CVE-2026-93203: batman-adv: bla: avoid CRC corruption due to parallel claim add

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

batman-adv: bla: avoid CRC corruption due to parallel claim add

batadvblaaddclaim() is used to add claims and modify the backbone of claims for CLAIM frames from remote backbones and local packets. When it handles a claim, it needs to either

add the new claim's CRC to the backbone CRC remove the already existing claim's CRC from the old backbone and add it to the new backbone

But when the "new" claim code was running in parallel to the "change backbone" code, it can happen that the CRC was invalid because the backbonegw of the claim was changed twice in the "new" claim code path:

CPU0 creates the claim for gateway A and publishes it in the claim hash. The crc16 of the address has not yet been added to A's crc at this point.

CPU1 processes a claim frame of gateway B for the same client, finds the just published claim, and performs the ownership change: it switches the pointer to B, removes the crc16 from A's crc - which never contained it - and adds it to B's crc.

CPU0 continues behind the creation branch, unconditionally switches the pointer back to A without compensating B's crc (its removecrc is false for the creation path), and finally adds the crc16 to A's crc

The CRC is then wrong for both:

claim belongs to A: but CRC is not part of backbone A's CRC claim doesn't belong to B: CRC is still part of backbone B's CRC

This wrong CRC is never recomputated from the stored claims. For local backbone claims, this can also not recovered using syncs.

To avoid this, split the functionality in clear separate parts:

new claim which always adds claim CRC to the backbone CRC (but never changes the already set backbonegw of the claim back)

update of existing claim which automatically changes the backbonegw entry and only updates both backbone CRCs when there was an actual change