Re: [PATCH v2 1/2] bpf: reject stack-argument callback subprograms

On 8/17/26 1:48 PM, Jérémy Jean wrote:
> Helper callbacks enter BPF subprograms through bpf_callback_t, whose
> runtime ABI supplies five arguments. BTF validation nevertheless permits
> static callback subprograms to declare more than five arguments when JIT
> stack arguments are supported.
>
> This lets verifier state for a callback use outgoing stack argument slots
> prepared at the helper call site. The helper does not pass those slots. On
> x86-64, callback loads of arguments seven and later therefore read the
> helper native frame instead of the synthetic values checked by the
> verifier. KASAN reports a slab OOB write.
>
> Reject callback subprograms with incoming stack arguments when processing
> callback calls.
>
> Fixes: 0f6bd5e7a804 ("bpf: Support stack arguments for bpf functions")
> Assisted-by: Codex:gpt-5
> Signed-off-by: Jérémy Jean <[email protected]>
> ---
>   kernel/bpf/verifier.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index fdc5fbb1f78c..5fcefc0eaba0 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9285,6 +9285,8 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
>   	err = btf_check_subprog_call(env, subprog, caller->regs);
>   	if (err == -EFAULT)
>   		return err;
> +	if (bpf_in_stack_arg_cnt(&env->subprog_info[subprog]))
> +		return -EINVAL;

This is not good as user will not know why it failed. Your v1 does have an error message.

But this is not needed. Without above verifer.c change, user will get an error message:
    func#0 writes 4 stack arg slots, but calls only require 0

NACK, see my v1 comment: https://lore.kernel.org/bpf/[email protected]/

>   
>   	/* set_callee_state is used for direct subprog calls, but we are
>   	 * interested in validating only BPF helpers that can call subprogs as