In the Linux kernel, the following vulnerability has been resolved: bpf: Fix REG INVARIANTS VIOLATION on speculative pointer arithmetic Take the following unprivileged program as an example: r0 = bpf_map_lookup_elem(...) /* PTR_TO_MA…

Overview

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

bpf: Fix REG INVARIANTS VIOLATION on speculative pointer arithmetic

Take the following unprivileged program as an example:

r0 = bpf_map_lookup_elem(...)	/* PTR_TO_MAP_VALUE, offset 0 */
...
14: r0 += r1			/* r1 is a bounded scalar */
15: r9 = r0

Loading it triggers a verifier warning from reg_bounds_sanity_check():

verifier bug: REG INVARIANTS VIOLATION (alu): const subreg tnum out
of sync with range bounds r64={.base=0x0, .size=0x0}
r32={.base=0x0, .size=0xffffffff} var_off=(0x0, 0x0)

What happens:

1. Processing insn 14 (r0 += r1) in adjust_ptr_min_max_vals(), the new offset is computed into dst_reg's var_off and 32/64-bit ranges.

2. Because pointer registers do not track 32-bit subregister bounds, __mark_reg32_unbounded() first sets r32 to the full range; r32 is re-derived from the offset at the end of the function by reg_bounds_sync().

3. On the unprivileged path, sanitize_ptr_alu() is called and, via sanitize_speculative_path() -> push_stack(), snapshots the current register state and schedules the next instruction (insn 15) to be verified directly as a speculative path.

4. That snapshot is taken between step 2 and the final reg_bounds_sync(): at this point dst_reg's var_off still holds the (const) original offset while r32 has just been blanked to the full range, i.e. the two are out of sync. When the speculative path later verifies insn 15 (r9 = r0), the inconsistent state reaches reg_bounds_sanity_check() and trips the warning.

var_off and the 32-bit range must always be consistent. There are two ways to keep the snapshot consistent:

1. sync var_off and r32 before the snapshot so they match, or
2. leave r32 at its original (already consistent) value and blank it only after the snapshot.

The whole point of sanitize_ptr_alu() is to insert a harmless masking sequence that keeps the access in bounds under speculation, so the state it snapshots should faithfully represent that. Take approach 2: move __mark_reg32_unbounded() to after sanitize_ptr_alu(), so the speculative snapshot keeps the pointer's original, consistent r32. The non-speculative path is unchanged: r32 is still blanked before the offset is applied and re-derived by reg_bounds_sync().

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.