bpf: Permits pointers on stack for helper calls

Currently, when checking stack memory accessed by helper calls,
for spills, only PTR_TO_BTF_ID and SCALAR_VALUE are
allowed.

Song discovered an issue where the below bpf program
  int dump_task(struct bpf_iter__task *ctx)
  {
    struct seq_file *seq = ctx->meta->seq;
    static char[] info = "abc";
    BPF_SEQ_PRINTF(seq, "%s\n", info);
    return 0;
  }
may cause a verifier failure.

The verifier output looks like:
  ; struct seq_file *seq = ctx->meta->seq;
  1: (79) r1 = *(u64 *)(r1 +0)
  ; BPF_SEQ_PRINTF(seq, "%s\n", info);
  2: (18) r2 = 0xffff9054400f6000
  4: (7b) *(u64 *)(r10 -8) = r2
  5: (bf) r4 = r10
  ;
  6: (07) r4 += -8
  ; BPF_SEQ_PRINTF(seq, "%s\n", info);
  7: (18) r2 = 0xffff9054400fe000
  9: (b4) w3 = 4
  10: (b4) w5 = 8
  11: (85) call bpf_seq_printf#126
   R1_w=ptr_seq_file(id=0,off=0,imm=0) R2_w=map_value(id=0,off=0,ks=4,vs=4,imm=0)
  R3_w=inv4 R4_w=fp-8 R5_w=inv8 R10=fp0 fp-8_w=map_value
  last_idx 11 first_idx 0
  regs=8 stack=0 before 10: (b4) w5 = 8
  regs=8 stack=0 before 9: (b4) w3 = 4
  invalid indirect read from stack off -8+0 size 8

Basically, the verifier complains the map_value pointer at "fp-8" location.
To fix the issue, if env->allow_ptr_leaks is true, let us also permit
pointers on the stack to be accessible by the helper.
Reported-by: Song Liu <songliubraving@fb.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201210013349.943719-1-yhs@fb.com

kernel/bpf/verifier.c

@@ -3769,7 +3769,8 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 			goto mark;
 
 		if (state->stack[spi].slot_type[0] == STACK_SPILL &&
-		    state->stack[spi].spilled_ptr.type == SCALAR_VALUE) {
+		    (state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
+		     env->allow_ptr_leaks)) {
 			__mark_reg_unknown(env, &state->stack[spi].spilled_ptr);
 			for (j = 0; j < BPF_REG_SIZE; j++)
 				state->stack[spi].slot_type[j] = STACK_MISC;