CVE-2025-39673

/// Technical description

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.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References