CVE-2026-46086

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.