Re: [PATCH bpf-next] bpf: make the verifier trace the "not qeual" for regs

[Andrii Nakryiko]

On Mon, Dec 11, 2023 at 6:16 PM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2023 at 3:16 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Sun, Dec 10, 2023 at 5:00 AM Menglong Dong <menglong8.dong@xxxxxxxxx> 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>
> > > ---
> > >  kernel/bpf/verifier.c | 45 ++++++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 44 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 727a59e4a647..7b074ac93190 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -1764,6 +1764,40 @@ static void __mark_reg_const_zero(struct bpf_reg_state *reg)
> > >         reg->type = SCALAR_VALUE;
> > >  }
> > >
> > > +#define CHECK_REG_MIN(value)                   \
> > > +do {                                           \
> > > +       if ((value) == (typeof(value))imm)      \
> > > +               value++;                        \
> > > +} while (0)
> > > +
> > > +#define CHECK_REG_MAX(value)                   \
> > > +do {                                           \
> > > +       if ((value) == (typeof(value))imm)      \
> > > +               value--;                        \
> > > +} while (0)
> > > +
> > > +static void mark_reg32_not_equal(struct bpf_reg_state *reg, u64 imm)
> > > +{
> > > +               CHECK_REG_MIN(reg->s32_min_value);
> > > +               CHECK_REG_MAX(reg->s32_max_value);
> > > +               CHECK_REG_MIN(reg->u32_min_value);
> > > +               CHECK_REG_MAX(reg->u32_max_value);
> > > +}
> > > +
> > > +static void mark_reg_not_equal(struct bpf_reg_state *reg, u64 imm)
> > > +{
> > > +               CHECK_REG_MIN(reg->smin_value);
> > > +               CHECK_REG_MAX(reg->smax_value);
> > > +
> > > +               CHECK_REG_MIN(reg->umin_value);
> > > +               CHECK_REG_MAX(reg->umax_value);
> > > +
> > > +               CHECK_REG_MIN(reg->s32_min_value);
> > > +               CHECK_REG_MAX(reg->s32_max_value);
> > > +               CHECK_REG_MIN(reg->u32_min_value);
> > > +               CHECK_REG_MAX(reg->u32_max_value);
> > > +}
> >
> > please don't use macros for this, this code is tricky enough without
> > having to jump around double-checking what exactly macros are doing.
> > Just code it explicitly.
> >
>
> Okay!
>
> > Also I don't see the need for mark_reg32_not_equal() and
> > mark_reg_not_equal() helper functions, there is just one place where
> > this logic is going to be called from, so let's add code right there.
> >
>
> Yeah, you are right. And I just found that you have already
> implemented the test case for this logic in reg_bounds.c/range_cond().
> I wonder why this logic is not implemented in the verifier yet?
> Am I missing something?

No, I just didn't want to add yet more verifier changes in my original
patch set on extending reg bounds logic.

>
> Thanks!
> Menglong Dong
>
> > > +
> > >  static void mark_reg_known_zero(struct bpf_verifier_env *env,
> > >                                 struct bpf_reg_state *regs, u32 regno)
> > >  {
> > > @@ -14332,7 +14366,16 @@ 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 */
> > > +               /* try to recompute the bound of reg1 if reg2 is a const and
> > > +                * is exactly the edge of reg1.
> > > +                */
> > > +               if (is_reg_const(reg2, is_jmp32)) {
> > > +                       val = reg_const_value(reg2, is_jmp32);
> > > +                       if (is_jmp32)
> > > +                               mark_reg32_not_equal(reg1, val);
> > > +                       else
> > > +                               mark_reg_not_equal(reg1, val);
> > > +               }
> > >                 break;
> > >         case BPF_JSET:
> > >                 if (!is_reg_const(reg2, is_jmp32))
> > > --
> > > 2.39.2
> > >