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ppp: fix race conditions in ppp_fill_forward_path

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ppp_fill_forward_path() has two race conditions:

1. The ppp->channels list can change between list_empty() and
   list_first_entry(), as ppp_lock() is not held. If the only channel
   is deleted in ppp_disconnect_channel(), list_first_entry() may
   access an empty head or a freed entry, and trigger a panic.

2. pch->chan can be NULL. When ppp_unregister_channel() is called,
   pch->chan is set to NULL before pch is removed from ppp->channels.

Fix these by using a lockless RCU approach:
- Use list_first_or_null_rcu() to safely test and access the first list
  entry.
- Convert list modifications on ppp->channels to their RCU variants and
  add synchronize_net() after removal.
- Check for a NULL pch->chan before dereferencing it.

Fixes: f6efc675c9 ("net: ppp: resolve forwarding path for bridge pppoe devices")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20250814012559.3705-2-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Qingfang Deng
2025-08-14 09:25:58 +08:00
committed by Paolo Abeni
parent 62c30c5443
commit 0417adf367
1 changed files with 11 additions and 6 deletions
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17
drivers/net/ppp/ppp_generic.c
View File

@@ -33,6 +33,7 @@
#include <linux/ppp_channel.h>
#include <linux/ppp-comp.h>
#include <linux/skbuff.h>
#include <linux/rculist.h>
#include <linux/rtnetlink.h>
#include <linux/if_arp.h>
#include <linux/ip.h>
@@ -1598,11 +1599,14 @@ static int ppp_fill_forward_path(struct net_device_path_ctx *ctx,
if (ppp->flags & SC_MULTILINK)
return -EOPNOTSUPP;
if (list_empty(&ppp->channels))
pch = list_first_or_null_rcu(&ppp->channels, struct channel, clist);
if (!pch)
return -ENODEV;
chan = READ_ONCE(pch->chan);
if (!chan)
return -ENODEV;
pch = list_first_entry(&ppp->channels, struct channel, clist);
chan = pch->chan;
if (!chan->ops->fill_forward_path)
return -EOPNOTSUPP;
@@ -2994,7 +2998,7 @@ ppp_unregister_channel(struct ppp_channel *chan)
*/
down_write(&pch->chan_sem);
spin_lock_bh(&pch->downl);
pch->chan = NULL;
WRITE_ONCE(pch->chan, NULL);
spin_unlock_bh(&pch->downl);
up_write(&pch->chan_sem);
ppp_disconnect_channel(pch);
@@ -3515,7 +3519,7 @@ ppp_connect_channel(struct channel *pch, int unit)
hdrlen = pch->file.hdrlen + 2; /* for protocol bytes */
if (hdrlen > ppp->dev->hard_header_len)
ppp->dev->hard_header_len = hdrlen;
list_add_tail(&pch->clist, &ppp->channels);
list_add_tail_rcu(&pch->clist, &ppp->channels);
++ppp->n_channels;
pch->ppp = ppp;
refcount_inc(&ppp->file.refcnt);
@@ -3545,10 +3549,11 @@ ppp_disconnect_channel(struct channel *pch)
if (ppp) {
/* remove it from the ppp unit's list */
ppp_lock(ppp);
list_del(&pch->clist);
list_del_rcu(&pch->clist);
if (--ppp->n_channels == 0)
wake_up_interruptible(&ppp->file.rwait);
ppp_unlock(ppp);
synchronize_net();
if (refcount_dec_and_test(&ppp->file.refcnt))
ppp_destroy_interface(ppp);
err = 0;