Re: [PATCH v2 bpf 1/2] bpf: verifier: check for packet data access based on target prog

[Alexei Starovoitov]

On Fri, Aug 21, 2020 at 1:53 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 8/21/20 12:07 PM, Udip Pant wrote:
> >
> >
> > > On 8/20/20, 11:17 PM, "Yonghong Song" <yhs@xxxxxx> wrote:
> >>
> >>
> >>
> >> On 8/20/20 11:13 PM, Yonghong Song wrote:
> >>>
> >>>
> >>> On 8/20/20 5:28 PM, Udip Pant wrote:
> >>>> While using dynamic program extension (of type BPF_PROG_TYPE_EXT), we
> >>>> need to check the program type of the target program to grant the read /
> >>>> write access to the packet data.
> >>>>
> >>>> The BPF_PROG_TYPE_EXT type can be used to extend types such as XDP, SKB
> >>>> and others. Since the BPF_PROG_TYPE_EXT program type on itself is just a
> >>>> placeholder for those, we need this extended check for those target
> >>>> programs to actually work while using this option.
> >>>>
> >>>> Tested this with a freplace xdp program. Without this patch, the
> >>>> verifier fails with error 'cannot write into packet'.
> >>>>
> >>>> Signed-off-by: Udip Pant <udippant@xxxxxx>
> >>>> ---
> >>>>    kernel/bpf/verifier.c | 6 +++++-
> >>>>    1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>>> index ef938f17b944..4d7604430994 100644
> >>>> --- a/kernel/bpf/verifier.c
> >>>> +++ b/kernel/bpf/verifier.c
> >>>> @@ -2629,7 +2629,11 @@ static bool may_access_direct_pkt_data(struct
> >>>> bpf_verifier_env *env,
> >>>>                           const struct bpf_call_arg_meta *meta,
> >>>>                           enum bpf_access_type t)
> >>>>    {
> >>>> -    switch (env->prog->type) {
> >>>> +    struct bpf_prog *prog = env->prog;
> >>>> +    enum bpf_prog_type prog_type = prog->aux->linked_prog ?
> >>>> +          prog->aux->linked_prog->type : prog->type;
> >>>
> >>> I checked the verifier code. There are several places where
> >>> prog->type is checked and EXT program type will behave differently
> >>> from the linked program.
> >>>
> >>> Maybe abstract the the above logic to one static function like
> >>>
> >>> static enum bpf_prog_type resolved_prog_type(struct bpf_prog *prog)
> >>> {
> >>>       return prog->aux->linked_prog ? prog->aux->linked_prog->type
> >>>                         : prog->type;
> >>> }
> >>>
> >
> > Sure.
> >
> >>> This function can then be used in different places to give the resolved
> >>> prog type.
> >>>
> >>> Besides here checking pkt access permission,
> >>> another possible places to consider is return value
> >>> in function check_return_code(). Currently,
> >>> for EXT program, the result value can be anything. This may need to
> >>> be enforced. Could you take a look? It could be others as well.
> >>> You can take a look at verifier.c by searching "prog->type".
> >>
> >
> > Yeah there are few other places in the verifier where it decides without resolving for the 'extended' type. But I am not too sure if it makes sense to extend this logic as part of this commit. For example, as you mentioned, in the check_return_code() it explicitly ignores the return type for the EXT prog (kernel/bpf/verifier.c#L7446).  Likewise, I noticed similar issue inside the check_ld_abs(), where it checks for may_access_skb(env->prog->type).
> >
> > I'm happy to extend this logic there as well if deemed appropriate.
>
> Thanks. I would like to see the verifier parity between original program
> and replace program. That is, if the original program and the replace
> program are the same, they should be both either accepted or rejected
> by verifier. Yes, this may imply more changes e.g., check_return_code()
> or check_ld_abs() than your original patch.
> Alexei or Daniel, what is your opinion on this?

The set was already marked as 'changes requested' in patchworks.
That's an indication that maintainers agree with the feedback :)
In this particular case it certainly makes sense to address all cases
instead of doing them one at a time.