CVE-2025-39673

About vulnerability

In the Linux kernel, the following vulnerability has been resolved:

ppp: fix race conditions in ppp_fill_forward_path

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.

Affected product:

AlmaLinux 9.2 ESU , TuxCare 9.6 ESU , Ubuntu 20.04 ELS

Affected packages:

linux @ 5.4.0 (+2 more)

In the Linux kernel, the following vulnerability has been resolved:

ppp: fix race conditions in ppp_fill_forward_path

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.