Re: [PATCH bpf-next v3 1/2] bpf: Enforce gotox targets against subprog bounds

[Anton Protopopov <a.s.protopopov@gmail.com>]

On 26/06/28 09:59PM, Nuoqi Gui wrote:
> During CFG construction, the verifier records the modeled gotox target set
> in insn_aux_data->jt. Later, check_indirect_jump() follows targets from
> the runtime PTR_TO_INSN register's actual INSN_ARRAY map.
> 
> This lets one gotox instruction observe different INSN_ARRAY maps on
> different paths and accept a target outside the calling subprog. The
> observed x86 JIT case can then enter another subprog without a matching
> BPF call frame and crash when executed.
> 
> Reject every target copied from the actual PTR_TO_INSN map if it is
> outside the calling subprog.
> 
> Fixes: 493d9e0d6083 ("bpf, x86: add support for indirect jumps")
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---
>  kernel/bpf/verifier.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index eb46a81a8c51..05a996a5ecdd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -17145,9 +17145,11 @@ static int indirect_jump_min_max_index(struct bpf_verifier_env *env,
>  static int check_indirect_jump(struct bpf_verifier_env *env, struct bpf_insn *insn)
>  {
>  	struct bpf_verifier_state *other_branch;
> +	struct bpf_subprog_info *subprog;
>  	struct bpf_reg_state *dst_reg;
>  	struct bpf_map *map;
>  	u32 min_index, max_index;
> +	int subprog_start, subprog_end;
>  	int err = 0;
>  	int n;
>  	int i;
> @@ -17188,6 +17190,23 @@ static int check_indirect_jump(struct bpf_verifier_env *env, struct bpf_insn *in
>  		return -EINVAL;
>  	}
>  
> +	subprog = bpf_find_containing_subprog(env, env->insn_idx);
> +	if (verifier_bug_if(!subprog, env,
> +			    "gotox insn %d is outside subprog bounds\n",
> +			    env->insn_idx))

Can this actually happen?

> +		return -EFAULT;
> +	subprog_start = subprog->start;
> +	subprog_end = (subprog + 1)->start;
> +
> +	for (i = 0; i < n; i++) {
> +		u32 target = env->gotox_tmp_buf->items[i];
> +
> +		if (target < subprog_start || target >= subprog_end) {
> +			verbose(env, "gotox target %u outside subprog\n", target);

In the previous patch there was more info printed (at least, subprog
boundaries looked ok, not 100% sure about map id).

> +			return -EINVAL;
> +		}
> +	}
> +
>  	for (i = 0; i < n - 1; i++) {
>  		mark_indirect_target(env, env->gotox_tmp_buf->items[i]);
>  		other_branch = push_stack(env, env->gotox_tmp_buf->items[i],
> 
> -- 
> 2.34.1
>