linux-kernel - Re: [PATCH bpf-next v7 02/24] bpf/verifier: allow kfunc to read user provided context

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Date:   Thu, 21 Jul 2022 22:15:14 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Joe Stringer <joe@...ium.io>, Jonathan Corbet <corbet@....net>,
        Tero Kristo <tero.kristo@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH bpf-next v7 02/24] bpf/verifier: allow kfunc to read user
 provided context

On Thu, 21 Jul 2022 at 17:36, Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> When a kfunc was trying to access data from context in a syscall eBPF
> program, the verifier was rejecting the call.
> This is because the syscall context is not known at compile time, and
> so we need to check this when actually accessing it.
>
> Check for the valid memory access and allow such situation to happen.
>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
>
> ---
>

LGTM, with just a couple more nits.
Acked-by: Kumar Kartikeya Dwivedi <memxor@...il.com>

> changes in v7:
> - renamed access_t into atype
> - allow zero-byte read
> - check_mem_access() to the correct offset/size
>
> new in v6
> ---
>  kernel/bpf/verifier.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7c1e056624f9..d5fe7e618c52 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -248,6 +248,7 @@ struct bpf_call_arg_meta {
>         struct bpf_map *map_ptr;
>         bool raw_mode;
>         bool pkt_access;
> +       bool is_kfunc;
>         u8 release_regno;
>         int regno;
>         int access_size;
> @@ -5170,6 +5171,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
>                                    struct bpf_call_arg_meta *meta)
>  {
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
> +       enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>         u32 *max_access;
>
>         switch (base_type(reg->type)) {
> @@ -5223,6 +5225,24 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
>                                 env,
>                                 regno, reg->off, access_size,
>                                 zero_size_allowed, ACCESS_HELPER, meta);
> +       case PTR_TO_CTX:
> +               /* in case of a kfunc called in a program of type SYSCALL, the context is
> +                * user supplied, so not computed statically.
> +                * Dynamically check it now
> +                */
> +               if (prog_type == BPF_PROG_TYPE_SYSCALL && meta && meta->is_kfunc) {
> +                       enum bpf_access_type atype = meta->raw_mode ? BPF_WRITE : BPF_READ;
> +                       int offset = access_size - 1;
> +
> +                       /* Allow zero-byte read from NULL or PTR_TO_CTX */

This will not be handling the case for NULL, only for kfunc(ptr_to_ctx, 0)
A null pointer has its reg->type as scalar, so it will be handled by
the default case.

> +                       if (access_size == 0)
> +                               return zero_size_allowed ? 0 : -EINVAL;

We should use -EACCES, just to be consistent.

> +
> +                       return check_mem_access(env, env->insn_idx, regno, offset, BPF_B,
> +                                               atype, -1, false);
> +               }
> +
> +               fallthrough;
>         default: /* scalar_value or invalid ptr */
>                 /* Allow zero-byte read from NULL, regardless of pointer type */
>                 if (zero_size_allowed && access_size == 0 &&
> @@ -5335,6 +5355,7 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
>         WARN_ON_ONCE(regno < BPF_REG_2 || regno > BPF_REG_5);
>
>         memset(&meta, 0, sizeof(meta));
> +       meta.is_kfunc = true;
>
>         if (may_be_null) {
>                 saved_reg = *mem_reg;
> --
> 2.36.1
>