Medium [CVE-2026-72426] Preserve pointer spill metadata during half-slot cleanup

In the Linux kernel, the following vulnerability has been resolved: bpf: Preserve pointer spill metadata during half-slot cleanup __clean_func_state() cleans dead stack slots in 4-byte halves.

When the high half of a STACK_SPILL slot is dead and the low half remains live, cleanup converts the live low half to STACK_MISC or STACK_ZERO and clears the saved spilled_ptr metadata. That conversion is safe only for scalar spills.

For a pointer spill, this metadata clear lets a later 32-bit fill from the still-live half avoid the normal non-scalar register-fill check and be treated as an ordinary scalar stack read. Leave non-scalar spill slots intact in this half-live shape.

This is conservative for pruning and preserves the existing check_stack_read_fixed_off() rejection path for partial fills from pointer spills. A flaw was found in the Linux kernel's Berkeley Packet Filter (BPF) subsystem.

The `__clean_func_state()` function, which manages stack slot cleanup, incorrectly clears metadata for pointer spills. This allows a subsequent read from a partially live stack slot to bypass crucial non-scalar register-fill checks, treating a pointer as a scalar value.

This vulnerability could lead to a security bypass, potentially enabling information disclosure or other memory corruption issues. Red Hat severity: Moderate — CVSS 5.5 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).