Re: [PATCH 2/3] bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)

[Andrii Nakryiko]

On Wed, Apr 5, 2023 at 5:40 PM Daniel Rosenberg <drosen@xxxxxxxxxx> wrote:
>
> bpf_dynptr_slice(_rw) uses a user provided buffer if it can not provide
> a pointer to a block of contiguous memory. This buffer is unused in the
> case of local dynptrs, and may be unused in other cases as well. There
> is no need to require the buffer, as the kfunc can just return NULL if
> it was needed and not provided.
>
> This adds another kfunc annotation, __opt, which combines with __sz and
> __szk to allow the buffer associated with the size to be NULL. If the
> buffer is NULL, the verifier does not check that the buffer is of
> sufficient size.
>
> Signed-off-by: Daniel Rosenberg <drosen@xxxxxxxxxx>
> ---
>  Documentation/bpf/kfuncs.rst | 23 ++++++++++++++++++++++-
>  kernel/bpf/helpers.c         | 32 ++++++++++++++++++++------------
>  kernel/bpf/verifier.c        | 17 +++++++++++++++++
>  3 files changed, 59 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> index d8a16c4bef7f..69573b511233 100644
> --- a/Documentation/bpf/kfuncs.rst
> +++ b/Documentation/bpf/kfuncs.rst
> @@ -100,7 +100,7 @@ Hence, whenever a constant scalar argument is accepted by a kfunc which is not a
>  size parameter, and the value of the constant matters for program safety, __k
>  suffix should be used.
>
> -2.2.2 __uninit Annotation
> +2.2.3 __uninit Annotation
>  -------------------------
>
>  This annotation is used to indicate that the argument will be treated as
> @@ -117,6 +117,27 @@ Here, the dynptr will be treated as an uninitialized dynptr. Without this
>  annotation, the verifier will reject the program if the dynptr passed in is
>  not initialized.
>
> +2.2.4 __opt Annotation
> +-------------------------
> +
> +This annotation is used to indicate that the buffer associated with an __sz or __szk
> +argument may be null. If the function is passed a nullptr in place of the buffer,
> +the verifier will not check that length is appropriate for the buffer. The kfunc is
> +responsible for checking if this buffer is null before using it.
> +
> +An example is given below::
> +
> +        __bpf_kfunc void *bpf_dynptr_slice(..., void *buffer__opt, u32 buffer__szk)
> +        {
> +        ...
> +        }
> +
> +Here, the buffer may be null. If buffer is not null, it at least of size buffer_szk.
> +Either way, the returned buffer is either NULL, or of size buffer_szk. Without this
> +annotation, the verifier will reject the program if a null pointer is passed in with
> +a nonzero size.
> +
> +
>  .. _BPF_kfunc_nodef:
>
>  2.3 Using an existing kernel function
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 6be16db9f188..f08556fd8b96 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2145,13 +2145,15 @@ __bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)
>   * bpf_dynptr_slice() - Obtain a read-only pointer to the dynptr data.
>   * @ptr: The dynptr whose data slice to retrieve
>   * @offset: Offset into the dynptr
> - * @buffer: User-provided buffer to copy contents into
> - * @buffer__szk: Size (in bytes) of the buffer. This is the length of the
> - *              requested slice. This must be a constant.
> + * @buffer__opt: User-provided buffer to copy contents into.  May be NULL
> + * @buffer__szk: Size (in bytes) of the buffer if present. This is the
> + *               length of the requested slice. This must be a constant.
>   *
>   * For non-skb and non-xdp type dynptrs, there is no difference between
>   * bpf_dynptr_slice and bpf_dynptr_data.
>   *
> + *  If buffer__opt is NULL, the call will fail if buffer_opt was needed.
> + *
>   * If the intention is to write to the data slice, please use
>   * bpf_dynptr_slice_rdwr.
>   *
> @@ -2168,7 +2170,7 @@ __bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)
>   * direct pointer)
>   */
>  __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset,
> -                                  void *buffer, u32 buffer__szk)
> +                                  void *buffer__opt, u32 buffer__szk)
>  {
>         enum bpf_dynptr_type type;
>         u32 len = buffer__szk;
> @@ -2188,15 +2190,19 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
>         case BPF_DYNPTR_TYPE_RINGBUF:
>                 return ptr->data + ptr->offset + offset;
>         case BPF_DYNPTR_TYPE_SKB:
> -               return skb_header_pointer(ptr->data, ptr->offset + offset, len, buffer);
> +               if (!buffer__opt)
> +                       return NULL;

should we always reject NULL even for SKB/XDP or only when the buffer
*would be* required? If the latter, we could use bpf_dynptr_slice()
with NULL buf to say "only return pointer if no byte copying is
required". As opposed to bpf_dynptr_data(), where I think we always
fail for SKB/XDP, because we are not sure whether users are aware of
this need to copy bytes. Here, users are aware, but chose to prevent
copying.

WDYT?

> +               return skb_header_pointer(ptr->data, ptr->offset + offset, len, buffer__opt);
>         case BPF_DYNPTR_TYPE_XDP:
>         {
>                 void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);
>                 if (xdp_ptr)
>                         return xdp_ptr;
>
> -               bpf_xdp_copy_buf(ptr->data, ptr->offset + offset, buffer, len, false);
> -               return buffer;
> +               if (!buffer__opt)
> +                       return NULL;
> +               bpf_xdp_copy_buf(ptr->data, ptr->offset + offset, buffer__opt, len, false);
> +               return buffer__opt;
>         }
>         default:
>                 WARN_ONCE(true, "unknown dynptr type %d\n", type);
> @@ -2208,13 +2214,15 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
>   * bpf_dynptr_slice_rdwr() - Obtain a writable pointer to the dynptr data.
>   * @ptr: The dynptr whose data slice to retrieve
>   * @offset: Offset into the dynptr
> - * @buffer: User-provided buffer to copy contents into
> - * @buffer__szk: Size (in bytes) of the buffer. This is the length of the
> - *              requested slice. This must be a constant.
> + * @buffer__opt: User-provided buffer to copy contents into. May be NULL
> + * @buffer__szk: Size (in bytes) of the buffer if present. This is the
> + *               length of the requested slice. This must be a constant.
>   *
>   * For non-skb and non-xdp type dynptrs, there is no difference between
>   * bpf_dynptr_slice and bpf_dynptr_data.
>   *
> + * If buffer__opt is NULL, the call will fail if buffer_opt was needed.
> + *
>   * The returned pointer is writable and may point to either directly the dynptr
>   * data at the requested offset or to the buffer if unable to obtain a direct
>   * data pointer to (example: the requested slice is to the paged area of an skb
> @@ -2245,7 +2253,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
>   * direct pointer)
>   */
>  __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 offset,
> -                                       void *buffer, u32 buffer__szk)
> +                                       void *buffer__opt, u32 buffer__szk)
>  {
>         if (!ptr->data || bpf_dynptr_is_rdonly(ptr))
>                 return NULL;
> @@ -2272,7 +2280,7 @@ __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 o
>          * will be copied out into the buffer and the user will need to call
>          * bpf_dynptr_write() to commit changes.
>          */
> -       return bpf_dynptr_slice(ptr, offset, buffer, buffer__szk);
> +       return bpf_dynptr_slice(ptr, offset, buffer__opt, buffer__szk);
>  }
>
>  __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 20beab52812a..b82faef389b1 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9428,6 +9428,19 @@ static bool is_kfunc_arg_const_mem_size(const struct btf *btf,
>         return __kfunc_param_match_suffix(btf, arg, "__szk");
>  }
>
> +static bool is_kfunc_arg_optional(const struct btf *btf,
> +                 const struct btf_param *arg,
> +                 const struct bpf_reg_state *reg)
> +{
> +       const struct btf_type *t;
> +
> +       t = btf_type_skip_modifiers(btf, arg->type, NULL);
> +       if (!btf_type_is_ptr(t) || reg->type != SCALAR_VALUE || reg->umax_value > 0)
> +               return false;
> +
> +       return __kfunc_param_match_suffix(btf, arg, "__opt");
> +}
> +
>  static bool is_kfunc_arg_constant(const struct btf *btf, const struct btf_param *arg)
>  {
>         return __kfunc_param_match_suffix(btf, arg, "__k");
> @@ -10539,10 +10552,14 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
>                         break;
>                 case KF_ARG_PTR_TO_MEM_SIZE:
>                 {
> +                       struct bpf_reg_state *buff_reg = &regs[regno];
> +                       const struct btf_param *buff_arg = &args[i];
>                         struct bpf_reg_state *size_reg = &regs[regno + 1];
>                         const struct btf_param *size_arg = &args[i + 1];
>
>                         ret = check_kfunc_mem_size_reg(env, size_reg, regno + 1);
> +                       if (ret < 0 && is_kfunc_arg_optional(meta->btf, buff_arg, buff_reg))
> +                               ret = 0;

would this work correctly if someone passes a non-null buffer with too
small size? Can you please add a test for this use case.

Also, I feel like for cases where we allow a NULL buffer, we need to
explicitly check that the register is a *known* NULL (SCALAR=0
basically). And also in that case the size of the buffer probably
should be enforced to zero, not just be allowed to be any value.

it's scary to just ignore some error, tbh, the number of error
conditions can grow overtime and we'll be masking them with this
is_kfunc_arg_optional() override. Let's be strict and explicit here.


>                         if (ret < 0) {
>                                 verbose(env, "arg#%d arg#%d memory, len pair leads to invalid memory access\n", i, i + 1);
>                                 return ret;
> --
> 2.40.0.577.gac1e443424-goog
>