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

[Daniel Borkmann]

On 5/8/20 11:53 PM, Alexei Starovoitov wrote:

From: Alexei Starovoitov <a...@kernel.org>

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