Re: [PATCH bpf-next v9 04/23] bpf/verifier: allow kfunc to return an allocated mem

[Benjamin Tissoires]

On Fri, Aug 26, 2022 at 3:25 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Wed, 24 Aug 2022 at 15:41, Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> wrote:
> >
> > For drivers (outside of network), the incoming data is not statically
> > defined in a struct. Most of the time the data buffer is kzalloc-ed
> > and thus we can not rely on eBPF and BTF to explore the data.
> >
> > This commit allows to return an arbitrary memory, previously allocated by
> > the driver.
> > An interesting extra point is that the kfunc can mark the exported
> > memory region as read only or read/write.
> >
> > So, when a kfunc is not returning a pointer to a struct but to a plain
> > type, we can consider it is a valid allocated memory assuming that:
> > - one of the arguments is either called rdonly_buf_size or
> >   rdwr_buf_size
> > - and this argument is a const from the caller point of view
> >
> > We can then use this parameter as the size of the allocated memory.
> >
> > The memory is either read-only or read-write based on the name
> > of the size parameter.
> >
> > Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > ---
> >
> > changes in v9:
> > - updated to match upstream (replaced kfunc_flag by a field in
> >   kfunc_meta)
> >
> > no changes in v8
> >
> > changes in v7:
> > - ensures btf_type_is_struct_ptr() checks for a ptr first
> >   (squashed from next commit)
> > - remove multiple_ref_obj_id need
> > - use btf_type_skip_modifiers instead of manually doing it in
> >   btf_type_is_struct_ptr()
> > - s/strncmp/strcmp/ in btf_is_kfunc_arg_mem_size()
> > - check for tnum_is_const when retrieving the size value
> > - have only one check for "Ensure only one argument is referenced
> >   PTR_TO_BTF_ID"
> > - add some more context to the commit message
> >
> > changes in v6:
> > - code review from Kartikeya:
> >   - remove comment change that had no reasons to be
> >   - remove handling of PTR_TO_MEM with kfunc releases
> >   - introduce struct bpf_kfunc_arg_meta
> >   - do rdonly/rdwr_buf_size check in btf_check_kfunc_arg_match
> >   - reverted most of the changes in verifier.c
> >   - make sure kfunc acquire is using a struct pointer, not just a plain
> >     pointer
> >   - also forward ref_obj_id to PTR_TO_MEM in kfunc to not use after free
> >     the allocated memory
> >
> > changes in v5:
> > - updated PTR_TO_MEM comment in btf.c to match upstream
> > - make it read-only or read-write based on the name of size
> >
> > new in v4
> >
> > change btf.h
> >
> > fix allow kfunc to return an allocated mem
> > ---
> >  include/linux/bpf.h   |  9 +++-
> >  include/linux/btf.h   | 10 +++++
> >  kernel/bpf/btf.c      | 98 ++++++++++++++++++++++++++++++++++---------
> >  kernel/bpf/verifier.c | 43 +++++++++++++------
> >  4 files changed, 128 insertions(+), 32 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 39bd36359c1e..90dd218e0199 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1932,13 +1932,20 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
> >                            const char *func_name,
> >                            struct btf_func_model *m);
> > [...]
> > +
> >  static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >                                     const struct btf *btf, u32 func_id,
> >                                     struct bpf_reg_state *regs,
> >                                     bool ptr_to_mem_ok,
> > -                                   u32 kfunc_flags)
> > +                                   struct bpf_kfunc_arg_meta *kfunc_meta)
> >  {
> >         enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
> >         bool rel = false, kptr_get = false, trusted_arg = false;
> > @@ -6207,12 +6232,12 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >                 return -EINVAL;
> >         }
> >
> > -       if (is_kfunc) {
> > +       if (is_kfunc && kfunc_meta) {
> >                 /* Only kfunc can be release func */
> > -               rel = kfunc_flags & KF_RELEASE;
> > -               kptr_get = kfunc_flags & KF_KPTR_GET;
> > -               trusted_arg = kfunc_flags & KF_TRUSTED_ARGS;
> > -               sleepable = kfunc_flags & KF_SLEEPABLE;
> > +               rel = kfunc_meta->flags & KF_RELEASE;
> > +               kptr_get = kfunc_meta->flags & KF_KPTR_GET;
> > +               trusted_arg = kfunc_meta->flags & KF_TRUSTED_ARGS;
> > +               sleepable = kfunc_meta->flags & KF_SLEEPABLE;
> >         }
> >
> >         /* check that BTF function arguments match actual types that the
> > @@ -6225,6 +6250,35 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >
> >                 t = btf_type_skip_modifiers(btf, args[i].type, NULL);
> >                 if (btf_type_is_scalar(t)) {
> > +                       if (is_kfunc && kfunc_meta) {
> > +                               bool is_buf_size = false;
> > +
> > +                               /* check for any const scalar parameter of name "rdonly_buf_size"
> > +                                * or "rdwr_buf_size"
> > +                                */
> > +                               if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> > +                                                             "rdonly_buf_size")) {
> > +                                       kfunc_meta->r0_rdonly = true;
> > +                                       is_buf_size = true;
> > +                               } else if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> > +                                                                    "rdwr_buf_size"))
> > +                                       is_buf_size = true;
> > +
> > +                               if (is_buf_size) {
> > +                                       if (kfunc_meta->r0_size) {
> > +                                               bpf_log(log, "2 or more rdonly/rdwr_buf_size parameters for kfunc");
> > +                                               return -EINVAL;
> > +                                       }
> > +
> > +                                       if (!tnum_is_const(reg->var_off)) {
> > +                                               bpf_log(log, "R%d is not a const\n", regno);
> > +                                               return -EINVAL;
> > +                                       }
> > +
> > +                                       kfunc_meta->r0_size = reg->var_off.value;
>
> Sorry for not pointing it out before, but you will need a call to
> mark_chain_precision here after this, since the value of the scalar is
> being used to decide the size of the returned pointer.

No worries.

I do however have a couple of questions (I have strictly no idea what
mark_chain_precision does):
- which register number should I call mark_chain_precision() as
parameter? r0 or regno (the one with the constant)?
- mark_chain_precision() is declared static in verifier.c. Should I
export it so btf.c can have access to it, or can I delay the call to
mark_chain_precision() in verifier.c when I set
regs[BPF_REG_0].mem_size?


>
> > +                               }
> > +                       }
> > +
> >                         if (reg->type == SCALAR_VALUE)
> >                                 continue;
> >                         bpf_log(log, "R%d is not a scalar\n", regno);
> > @@ -6255,6 +6309,19 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >                 if (ret < 0)
> >                         return ret;
> >
> > +               if (is_kfunc && reg->type == PTR_TO_BTF_ID) {
>
> I think you can drop this extra check 'reg->type == PTR_TO_BTF_ID),
> this condition of only one ref_obj_id should hold regardless of the
> type.

Ack.

Cheers,
Benjamin

>
> > [...]
>