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

[Eduard Zingerman]

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.
      */