ppp: fix race conditions in ppp_fill_forward_path

Vulnerability 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.

References

• https://git.kernel.org/stable/c/9a1969fbffc1f1900d92d7594b1b7d8d72ef3dc7
• https://git.kernel.org/stable/c/0f1630be6fcca3f0c63e4b242ad202e5cde28a40
• https://git.kernel.org/stable/c/ca18d751bcc9faf5b7e82e9fae1223d103928181
• https://git.kernel.org/stable/c/94731cc551e29511d85aa8dec61a6c071b1f2430
• https://git.kernel.org/stable/c/f97f6475fdcb3c28ff3c55cc4b7bde632119ec08
• https://git.kernel.org/stable/c/0417adf367a0af11adf7ace849af4638cfb573f7