Re: [PATCH bpf-next v8 02/24] bpf/verifier: allow kfunc to read user provided context

[Benjamin Tissoires]

On Fri, Jul 22, 2022 at 6:16 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Fri, Jul 22, 2022 at 1:46 AM Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> 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.
> >
> > Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > ---
> >
> > changes in v8:
> > - fixup comment
> > - return -EACCESS instead of -EINVAL for consistency
> >
> > 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..c807c5d7085a 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) {
>
> prog_type check looks a bit odd here.
> Can we generalize with
> if (!env->ops->convert_ctx_access

Yep, seems to be working fine for my use case and the test cases I
have in this series.

>
> In other words any program type that doesn't have ctx rewrites can
> use helpers to access ctx fields ?
>
> Also why kfunc only?
> It looks safe to allow normal helpers as well.

Well, not sure what is happening here, but if I remove the check for
kfunc, the test for PTR_TO_CTX == NULL and size == 0 gives me a
-EINVAL.

The original reason for kfunc only was because I wanted to scope the
changes to something I can control, but now I am completely out of
ideas on why the NULL test fails if it enters the if branch.

Unfortunately I won't have a lot of time this week to tackle this (I
am on holiday with my family), and next will be tough too (at home but
doing renovations).

I can send the fixup to remove the prog_type check as I just made sure
it works with the selftests. But I won't be able to dig further why it
fails without the kfunc check, because not enough time and
concentration.

Cheers,
Benjamin