Re: [PATCH bpf-next v3 1/2] bpf: permits pointers on stack for helper calls

[Daniel Borkmann <daniel@iogearbox.net>]

On 12/11/20 4:41 AM, Yonghong Song wrote:
> 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.
> 
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Reported-by: Song Liu <songliubraving@fb.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>   kernel/bpf/verifier.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 93def76cf32b..eebb2d3e16bf 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3769,7 +3769,9 @@ 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 ||
> +		     (state->stack[spi].spilled_ptr.type != NOT_INIT &&

Thinking more on this, your v2 was actually correct since in such case stype
would have been STACK_MISC or STACK_ZERO and we would have jumped to goto mark
here instead, so the above is not reachable under NOT_INIT. Anyway, I took the
v2 in, thanks!

> +		      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;
>