CVE-2026-53081

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

bpf: Enforce regsafe base id consistency for BPF_ADD_CONST scalars

When regsafe() compares two scalar registers that both carry

BPF_ADD_CONST, check_scalar_ids() maps their full compound id

(aka base | BPF_ADD_CONST flag) as one idmap entry. However,

it never verifies that the underlying base ids, that is, with

the flag stripped are consistent with existing idmap mappings.

This allows construction of two verifier states where the old

state has R3 = R2 + 10 (both sharing base id A) while the current

state has R3 = R4 + 10 (base id C, unrelated to R2). The idmap

creates two independent entries: A->B (for R2) and A|flag->C|flag

(for R3), without catching that A->C conflicts with A->B. State

pruning then incorrectly succeeds.

Fix this by additionally verifying base ID mapping consistency

whenever BPF_ADD_CONST is set: after mapping the compound ids,

also invoke check_ids() on the base IDs (flag bits stripped).

This ensures that if A was already mapped to B from comparing

the source register, any ADD_CONST derivative must also derive

from B, not an unrelated C.