Re: [PATCH bpf-next v9 01/23] bpf/verifier: allow all functions to read user provided context

[Kumar Kartikeya Dwivedi]

On Fri, 26 Aug 2022 at 03:42, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Aug 24, 2022 at 6:41 AM Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> wrote:
> >
> > When a function was trying to access data from context in a syscall eBPF
> > program, the verifier was rejecting the call unless it was accessing the
> > first element.
> > 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 if there is no convert_ctx callback,
> > and allow such situation to happen.
> >
> > There is a slight hiccup with subprogs. btf_check_subprog_arg_match()
> > will check that the types are matching, which is a good thing, but to
> > have an accurate result, it hides the fact that the context register may
> > be null. This makes env->prog->aux->max_ctx_offset being set to the size
> > of the context, which is incompatible with a NULL context.
> >
> > Solve that last problem by storing max_ctx_offset before the type check
> > and restoring it after.
> >
> > Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > ---
> >
> > changes in v9:
> > - rewrote the commit title and description
> > - made it so all functions can make use of context even if there is
> >   no convert_ctx
> > - remove the is_kfunc field in bpf_call_arg_meta
> >
> > 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/btf.c      | 11 ++++++++++-
> >  kernel/bpf/verifier.c | 19 +++++++++++++++++++
> >  2 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 903719b89238..386300f52b23 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6443,8 +6443,8 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
> >  {
> >         struct bpf_prog *prog = env->prog;
> >         struct btf *btf = prog->aux->btf;
> > +       u32 btf_id, max_ctx_offset;
> >         bool is_global;
> > -       u32 btf_id;
> >         int err;
> >
> >         if (!prog->aux->func_info)
> > @@ -6457,9 +6457,18 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
> >         if (prog->aux->func_info_aux[subprog].unreliable)
> >                 return -EINVAL;
> >
> > +       /* subprogs arguments are not actually accessing the data, we need
> > +        * to check for the types if they match.
> > +        * Store the max_ctx_offset and restore it after btf_check_func_arg_match()
> > +        * given that this function will have a side effect of changing it.
> > +        */
> > +       max_ctx_offset = env->prog->aux->max_ctx_offset;
> > +
> >         is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
> >         err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, 0);
> >
> > +       env->prog->aux->max_ctx_offset = max_ctx_offset;
>
> I don't understand this.
> If we pass a ctx into a helper and it's going to
> access [0..N] bytes from it why do we need to hide it?
> max_ctx_offset will be used later raw_tp, tp, syscall progs
> to determine whether it's ok to load them.
> By hiding the actual size of access somebody can construct
> a prog that reads out of bounds.
> How is this related to NULL-ness property?

Same question, was just typing exactly the same thing.

>
> > +
> >         /* Compiler optimizations can remove arguments from static functions
> >          * or mismatched type can be passed into a global function.
> >          * In such cases mark the function as unreliable from BTF point of view.
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2c1f8069f7b7..d694f43ab911 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5229,6 +5229,25 @@ 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 the function doesn't know how to access the context,
> > +                * (because we are in a program of type SYSCALL for example), we
> > +                * can not statically check its size.
> > +                * Dynamically check it now.
> > +                */
> > +               if (!env->ops->convert_ctx_access) {
> > +                       enum bpf_access_type atype = meta && meta->raw_mode ? BPF_WRITE : BPF_READ;
> > +                       int offset = access_size - 1;
> > +
> > +                       /* Allow zero-byte read from PTR_TO_CTX */
> > +                       if (access_size == 0)
> > +                               return zero_size_allowed ? 0 : -EACCES;
> > +
> > +                       return check_mem_access(env, env->insn_idx, regno, offset, BPF_B,
> > +                                               atype, -1, false);
> > +               }
>
> This part looks good alone. Without max_ctx_offset save/restore.

+1, save/restore would be incorrect.