Re: [PATCH v3 bpf-next 06/11] bpf: implement accurate raw_tp context access via BTF

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Oct 16, 2019 at 4:16 AM Alexei Starovoitov <ast@kernel.org> wrote:
>
> libbpf analyzes bpf C program, searches in-kernel BTF for given type name
> and stores it into expected_attach_type.
> The kernel verifier expects this btf_id to point to something like:
> typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, void *loc);
> which represents signature of raw_tracepoint "kfree_skb".
>
> Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb'
> and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint.
> In first case it passes btf_id of 'struct sk_buff *' back to the verifier core
> and 'void *' in second case.
>
> Then the verifier tracks PTR_TO_BTF_ID as any other pointer type.
> Like PTR_TO_SOCKET points to 'struct bpf_sock',
> PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on.
> PTR_TO_BTF_ID points to in-kernel structs.
> If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF
> then PTR_TO_BTF_ID#1234 points to one of in kernel skbs.
>
> When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32)
> the btf_struct_access() checks which field of 'struct sk_buff' is
> at offset 32. Checks that size of access matches type definition
> of the field and continues to track the dereferenced type.
> If that field was a pointer to 'struct net_device' the r2's type
> will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device'
> in vmlinux's BTF.
>
> Such verifier analysis prevents "cheating" in BPF C program.
> The program cannot cast arbitrary pointer to 'struct sk_buff *'
> and access it. C compiler would allow type cast, of course,
> but the verifier will notice type mismatch based on BPF assembly
> and in-kernel BTF.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  include/linux/bpf.h          |  17 +++-
>  include/linux/bpf_verifier.h |   4 +
>  kernel/bpf/btf.c             | 190 +++++++++++++++++++++++++++++++++++
>  kernel/bpf/verifier.c        |  88 +++++++++++++++-
>  kernel/trace/bpf_trace.c     |   2 +-
>  5 files changed, 296 insertions(+), 5 deletions(-)
>

Maybe it's just me reading this code for Nth time, but I find
btf_struct_access() much easier to follow now. Thanks!

Acked-by: Andrii Nakryiko <andriin@fb.com>

[...]

>  static void print_verifier_state(struct bpf_verifier_env *env,
>                                  const struct bpf_func_state *state)
>  {
> @@ -460,6 +480,8 @@ static void print_verifier_state(struct bpf_verifier_env *env,
>                         /* reg->off should be 0 for SCALAR_VALUE */
>                         verbose(env, "%lld", reg->var_off.value + reg->off);
>                 } else {
> +                       if (t == PTR_TO_BTF_ID)
> +                               verbose(env, "%s", kernel_type_name(reg->btf_id));
>                         verbose(env, "(id=%d", reg->id);

not related to specific changes in this patch set, just to bring this
up, but this extra id=%d part is quite confusing for register types
that shouldn't really have id associated with it. We should probably
add some filter here to print this only for ref-tracked register
types.

>                         if (reg_type_may_be_refcounted_or_null(t))
>                                 verbose(env, ",ref_obj_id=%d", reg->ref_obj_id);
> @@ -2337,10 +2359,12 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
>
>  /* check access to 'struct bpf_context' fields.  Supports fixed offsets only */
>  static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off, int size,

[...]