Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

[Yonghong Song <yhs@fb.com>]

On 6/5/21 8:01 AM, Kurt Manucredo wrote:
> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
> kernel/bpf/core.c:1414:2.

This is not enough. We need more information on why this happens
so we can judge whether the patch indeed fixed the issue.

> 
> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
> missing them and return with error when detected.
> 
> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com
> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com>
> ---
> 
> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
> 
> Changelog:
> ----------
> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
>       Fix commit message.
> v3 - Make it clearer what the fix is for.
> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
>       check in check_alu_op() in verifier.c.
> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
>       check in ___bpf_prog_run().
> 
> thanks
> 
> kind regards
> 
> Kurt
> 
>   kernel/bpf/verifier.c | 30 +++++++++---------------------
>   1 file changed, 9 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 94ba5163d4c5..ed0eecf20de5 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>   	u32_min_val = src_reg.u32_min_value;
>   	u32_max_val = src_reg.u32_max_value;
>   
> +	if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
> +			umax_val >= insn_bitness) {
> +		/* Shifts greater than 31 or 63 are undefined.
> +		 * This includes shifts by a negative number.
> +		 */
> +		verbose(env, "invalid shift %lld\n", umax_val);
> +		return -EINVAL;
> +	}

I think your fix is good. I would like to move after
the following code though:

         if (!src_known &&
             opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
                 __mark_reg_unknown(env, dst_reg);
                 return 0;
         }

> +
>   	if (alu32) {
>   		src_known = tnum_subreg_is_const(src_reg.var_off);
>   		if ((src_known &&
> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>   		scalar_min_max_xor(dst_reg, &src_reg);
>   		break;
>   	case BPF_LSH:
> -		if (umax_val >= insn_bitness) {
> -			/* Shifts greater than 31 or 63 are undefined.
> -			 * This includes shifts by a negative number.
> -			 */
> -			mark_reg_unknown(env, regs, insn->dst_reg);
> -			break;
> -		}

I think this is what happens. For the above case, we simply
marks the dst reg as unknown and didn't fail verification.
So later on at runtime, the shift optimization will have wrong
shift value (> 31/64). Please correct me if this is not right
analysis. As I mentioned in the early please write detailed
analysis in commit log.

Please also add a test at tools/testing/selftests/bpf/verifier/.


>   		if (alu32)
>   			scalar32_min_max_lsh(dst_reg, &src_reg);
>   		else
>   			scalar_min_max_lsh(dst_reg, &src_reg);
>   		break;
>   	case BPF_RSH:
> -		if (umax_val >= insn_bitness) {
> -			/* Shifts greater than 31 or 63 are undefined.
> -			 * This includes shifts by a negative number.
> -			 */
> -			mark_reg_unknown(env, regs, insn->dst_reg);
> -			break;
> -		}
>   		if (alu32)
>   			scalar32_min_max_rsh(dst_reg, &src_reg);
>   		else
>   			scalar_min_max_rsh(dst_reg, &src_reg);
>   		break;
>   	case BPF_ARSH:
> -		if (umax_val >= insn_bitness) {
> -			/* Shifts greater than 31 or 63 are undefined.
> -			 * This includes shifts by a negative number.
> -			 */
> -			mark_reg_unknown(env, regs, insn->dst_reg);
> -			break;
> -		}
>   		if (alu32)
>   			scalar32_min_max_arsh(dst_reg, &src_reg);
>   		else
>