In the Linux kernel, the following vulnerability has been resolved:
bpf: Reset register bounds before narrowing retval range in check_mem_access()
When the BPF verifier processes a context load of an LSM hook return value, it calls __mark_reg_s32_range() to narrow the register to the hooks valid range. However, __mark_reg_s32_range() intersects the new range with the registers existing bounds using max_t()/min_t() rather than replacing them.
If the destination register carries stale bounds from a prior instruction (e.g. BPF_MOV64_IMM), the intersection can produce a range narrower than reality. The verifier then believes it knows the registers exact value, while at runtime the actual hook return value is loaded, creating a verifier/runtime mismatch that can be used to bypass BPF memory safety checks.
The else branch already calls mark_reg_unknown() to reset register state before any narrowing. Apply the same reset in the is_retval path so stale bounds are cleared before __mark_reg_s32_range() intersects.