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([2a01:e0a:ab7:2110:6a1d:efff:fe52:1959]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6b497cfd6ffsm2575185a12.24.2026.10.29.08.44.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Oct 2026 08:44:47 -0700 (PDT) Message-ID: <7fc6fa79-af42-49e8-8b41-00ad4a4d9b74@6wind.com> Date: Thu, 29 Oct 2026 16:44:46 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: nicolas.dichtel@6wind.com Subject: Re: [PATCH net] genetlink: Reject joining multicast groups no family owns To: Anil Kaushik , netdev@vger.kernel.org Cc: "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Ido Schimmel , XingWang Xiang , Alice Ryhl , linux-kernel@vger.kernel.org References: <20261029062214.3407223-1-anilkaushikwireless@gmail.com> From: Nicolas Dichtel Content-Language: en-US Organization: 6WIND In-Reply-To: <20261029062214.3407223-1-anilkaushikwireless@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Le 29/10/2026 à 07:22, Anil Kaushik a écrit : > genl_bind() checks the GENL_MCAST_CAP_* flags of a group only when a > family owns it. For a group ID that no family owns, it returns 0, so any > user can join it. > > A family registered later can get that ID, and the socket stays a member: > nothing checks it again. An unprivileged user can join the free IDs, load > a family module with CTRL_CMD_GETFAMILY, which calls request_module(), > and then receive the notifications of a group with > GENL_MCAST_CAP_NET_ADMIN or GENL_MCAST_CAP_SYS_ADMIN. For example the > drop_monitor "events" group always gets ID 1, and is free until the > drop_monitor module is loaded. > > Fix by returning -ENOENT from genl_bind() when no family owns the group. > Nothing is ever sent to such a group, so joining it has no use. Moreover, a user should retrieve the group index before joining it. It seems weird to join it without this step. > > Found by the Sashiko review of the gtp multicast group fix. Tested as an > unprivileged user who joins all free IDs, then loads gtp and > drop_monitor: > > Before: > joins: ok=62 EPERM=2 > gtp/gtp (ID 15): member > NET_DM/events (ID 1): member > > After: > joins: ok=19 ENOENT=43 EPERM=2 > gtp/gtp (ID 15): not member > NET_DM/events (ID 1): not member > > Link: https://lore.kernel.org/netdev/179324692107.3134.14443689892607921337@kernel.org/ > Fixes: e03781879a0d ("drop_monitor: Require 'CAP_SYS_ADMIN' when joining "events" group") > Signed-off-by: Anil Kaushik Reviewed-by: Nicolas Dichtel