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

[Yonghong Song]

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;
> }
>
> 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".

Note that if the EXT program tries to replace a global subprogram,
then return value cannot be enforced, just as what Patch #2 example shows.

> > +
> > +    switch (prog_type) {
> >       /* Program types only with direct read access go here! */
> >       case BPF_PROG_TYPE_LWT_IN:
> >       case BPF_PROG_TYPE_LWT_OUT: