On Wed, 4 Dec 2024 at 21:19, Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> On Wed, 4 Dec 2024 at 21:12, Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > On Tue, 2024-12-03 at 18:41 -0800, Kumar Kartikeya Dwivedi wrote:
> >
> > [...]
> >
> > > +/* r2 with offset is checked, which marks r1 with off=0 as non-NULL */
> > > +SEC("tp_btf/bpf_testmod_test_raw_tp_null")
> > > +__failure
> > > +__msg("3: (07) r2 += 8 ; R2_w=trusted_ptr_or_null_sk_buff(id=1,off=8)")
> > > +__msg("4: (15) if r2 == 0x0 goto pc+2 ; R2_w=trusted_ptr_or_null_sk_buff(id=2,off=8)")
> > > +__msg("5: (bf) r1 = r1 ; R1_w=trusted_ptr_sk_buff()")
> >
> > This looks like a bug.
> > 'r1 != 0' does not follow from 'r2 == r1 + 8 and r2 != 0'.
> >
>
> Hmm, yes, it's broken.
> I am realizing where we do it now will walk r1 first and we'll not see
> r2 off != 0 until after we mark it already.
> I guess we need to do the check sooner outside this function in
> mark_ptr_or_null_regs.
> There we have the register being operated on, so if off != 0 we don't
> walk all regs in state.
What this will do in both cases::
First, avoid walking states when off != 0, and reset id.
If off == 0, go inside mark_ptr_or_null_reg and walk all regs, and
remove marks for those with off != 0.
>
> Do you think that should fix this?
>
> > > +int BPF_PROG(test_raw_tp_null_copy_check_with_off, struct sk_buff *skb)
> > > +{
> > > + asm volatile (
> > > + "r1 = *(u64 *)(r1 +0); \
> > > + r2 = r1; \
> > > + r3 = 0; \
> > > + r2 += 8; \
> > > + if r2 == 0 goto jmp2; \
> > > + r1 = r1; \
> > > + *(u64 *)(r3 +0) = r3; \
> > > + jmp2: "
> > > + ::
> > > + : __clobber_all
> > > + );
> > > + return 0;
> > > +}
> >
> > [...]
