https://docs.aws.amazon.com/ja_jp/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourceip
大事なことはすべてここにある

まとめ

aws:SourceIpはリストで指定NG
VPCエンドポイント経由のときはaws:SourceIpではなくaws:VpcSourceIp

初歩的ミス

リストでaws:SourceIpだめ

{
	"Version": "2012-10-17",
	"Statement": [
		{
			"Sid": "AllowSSLRequestsOnly",
			"Effect": "Deny",
			"Principal": "*",
			"Action": "s3:*",
			"Resource": [
				"arn:aws:s3:::s3-a",
				"arn:aws:s3:::s3-a/*"
			],
			"Condition": {
				"Bool": {
					"aws:SecureTransport": "false"
				}
			}
		},
		{
			"Sid": "AllowCrossAccountEC2Access",
			"Effect": "Allow",
			"Principal": {
				"AWS": "arn:aws:iam::123456789012:role/iam-a"
			},
			"Action": [
				"s3:GetObject",
				"s3:PutObject",
				"s3:ListBucket"
			],
			"Resource": [
				"arn:aws:s3:::s3-a",
				"arn:aws:s3:::s3-a/*"
			],
			"Condition": {
				"IpAddress": {
					"aws:SourceIp": [
						"172.31.XXX.0/27"
					]
				}
			}
		}
	]
}

↑機能しない

リスト消したらいけた
↓機能した

コマンド

aws s3api put-object --bucket s3-a --key test.txt
{
    "VersionId": "fzwymj0plYsbv6vcD0ckKvsXefKXAqDF",
    "ETag": "\"d41d8cd98f00b204e9800998ecf8427e\"",
    "ServerSideEncryption": "AES256"
}

ポリシー

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowSSLRequestsOnly",
            "Effect": "Deny",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "Bool": {
                    "aws:SecureTransport": "false"
                }
            }
        },
        {
            "Sid": "AllowCrossAccountEC2Access",
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "arn:aws:iam::123456789012:role/iam-a"
                ]
            },
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "IpAddress": {
                    "aws:SourceIp": "172.31.XXX.0/27"←ここ
                }
            }
        }
    ]
}

応用

だけれどもVPCエンドポイント（gateway）をルートに加えると・・・

aws s3api put-object --bucket s3-a --key test.txt

An error occurred (AccessDenied) when calling the PutObject operation: Access Denied

これはVPCエンドポイントの経路が優先されるので、接続元のIPアドレスがプライベートIPアドレスとして接続しにいった
よって、許可IPとフィールドの変更（VpcSourceIp）を行う

誤

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowSSLRequestsOnly",
            "Effect": "Deny",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "Bool": {
                    "aws:SecureTransport": "false"
                }
            }
        },
        {
            "Sid": "AllowCrossAccountEC2Access",
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "arn:aws:iam::123456789012:role/iam-a"
                ]
            },
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "IpAddress": {
                    "aws:SourceIp": "10.XX.XX.0/27"
                }
            }
        }
    ]
}

正しい方にしたらできたね
コマンド

aws s3api put-object --bucket s3-a --key test.txt
{
    "VersionId": "q_iSzdIbEdrQRVOqwnNZuQgnyYQJBXER",
    "ETag": "\"d41d8cd98f00b204e9800998ecf8427e\"",
    "ServerSideEncryption": "AES256"
}

ポリシー

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowSSLRequestsOnly",
            "Effect": "Deny",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "Bool": {
                    "aws:SecureTransport": "false"
                }
            }
        },
        {
            "Sid": "AllowCrossAccountEC2Access",
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "arn:aws:iam::123456789012:role/iam-a"
                ]
            },
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::s3-a",
                "arn:aws:s3:::s3-a/*"
            ],
            "Condition": {
                "IpAddress": {
                    "aws:VpcSourceIp": "10.XX.XX.0/27"
                }
            }
        }
    ]
}