Re: [PATCH bpf-next v3 1/2] bpf: make the verifier tracks the "not equal" for regs

[Andrii Nakryiko]

On Wed, Dec 13, 2023 at 10:28 PM 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>
> ---
> v2:
> - fix a typo in the subject
> - add some comments, as Eduard advised
> ---
>  kernel/bpf/verifier.c | 38 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 37 insertions(+), 1 deletion(-)
>

The logic looks good

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 727a59e4a647..9b1932e51823 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14332,7 +14332,43 @@ 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) {
> +                       /* 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.
> +                        */
> +                       if (reg1->u32_min_value == (u32)val)
> +                               reg1->u32_min_value++;
> +                       if (reg1->u32_max_value == (u32)val)
> +                               reg1->u32_max_value--;
> +                       if (reg1->s32_min_value == (s32)val)
> +                               reg1->s32_min_value++;
> +                       if (reg1->s32_max_value == (s32)val)
> +                               reg1->s32_max_value--;
> +               } else {
> +                       if (reg1->umin_value == (u64)val)
> +                               reg1->umin_value++;
> +                       if (reg1->umax_value == (u64)val)
> +                               reg1->umax_value--;
> +                       if (reg1->smin_value == (s64)val)
> +                               reg1->smin_value++;
> +                       if (reg1->smax_value == (s64)val)
> +                               reg1->smax_value--;
> +               }
>                 break;
>         case BPF_JSET:
>                 if (!is_reg_const(reg2, is_jmp32))
> --
> 2.39.2
>