Re: [PATCH net-next v2 1/2] bpf: make the verifier trace the "not qeual" for regs

[Menglong Dong]

On Wed, Dec 13, 2023 at 7:23 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Tue, 2023-12-12 at 21:10 +0800, Menglong Dong wrote:
> > We can derive some new information for BPF_JNE in regs_refine_cond_op().
> > Take following code for example:
> >
> >   /* The type of "a" is u16 */
> >   if (a > 0 && a < 100) {
> >     /* the range of the register for a is [0, 99], not [1, 99],
> >      * and will cause the following error:
> >      *
> >      *   invalid zero-sized read
> >      *
> >      * as a can be 0.
> >      */
> >     bpf_skb_store_bytes(skb, xx, xx, a, 0);
> >   }
> >
> > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> > TRUE branch, the dst_reg will be marked as known to 0. However, in the
> > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> > the [min, max] for a is [0, 99], not [1, 99].
> >
> > For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
> > const and is exactly the edge of the dst reg.
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
> > ---
>
> Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
>
> >  kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++-
> >  1 file changed, 28 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 727a59e4a647..08ee0e02df96 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
> >               }
> >               break;
> >       case BPF_JNE:
> > -             /* we don't derive any new information for inequality yet */
> > +             if (!is_reg_const(reg2, is_jmp32))
> > +                     swap(reg1, reg2);
> > +             if (!is_reg_const(reg2, is_jmp32))
> > +                     break;
> > +
> > +             /* try to recompute the bound of reg1 if reg2 is a const and
> > +              * is exactly the edge of reg1.
> > +              */
> > +             val = reg_const_value(reg2, is_jmp32);
> > +             if (is_jmp32) {
> > +                     if (reg1->u32_min_value == (u32)val)
> > +                             reg1->u32_min_value++;
>
> Nit: I spent an unreasonable amount of time trying to figure out if
>      overflow might be an issue here. Would it be helpful to add a
>      comment like below? (not sure, maybe it's obvious and I'm being slow)
>
>      /* u32_min_value is not equal to 0xffffffff at this point,
>       * because otherwise u32_max_value is 0xffffffff as well,
>       * in such a case both reg1 and reg2 would be constants,
>       * jump would be predicted and reg_set_min_max() won't
>       * be called.
>       * Same reasoning works for all {u,s}{min,max}{32,64} cases below.
>       */

Okay, I'll add this comment in the next version.

Thanks!
Menglong Dong