Re: [PATCH v5 bpf-next 2/3] bpf: implement CAP_BPF

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

On 5/8/20 11:53 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> Implement permissions as stated in uapi/linux/capability.h
> In order to do that the verifier allow_ptr_leaks flag is split
> into allow_ptr_leaks and bpf_capable flags and they are set as:
>    env->allow_ptr_leaks = perfmon_capable();
>    env->bpf_capable = bpf_capable();

[...]
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 70ad009577f8..a6893746cd87 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1293,7 +1293,7 @@ static void __mark_reg_unknown(const struct bpf_verifier_env *env,
>   	reg->type = SCALAR_VALUE;
>   	reg->var_off = tnum_unknown;
>   	reg->frameno = 0;
> -	reg->precise = env->subprog_cnt > 1 || !env->allow_ptr_leaks;
> +	reg->precise = env->subprog_cnt > 1 || !env->bpf_capable;
>   	__mark_reg_unbounded(reg);
>   }
>   
> @@ -1425,8 +1425,9 @@ static int check_subprogs(struct bpf_verifier_env *env)
>   			continue;
>   		if (insn[i].src_reg != BPF_PSEUDO_CALL)
>   			continue;
> -		if (!env->allow_ptr_leaks) {
> -			verbose(env, "function calls to other bpf functions are allowed for root only\n");
> +		if (!env->bpf_capable) {
> +			verbose(env,
> +				"function calls to other bpf functions are allowed for CAP_BPF and CAP_SYS_ADMIN\n");
>   			return -EPERM;
>   		}
>   		ret = add_subprog(env, i + insn[i].imm + 1);
> @@ -1960,7 +1961,7 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int regno,
>   	bool new_marks = false;
>   	int i, err;
>   
> -	if (!env->allow_ptr_leaks)
> +	if (!env->bpf_capable)
>   		/* backtracking is root only for now */
>   		return 0;
>   
> @@ -2208,7 +2209,7 @@ static int check_stack_write(struct bpf_verifier_env *env,
>   		reg = &cur->regs[value_regno];
>   
>   	if (reg && size == BPF_REG_SIZE && register_is_const(reg) &&
> -	    !register_is_null(reg) && env->allow_ptr_leaks) {
> +	    !register_is_null(reg) && env->bpf_capable) {
>   		if (dst_reg != BPF_REG_FP) {
>   			/* The backtracking logic can only recognize explicit
>   			 * stack slot address like [fp - 8]. Other spill of
> @@ -3428,7 +3429,7 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
>   		 * Spectre masking for stack ALU.
>   		 * See also retrieve_ptr_limit().
>   		 */
> -		if (!env->allow_ptr_leaks) {
> +		if (!env->bpf_capable) {
>   			char tn_buf[48];
>   
>   			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
> @@ -7229,7 +7230,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
>   		insn_stack[env->cfg.cur_stack++] = w;
>   		return 1;
>   	} else if ((insn_state[w] & 0xF0) == DISCOVERED) {
> -		if (loop_ok && env->allow_ptr_leaks)
> +		if (loop_ok && env->bpf_capable)
>   			return 0;
>   		verbose_linfo(env, t, "%d: ", t);
>   		verbose_linfo(env, w, "%d: ", w);
> @@ -8338,7 +8339,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
>   	if (env->max_states_per_insn < states_cnt)
>   		env->max_states_per_insn = states_cnt;
>   
> -	if (!env->allow_ptr_leaks && states_cnt > BPF_COMPLEXITY_LIMIT_STATES)
> +	if (!env->bpf_capable && states_cnt > BPF_COMPLEXITY_LIMIT_STATES)
>   		return push_jmp_history(env, cur);
>   
>   	if (!add_new_state)
> @@ -9998,7 +9999,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
>   			insn->code = BPF_JMP | BPF_TAIL_CALL;
>   
>   			aux = &env->insn_aux_data[i + delta];
> -			if (env->allow_ptr_leaks && !expect_blinding &&
> +			if (env->bpf_capable && !expect_blinding &&
>   			    prog->jit_requested &&
>   			    !bpf_map_key_poisoned(aux) &&
>   			    !bpf_map_ptr_poisoned(aux) &&
> @@ -10725,7 +10726,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
>   		env->insn_aux_data[i].orig_idx = i;
>   	env->prog = *prog;
>   	env->ops = bpf_verifier_ops[env->prog->type];
> -	is_priv = capable(CAP_SYS_ADMIN);
> +	is_priv = bpf_capable();
>   
>   	if (!btf_vmlinux && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) {
>   		mutex_lock(&bpf_verifier_lock);
> @@ -10766,7 +10767,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
>   	if (attr->prog_flags & BPF_F_ANY_ALIGNMENT)
>   		env->strict_alignment = false;
>   
> -	env->allow_ptr_leaks = is_priv;
> +	env->allow_ptr_leaks = perfmon_capable();
> +	env->bpf_capable = bpf_capable();

Probably more of a detail, but it feels weird to tie perfmon_capable() into the BPF
core and use it in various places there. I would rather make this a proper bpf_*
prefixed helper and add a more descriptive name (what does it have to do with perf
or monitoring directly?). For example, all the main functionality could be under
`bpf_base_capable()` and everything with potential to leak pointers or mem to user
space as `bpf_leak_capable()`. Then inside include/linux/capability.h this can still
resolve under the hood to something like:

static inline bool bpf_base_capable(void)
{
	return capable(CAP_BPF) || capable(CAP_SYS_ADMIN);
}

static inline bool bpf_leak_capable(void)
{
	return perfmon_capable();
}

Thanks,
Daniel