On Thu, Jul 28, 2022 at 4:39 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
>
> On Tue, Jul 26, 2022 at 11:47:04AM -0700, Joanne Koong wrote:
> > @@ -1567,6 +1607,18 @@ BPF_CALL_3(bpf_dynptr_data, struct bpf_dynptr_kern *, ptr, u32, offset, u32, len
> > if (bpf_dynptr_is_rdonly(ptr))
> Is it possible to allow data slice for rdonly dynptr-skb?
> and depends on the may_access_direct_pkt_data() check in the verifier.
Ooh great idea. This should be very simple to do, since the data slice
that gets returned is assigned as PTR_TO_PACKET. So any stx operations
on it will by default go through the may_access_direct_pkt_data()
check. I'll add this for v2.
>
> > return 0;
> >
> > + type = bpf_dynptr_get_type(ptr);
> > +
> > + if (type == BPF_DYNPTR_TYPE_SKB) {
> > + struct sk_buff *skb = ptr->data;
> > +
> > + /* if the data is paged, the caller needs to pull it first */
> > + if (ptr->offset + offset + len > skb->len - skb->data_len)
> > + return 0;
> > +
> > + return (unsigned long)(skb->data + ptr->offset + offset);
> > + }
> > +
> > return (unsigned long)(ptr->data + ptr->offset + offset);
> > }
>
> [ ... ]
>
> > -static u32 stack_slot_get_id(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> > +static void stack_slot_get_dynptr_info(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> > + struct bpf_call_arg_meta *meta)
> > {
> > struct bpf_func_state *state = func(env, reg);
> > int spi = get_spi(reg->off);
> >
> > - return state->stack[spi].spilled_ptr.id;
> > + meta->ref_obj_id = state->stack[spi].spilled_ptr.id;
> > + meta->type = state->stack[spi].spilled_ptr.dynptr.type;
> > }
> >
> > static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > @@ -6052,6 +6057,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > case DYNPTR_TYPE_RINGBUF:
> > err_extra = "ringbuf ";
> > break;
> > + case DYNPTR_TYPE_SKB:
> > + err_extra = "skb ";
> > + break;
> > default:
> > break;
> > }
> > @@ -6065,8 +6073,10 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > verbose(env, "verifier internal error: multiple refcounted args in BPF_FUNC_dynptr_data");
> > return -EFAULT;
> > }
> > - /* Find the id of the dynptr we're tracking the reference of */
> > - meta->ref_obj_id = stack_slot_get_id(env, reg);
> > + /* Find the id and the type of the dynptr we're tracking
> > + * the reference of.
> > + */
> > + stack_slot_get_dynptr_info(env, reg, meta);
> > }
> > }
> > break;
> > @@ -7406,7 +7416,11 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> > regs[BPF_REG_0].type = PTR_TO_TCP_SOCK | ret_flag;
> > } else if (base_type(ret_type) == RET_PTR_TO_ALLOC_MEM) {
> > mark_reg_known_zero(env, regs, BPF_REG_0);
> > - regs[BPF_REG_0].type = PTR_TO_MEM | ret_flag;
> > + if (func_id == BPF_FUNC_dynptr_data &&
> > + meta.type == BPF_DYNPTR_TYPE_SKB)
> > + regs[BPF_REG_0].type = PTR_TO_PACKET | ret_flag;
> > + else
> > + regs[BPF_REG_0].type = PTR_TO_MEM | ret_flag;
> > regs[BPF_REG_0].mem_size = meta.mem_size;
> check_packet_access() uses range.
> It took me a while to figure range and mem_size is in union.
> Mentioning here in case someone has similar question.
For v2, I'll add this as a comment in the code or I'll include
"regs[BPF_REG_0].range = meta.mem_size" explicitly to make it more
obvious :)
>
> > } else if (base_type(ret_type) == RET_PTR_TO_MEM_OR_BTF_ID) {
> > const struct btf_type *t;
> > @@ -14132,6 +14146,25 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> > goto patch_call_imm;
> > }
> >
> > + if (insn->imm == BPF_FUNC_dynptr_from_skb) {
> > + if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE))
> > + insn_buf[0] = BPF_MOV32_IMM(BPF_REG_4, true);
> > + else
> > + insn_buf[0] = BPF_MOV32_IMM(BPF_REG_4, false);
> > + insn_buf[1] = *insn;
> > + cnt = 2;
> > +
> > + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> > + if (!new_prog)
> > + return -ENOMEM;
> > +
> > + delta += cnt - 1;
> > + env->prog = new_prog;
> > + prog = new_prog;
> > + insn = new_prog->insnsi + i + delta;
> > + goto patch_call_imm;
> > + }
> Have you considered to reject bpf_dynptr_write()
> at prog load time?
It's possible to reject bpf_dynptr_write() at prog load time but would
require adding tracking in the verifier for whether a dynptr is
read-only or not. Do you think it's better to reject it at load time
instead of returning NULL at runtime?
