentation/networking/ppp_generic.rst
View File

@ -314,6 +314,22 @@ channel are:
it is connected to. It will return an EINVAL error if the channel
is not connected to an interface.
* PPPIOCBRIDGECHAN bridges a channel with another. The argument should
point to an int containing the channel number of the channel to bridge
to. Once two channels are bridged, frames presented to one channel by
ppp_input() are passed to the bridge instance for onward transmission.
This allows frames to be switched from one channel into another: for
example, to pass PPPoE frames into a PPPoL2TP session. Since channel
bridging interrupts the normal ppp_input() path, a given channel may
not be part of a bridge at the same time as being part of a unit.
This ioctl will return an EALREADY error if the channel is already
part of a bridge or unit, or ENXIO if the requested channel does not
exist.
* PPPIOCUNBRIDGECHAN performs the inverse of PPPIOCBRIDGECHAN, unbridging
a channel pair. This ioctl will return an EINVAL error if the channel
does not form part of a bridge.
* All other ioctl commands are passed to the channel ioctl() function.
The ioctl calls that are available on an instance that is attached to

152
drivers/net/ppp/ppp_generic.c
View File

@ -174,7 +174,8 @@ struct channel {
struct ppp *ppp; /* ppp unit we're connected to */
struct net *chan_net; /* the net channel belongs to */
struct list_head clist; /* link in list of channels per unit */
rwlock_t upl; /* protects `ppp' */
rwlock_t upl; /* protects `ppp' and 'bridge' */
struct channel __rcu *bridge; /* "bridged" ppp channel */
#ifdef CONFIG_PPP_MULTILINK
u8 avail; /* flag used in multilink stuff */
u8 had_frag; /* >= 1 fragments have been sent */
@ -606,6 +607,83 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p)
#endif
#endif
/* Bridge one PPP channel to another.
* When two channels are bridged, ppp_input on one channel is redirected to
* the other's ops->start_xmit handler.
* In order to safely bridge channels we must reject channels which are already
* part of a bridge instance, or which form part of an existing unit.
* Once successfully bridged, each channel holds a reference on the other
* to prevent it being freed while the bridge is extant.
*/
static int ppp_bridge_channels(struct channel *pch, struct channel *pchb)
{
write_lock_bh(&pch->upl);
if (pch->ppp ||
rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) {
write_unlock_bh(&pch->upl);
return -EALREADY;
}
rcu_assign_pointer(pch->bridge, pchb);
write_unlock_bh(&pch->upl);
write_lock_bh(&pchb->upl);
if (pchb->ppp ||
rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl))) {
write_unlock_bh(&pchb->upl);
goto err_unset;
}
rcu_assign_pointer(pchb->bridge, pch);
write_unlock_bh(&pchb->upl);
refcount_inc(&pch->file.refcnt);
refcount_inc(&pchb->file.refcnt);
return 0;
err_unset:
write_lock_bh(&pch->upl);
RCU_INIT_POINTER(pch->bridge, NULL);
write_unlock_bh(&pch->upl);
synchronize_rcu();
return -EALREADY;
}
static int ppp_unbridge_channels(struct channel *pch)
{
struct channel *pchb, *pchbb;
write_lock_bh(&pch->upl);
pchb = rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl));
if (!pchb) {
write_unlock_bh(&pch->upl);
return -EINVAL;
}
RCU_INIT_POINTER(pch->bridge, NULL);
write_unlock_bh(&pch->upl);
/* Only modify pchb if phcb->bridge points back to pch.
* If not, it implies that there has been a race unbridging (and possibly
* even rebridging) pchb. We should leave pchb alone to avoid either a
* refcount underflow, or breaking another established bridge instance.
*/
write_lock_bh(&pchb->upl);
pchbb = rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl));
if (pchbb == pch)
RCU_INIT_POINTER(pchb->bridge, NULL);
write_unlock_bh(&pchb->upl);
synchronize_rcu();
if (pchbb == pch)
if (refcount_dec_and_test(&pch->file.refcnt))
ppp_destroy_channel(pch);
if (refcount_dec_and_test(&pchb->file.refcnt))
ppp_destroy_channel(pchb);
return 0;
}
static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct ppp_file *pf;
@ -641,8 +719,9 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
}
if (pf->kind == CHANNEL) {
struct channel *pch;
struct channel *pch, *pchb;
struct ppp_channel *chan;
struct ppp_net *pn;
pch = PF_TO_CHANNEL(pf);
@ -657,6 +736,31 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
err = ppp_disconnect_channel(pch);
break;
case PPPIOCBRIDGECHAN:
if (get_user(unit, p))
break;
err = -ENXIO;
pn = ppp_pernet(current->nsproxy->net_ns);
spin_lock_bh(&pn->all_channels_lock);
pchb = ppp_find_channel(pn, unit);
/* Hold a reference to prevent pchb being freed while
* we establish the bridge.
*/
if (pchb)
refcount_inc(&pchb->file.refcnt);
spin_unlock_bh(&pn->all_channels_lock);
if (!pchb)
break;
err = ppp_bridge_channels(pch, pchb);
/* Drop earlier refcount now bridge establishment is complete */
if (refcount_dec_and_test(&pchb->file.refcnt))
ppp_destroy_channel(pchb);
break;
case PPPIOCUNBRIDGECHAN:
err = ppp_unbridge_channels(pch);
break;
default:
down_read(&pch->chan_sem);
chan = pch->chan;
@ -2089,6 +2193,40 @@ static bool ppp_decompress_proto(struct sk_buff *skb)
return pskb_may_pull(skb, 2);
}
/* Attempt to handle a frame via. a bridged channel, if one exists.
* If the channel is bridged, the frame is consumed by the bridge.
* If not, the caller must handle the frame by normal recv mechanisms.
* Returns true if the frame is consumed, false otherwise.
*/
static bool ppp_channel_bridge_input(struct channel *pch, struct sk_buff *skb)
{
struct channel *pchb;
rcu_read_lock();
pchb = rcu_dereference(pch->bridge);
if (!pchb)
goto out_rcu;
spin_lock(&pchb->downl);
if (!pchb->chan) {
/* channel got unregistered */
kfree_skb(skb);
goto outl;
}
skb_scrub_packet(skb, !net_eq(pch->chan_net, pchb->chan_net));
if (!pchb->chan->ops->start_xmit(pchb->chan, skb))
kfree_skb(skb);
outl:
spin_unlock(&pchb->downl);
out_rcu:
rcu_read_unlock();
/* If pchb is set then we've consumed the packet */
return !!pchb;
}
void
ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
{
@ -2100,6 +2238,10 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
return;
}
/* If the channel is bridged, transmit via. bridge */
if (ppp_channel_bridge_input(pch, skb))
return;
read_lock_bh(&pch->upl);
if (!ppp_decompress_proto(skb)) {
kfree_skb(skb);
@ -2796,8 +2938,11 @@ ppp_unregister_channel(struct ppp_channel *chan)
list_del(&pch->list);
spin_unlock_bh(&pn->all_channels_lock);
ppp_unbridge_channels(pch);
pch->file.dead = 1;
wake_up_interruptible(&pch->file.rwait);
if (refcount_dec_and_test(&pch->file.refcnt))
ppp_destroy_channel(pch);
}
@ -3270,7 +3415,8 @@ ppp_connect_channel(struct channel *pch, int unit)
goto out;
write_lock_bh(&pch->upl);
ret = -EINVAL;
if (pch->ppp)
if (pch->ppp ||
rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl)))
goto outl;
ppp_lock(ppp);

2
include/uapi/linux/ppp-ioctl.h
View File

@ -115,6 +115,8 @@ struct pppol2tp_ioc_stats {
#define PPPIOCATTCHAN _IOW('t', 56, int) /* attach to ppp channel */
#define PPPIOCGCHAN _IOR('t', 55, int) /* get ppp channel number */
#define PPPIOCGL2TPSTATS _IOR('t', 54, struct pppol2tp_ioc_stats)
#define PPPIOCBRIDGECHAN _IOW('t', 53, int) /* bridge one channel to another */
#define PPPIOCUNBRIDGECHAN _IO('t', 54) /* unbridge channel */
#define SIOCGPPPSTATS (SIOCDEVPRIVATE + 0)
#define SIOCGPPPVER (SIOCDEVPRIVATE + 1) /* NEVER change this!! */