Re: [PATCH v13 bpf-next 03/10] bpf: Allow initializing dynptrs in kfuncs

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

On Mon, Mar 6, 2023 at 10:54 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Sun, Mar 5, 2023 at 11:36 PM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
> >
> > On Wed, Mar 01, 2023 at 04:49:46PM CET, Joanne Koong wrote:
> > > This change allows kfuncs to take in an uninitialized dynptr as a
> > > parameter. Before this change, only helper functions could successfully
> > > use uninitialized dynptrs. This change moves the memory access check
> > > (including stack state growing and slot marking) into
> > > process_dynptr_func(), which both helpers and kfuncs call into.
> > >
> > > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > > ---
> > >  kernel/bpf/verifier.c | 67 ++++++++++++++-----------------------------
> > >  1 file changed, 22 insertions(+), 45 deletions(-)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index e0e00509846b..82e39fc5ed05 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -268,7 +268,6 @@ struct bpf_call_arg_meta {
> > >       u32 ret_btf_id;
> > >       u32 subprogno;
> > >       struct btf_field *kptr_field;
> > > -     u8 uninit_dynptr_regno;
> > >  };
> > >
> > >  struct btf *btf_vmlinux;
> > > @@ -6225,10 +6224,11 @@ static int process_kptr_func(struct bpf_verifier_env *env, int regno,
> > >   * Helpers which do not mutate the bpf_dynptr set MEM_RDONLY in their argument
> > >   * type, and declare it as 'const struct bpf_dynptr *' in their prototype.
> > >   */
> > > -static int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> > > -                            enum bpf_arg_type arg_type, struct bpf_call_arg_meta *meta)
> > > +static int process_dynptr_func(struct bpf_verifier_env *env, int regno, int insn_idx,
> > > +                            enum bpf_arg_type arg_type)
> > >  {
> > >       struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
> > > +     int err;
> > >
> > >       /* MEM_UNINIT and MEM_RDONLY are exclusive, when applied to an
> > >        * ARG_PTR_TO_DYNPTR (or ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_*):
> > > @@ -6254,23 +6254,23 @@ static int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> > >        *               to.
> > >        */
> > >       if (arg_type & MEM_UNINIT) {
> > > +             int i;
> > > +
> > >               if (!is_dynptr_reg_valid_uninit(env, reg)) {
> > >                       verbose(env, "Dynptr has to be an uninitialized dynptr\n");
> > >                       return -EINVAL;
> > >               }
> > >
> > > -             /* We only support one dynptr being uninitialized at the moment,
> > > -              * which is sufficient for the helper functions we have right now.
> > > -              */
> > > -             if (meta->uninit_dynptr_regno) {
> > > -                     verbose(env, "verifier internal error: multiple uninitialized dynptr args\n");
> > > -                     return -EFAULT;
> > > +             /* we write BPF_DW bits (8 bytes) at a time */
> > > +             for (i = 0; i < BPF_DYNPTR_SIZE; i += 8) {
> > > +                     err = check_mem_access(env, insn_idx, regno,
> > > +                                            i, BPF_DW, BPF_WRITE, -1, false);
> > > +                     if (err)
> > > +                             return err;
> > >               }
> >
> > I am not sure moving check_mem_access into process_dynptr_func is the right
> > thing to do. Not sure if a problem already, but sooner or later it might be.
> >
> > The side effects of the call should take effect on the current state only after
> > we have gone through all arguments for the helper/kfunc call. In this case we
> > will now do stack access while processing the dynptr arg, which may affect the
> > state of stack we see through other memory arguments coming later.
> >
> > I think it is better to do it after argument processing is done, similar to
> > existing meta.access_size handling which is done after check_func_arg loop (for
> > the same reasons).
> >
>
> Thanks for taking a look. I don't have a strong preference for either
> so if you do feel strongly about doing the check_mem_access() only
> after argument processing, I'm happy to change it. The
> check_mem_access() call on the dyntpr will mark only the dynptr stack
> slots, so I don't fully see how it may affect the state of stack
> through other memory arguments coming later, but I do see your point
> about keeping the logic more separated out.

FWIW, I did a similar approach for iters as well. And I suspect it's
not the only place where we do similar things while processing helper
arguments, etc.

Let's keep this in mind, but I wouldn't necessarily go complicating
code right now with more of "let's record some info for later" and
then "ok, we recorded something before, let's act on it".

>
> > > [...]