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From:   Benjamin Tissoires <benjamin.tissoires@redhat.com>
Date:   Mon, 18 Jul 2022 15:53:55 +0200
Message-ID: <CAO-hwJ+RU89t=w5RgJNG=G46veHHF6NiR1zAsec9YuYrX=FF8A@mail.gmail.com>
Subject: Re: [PATCH bpf-next v6 02/23] bpf/verifier: allow kfunc to read user
 provided context
To:     Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Jiri Kosina <jikos@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Shuah Khan <shuah@kernel.org>,
        Dave Marchevsky <davemarchevsky@fb.com>,
        Joe Stringer <joe@cilium.io>, Jonathan Corbet <corbet@lwn.net>,
        Tero Kristo <tero.kristo@linux.intel.com>,
        lkml <linux-kernel@vger.kernel.org>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>
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On Sat, Jul 16, 2022 at 9:48 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Tue, 12 Jul 2022 at 17:02, Benjamin Tissoires
> <benjamin.tissoires@redhat.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@redhat.com>
> >
> > ---
> >
> > new in v6
> > ---
> >  kernel/bpf/verifier.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 328cfab3af60..f6af57a84247 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 +5225,19 @@ 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 access_t = meta->raw_mode ? BPF_WRITE : BPF_READ;
>
> small nit: _t suffix is used for types, so you could probably rename
> this. maybe atype?

Ack, fixed locally.

>
> > +
> > +                       return check_mem_access(env, env->insn_idx, regno, access_size, BPF_B,
> > +                                               access_t, -1, false);
>
> If I read the code correctly, this makes the max_ctx_offset of prog
> access_size + 1 (off + size_to_bytes(BPF_B)), which is 1 more than the
> actual size being accessed.

Oh, correct. I am mixing offset and access_size, which creates this :(

>
> This also messes up check_helper_mem_access when it allows NULL, 0
> pair to pass (because check is against actual size + 1). We do allow
> passing NULL when size is 0 for kfuncs (see zero_size_allowed is true

I am a little bit confused by how check_mem_size_reg() treats the case
when reg->umin_value == 0.

What does it mean to call check_helper_mem_access() with a 0 size if
we have zero_size_allowed?

Can I just have in the PTR_TO_CTX case: "if (access_size == 0) return
zero_size_allowed ? 0 : -EINVAL;" or should I only allow the call if
the ptr in the register is null?

> in check_mem_size_reg), so your hid_hw_request function is missing
> that NULL check for buf too.

Actually, in hid_hw_request() we ensure buf__sz is greater than 1, so
buf can not be null. But I agree it doesn't hurt to have that extra
check to be sure (we are called from a syscall program, so not time
sensitive).

>
> In the selftest that checks for failure in loading
> + bpf_kfunc_call_test_mem_len_pass1(&args->data, sizeof(*args) + 1);
> so it will still fail with just sizeof(*args).

Good point.

>
> Also please add coverage for this case in the next version.

I added both (NULL, 0) and (&args->data, sizeof(*args)) as passing
tests locally.

And thanks for the review!

Cheers,
Benjamin


>
> > +               }
> > +
> > +               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 +5350,7 @@ 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
> >
>