Re: [PATCH bpf v3 2/3] bpf: Do not mark NULL-checked raw_tp arg as scalar

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Dec 06, 2024 at 07:10:48PM +0100, Kumar Kartikeya Dwivedi wrote:
> On Fri, 6 Dec 2024 at 18:59, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Dec 6, 2024 at 8:11 AM Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
> > >
> > > An implication of this fix, which follows from the way the raw_tp fixes
> > > were implemented, is that all PTR_MAYBE_NULL trusted PTR_TO_BTF_ID are
> > > engulfed by these checks, and PROBE_MEM will apply to all of them, incl.
> > > those coming from helpers with KF_ACQUIRE returning maybe null trusted
> > > pointers. This NULL tagging after this commit will be sticky. Compared
> > > to a solution which only specially tagged raw_tp args with a different
> > > special maybe null tag (like PTR_SOFT_NULL), it's a consequence of
> > > overloading PTR_MAYBE_NULL with this meaning.
> > >
> > > Fixes: cb4158ce8ec8 ("bpf: Mark raw_tp arguments with PTR_MAYBE_NULL")
> > > Reported-by: Manu Bretelle <chantra@meta.com>
> > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > > ---
> > >  kernel/bpf/verifier.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 82f40d63ad7b..556fb609d4a4 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -15365,6 +15365,12 @@ static void mark_ptr_or_null_reg(struct bpf_verifier_env *env,
> > >                         return;
> > >
> > >                 if (is_null) {
> > > +                       /* We never mark a raw_tp trusted pointer as scalar, to
> > > +                        * preserve backwards compatibility, instead just leave
> > > +                        * it as is.
> > > +                        */
> > > +                       if (mask_raw_tp_reg_cond(env, reg))
> > > +                               return;
> >
> > The blast radius is getting too big.
> > Patch 1 is ok, but here we're doubling down on
> > the hack in commit
> > cb4158ce8ec8 ("bpf: Mark raw_tp arguments with PTR_MAYBE_NULL")
> 
> There are two concerns:
> First, it applies whether or not a register is a raw_tp arg. There is
> a way to detect that (with some register state, instead of using a
> separate tag).
> Second, we treat the program in the == NULL branch as if the pointer
> _maybe_ null, and in the != NULL as definitively not NULL.
> I don't really see how that's too different, given we already allow direct
> access etc. when the pointer is _unchecked_ after entry, and the state
> is same as
> the case where == NULL branch is explored.
> 
> >
> > I think we need to revert the raw_tp masking hack and
> > go with denylist the way Jiri proposed:
> > https://lore.kernel.org/bpf/ZrIj9jkXqpKXRuS7@krava/
> >
> > denylist is certainly less safer and it's a whack-a-mole
> > comparing to allowlist, but it's much much shorter
> > according to Jiri's analysis:
> > https://lore.kernel.org/bpf/Zr3q8ihbe8cUdpfp@krava/
> 
> Ok, let's revert.
> Jiri, do you have the diff around for that attempt? Could you post a
> revert of the patches and then the diff you shared?
> If not, I can carry it as well with the revert, if you share it with
> me (keeping the attribution etc.). Either is fine, lmk.

hi,
sorry for late reply.. I rebased it, there were some conflicts, it's compile tested,
and perhaps not up2date:
  https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/log/?h=bpf/tp_fix

jirka