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

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-12-06 at 09:59 -0800, Alexei Starovoitov 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")
> 
> 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/
> 
> Eduard had an idea how to auto generate such allow/denylist
> during the build.
> That could be a follow up.

If the sole goal is to avoid dead code elimination for tracepoint
parameter null check, there might be another hack. Not sure if it was
discussed:
- don't add PTR_MAYBE_NULL (but maybe add a new tag, PTR_SOFT_NULL
  from Kumar's original RFC);
- in is_branch_taken() don't predict anything when tracepoint
  parameters are compared;
- in mark_ptr_or_null_regs() don't propagate null for pointers to
  tracepoint parameters (as in this patch).

Seems pretty confined but can't catch nullable tracepoint parameters
being passed to kfuncs.