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
>
