In the Linux kernel, the following vulnerability has been resolved: net: bridge: use a stable FDB dst snapshot in RCU readers Local FDB entries can be rewritten in place by `fdb_delete_local()`, which updates `f->dst` to another port o…

Overview

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

net: bridge: use a stable FDB dst snapshot in RCU readers

Local FDB entries can be rewritten in place by fdb_delete_local(), which updates f->dst to another port or to NULL while keeping the entry alive. Several bridge RCU readers inspect f->dst, including br_fdb_fillbuf() through the brforward_read() sysfs path.

These readers currently load f->dst multiple times and can therefore observe inconsistent values across the check and later dereference. In br_fdb_fillbuf(), this means a concurrent local-FDB update can change f->dst after the NULL check and before the port_no dereference, leading to a NULL-ptr-deref.

Fix this by taking a single READ_ONCE() snapshot of f->dst in each affected RCU reader and using that snapshot for the rest of the access sequence. Also publish the in-place f->dst updates in fdb_delete_local() with WRITE_ONCE() so the readers and writer use matching access patterns.

Affected

• linux_kernel >= 3.14.1, < 5.10.259
• linux_kernel >= 5.11, < 5.15.210
• linux_kernel >= 5.16, < 6.1.176
• linux_kernel >= 6.2, < 6.6.140
• linux_kernel >= 6.7, < 6.12.86
• linux_kernel >= 6.13, < 6.18.27
• linux_kernel >= 6.19, < 7.0.4
• linux_kernel = 3.14

Remediation

Upgrade past the affected range:

• linux_kernel 7.0.4