Re: [PATCH bpf-next v7 02/24] bpf/verifier: allow kfunc to read user provided context

Benjamin Tissoires
2022-07-22T08:44:18Z

On Thu, Jul 21, 2022 at 10:15 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Thu, 21 Jul 2022 at 17:36, 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>
> >
> > ---
> >
>
> LGTM, with just a couple more nits.
> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Thanks a lot for the quick review.

Sending the new version of just this patch now.

Cheers,
Benjamin

>
> > 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..d5fe7e618c52 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) {
> > + enum bpf_access_type atype = meta->raw_mode ? BPF_WRITE : BPF_READ;
> > + int offset = access_size - 1;
> > +
> > + /* Allow zero-byte read from NULL or PTR_TO_CTX */
>
> This will not be handling the case for NULL, only for kfunc(ptr_to_ctx, 0)
> A null pointer has its reg->type as scalar, so it will be handled by
> the default case.
>
> > + if (access_size == 0)
> > + return zero_size_allowed ? 0 : -EINVAL;
>
> We should use -EACCES, just to be consistent.
>
> > +
> > + return check_mem_access(env, env->insn_idx, regno, offset, BPF_B,
> > + atype, -1, false);
> > + }
> > +
> > + 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 +5355,7 @@ 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
> >
>