CVE-2026-46086net: bridge: use a stable FDB dst snapshot in RCU readers

Official description

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 packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/c502fa9f094cb03d1d1685c71e2105ab359bc2b8
• https://git.kernel.org/stable/c/a6ae4511c07b91f597e461406c6330f0d4ff810e
• https://git.kernel.org/stable/c/1406c4e0ed1eaf8a29801ab1163d00fb7ee4359a
• https://git.kernel.org/stable/c/0b9e4bbfb7c949151e3acd44ed4aa33614d2e110
• https://git.kernel.org/stable/c/81af4137a30c4c2dc694dea8cacb180bd66000ef
• https://git.kernel.org/stable/c/5424e678f9b304e148cf5dcc047cffc7a56a3bb5
• https://git.kernel.org/stable/c/9a2d9d4e657b23dc21f24cf139e3aeff0b61341f
• https://git.kernel.org/stable/c/df4601653201de21b487c3e7fffd464790cab808
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html
• https://cert-portal.siemens.com/productcert/html/ssa-784472.html