Re: [PATCH bpf-next 1/2] bpf: Skip bounds adjustment for conditional jumps on same register

On Fri, Oct 24, 2025 at 9:38 AM KaFai Wan kafai.wan@linux.dev wrote:

On Fri, 2025-10-24 at 09:21 -0700, Eduard Zingerman wrote:

...

On Sat, 2025-10-25 at 00:13 +0800, KaFai Wan wrote:

[...]

...

For non-scalar cases we only allow pointer comparison on pkt_ptr, this check is before is_branch_taken()

src_reg = &regs[insn->src_reg];
if (!(reg_is_pkt_pointer_any(dst_reg) && reg_is_pkt_pointer_any(src_reg)) &&
    is_pointer_value(env, insn->src_reg)) {
        verbose(env, "R%d pointer comparison prohibited\n",
                insn->src_reg);
        return -EACCES;
}

and in the end of check_cond_jmp_op() (after is_branch_taken()), we checked again

} else if (!try_match_pkt_pointers(insn, dst_reg, &regs[insn->src_reg],
                                   this_branch, other_branch) &&
           is_pointer_value(env, insn->dst_reg)) {
        verbose(env, "R%d pointer comparison prohibited\n",
                insn->dst_reg);
        return -EACCES;
}

this time we check if it is valid comparison on pkt_ptr in try_match_pkt_pointers().

Currently we just allow 4 opcode (BPF_JGT, BPF_JLT, BPF_JGE, BPF_JLE) on pkt_ptr, and with conditions. But we bypass these prohibits in privileged mode (is_pointer_value() always return false in privileged mode).

So the logic skip these prohibits for pkt_ptr in unprivileged mode.

Well, yes, but do you really need to do forbid `if r0 > r0 goto ...` in unpriv?

Currently `if r0 > r0 goto ...` is forbid in unpriv, but we can allow it.

Let's not relax unpriv. We don't need new threads with researchers whether such things can be exploited.